Bac to the future

Over the last few years, there has been a growing body of research – and tremendous interest – focused on the role/influence of the gastrointestinal system in Parkinson’s.

In today’s post we are going to look at a recent piece of research that suggests some of the bacteria in our gut can influence the availability of the medication we use to treat Parkinson’s. 

In addition, we will look at a novel way researchers are re-engineering bacteria in the gut to correct other medical conditions (such as phenylketonuria) and we will ask if the same can not be applied to Parkinson’s.


The Platypus. Source: National geographic

The interesting, but utterly useless fact of the day: The duck-billed Platypus of Australia does not have a stomach.


No really. These oddities of evolution have no stomach. There’s no sac in the middle of their bodies that secrete powerful acids and digestive enzymes. The oesophagus (the tube from the mouth) of the platypus connects directly to its intestines.

The platypus. Source: Topimage

And believe it or not, platypus are not alone on this ‘sans estomac‘ trend. At least a 1/4 of the fish species on this planet do not have a stomach (Source).


And this absense of the stomach isn’t even remotely weird in the animal kingdom. Some creatures don’t even have a gastrointestinal system. No mouth. No anus. No intestines. Nothing.


The giant tube worm – Riftia pachyptila – lives on the floor of the Pacific Ocean, next to hot hydrothermal vents and can tolerate extremely high levels of hydrogen sulfide (hazardous for you and I). These creatures – which can grow up to 2.4 meters (or 7+ feet) in length – have no gastrointestinal tract whatsoever. Zip, zero, nada.

Rather they have an internal cavity – called a trophosome – filled with bacteria which live symbiotically with them.

Watch this video of Ed Yong explaining it all (great video!):


WOW! Fascinating! But what does ANY of this have to do with Parkinson’s?

Well, human beings are on the other end of this gut spectrum. We have a VERY complicated gastrointestinal system.

We are not quite up there with cows and other ruminants (mammals that are able to acquire nutrients from plant-based food) which all have more than mutliple stomachs.

“We all have multiple stomachs”. Source: Quora

But we do have a complex system of breaking down food and getting what we need from it.

And new research suggests that our gastrointestinal system may be having an impact on how our body absorbs medication like Levodopa – which is used to treat Parkinson’s.

Remind me again, what is Levodopa?

Levodopa (or L-DOPA) is the precursor to the chemical dopamine. Dopamine producing cells are lost in Parkinson’s, so replacing the missing dopamine is one way to treat the motor features of the condition.

Unfortunately, simply giving people pills of dopamine is a non-starter though: dopamine is unstable, breaks down too quickly, and (strangely) has a very hard time getting into the brain (it is blocked by the protective layer called the blood-brain-barrier).

Levodopa, on the other hand, is very robust and has no problem entering the brain.

We use tablets of levodopa to replace the missing dopamine in individuals with Parkinson’s. Brand names of levodopa include Sinemet, Stalevo, and Atamet.

Once inside the brain, levodopa is quickly converted into dopamine. It is changed into dopamine by an enzyme called DOPA decarboxylase, and this change rapidly increases the levels of dopamine in the brain, allowing the locomotion parts of the brain to function more normally.


The chemical conversion of L-DOPA to dopamine. Source: Nootrobox

Ok, so what did you mean by “our gastrointestinal system may be having an impact on how our body receives Levodopa”?

This research report was recently published:

Title: Gut bacterial tyrosine decarboxylases restrict levels of levodopa in the treatment of Parkinson’s disease.
Authors: van Kessel SP, Frye AK, El-Gendy AO, Castejon M, Keshavarzian A, van Dijk G, El Aidy S.
Journal: Nat Commun. 2019 Jan 18;10(1):310. doi: 10.1038/s41467-019-08294-y.
PMID: 30659181               (This report is OPEN ACCESS if you would like to read it)

In this study, the researchers were interested in the influence that the microbiota of the gut could have on how well levodopa is absorbed.

What is the microbiota of the gut?

You may think of yourself as an individual, but in reality you are not. Not even remotely singular.

Ours bodies are made up of microbiota – that is,  collections of microbes or microorganisms inhabiting particular environments (or region of our body) and creating “mini-ecosystems”. And whether you like this idea or not, you need them. Many of them are essential to our survival.

The human gut in particular hosts tens of trillions of microorganisms, including at least 1000 species of bacteria (which is a guess-timate as we are not really sure how many species there are).


Bacteria in the gut. Source: Huffington Post

The microorganisms in the human gut perform all manner of tasks for you to make your life easier. From helping to break down food, to aiding with the production of some vitamins (in particular B and K).

And the resarchers think that some of these bacteria may affect levodopa treatment in people with Parkinson’s?

Exactly. And this is what they were interested in investigating.

It you will recall, levodopa is converted into dopamine by an enzyme called DOPA decarboxylase (or DDC), but gut bacteria do not produce this enzyme. They do, however, produce other ‘decarboxylases’, including one called tyrosine decarboxylase (or TDC)

Tyrosine decarboxylase is an enzyme which converts tyrosine (an amino acid) into tyramine.

Source: Semanticscholar

Something you may have noticed about tyrosine and levodopa is that they are very similar in the chemical structure.

Tyrosine (left) and levodopa (right). Source: Wikipedia

And the researchers were wondering if – given this similarity – tyrosine decarboxylase might also have the ability to decarboxylate levodopa (that is, convert it into dopamine).

They decided to test this idea, and they began by collecting the microbiota from the jejunum.

What is the jejunum?

The human digestive system is about 26 feet long – approximately 8 meters from mouth to anus – and it can be divided into different regions.

The jejunum is the second part of the small intestine in humans. It measures on average 2.5 meters in length and it lies between the duodenum and the ileum.

Source: Wikipedia

Importantly, the jejunum is recognised as the primary site of levodopa absorption (Click here to read more about this).

The researchers collected microbiota from the small intestine and grew the bacteria in culture with levodopa. They found that the bacteria were efficient at converting the levodopa into dopamine.

And they confirmed this result by repeating the experiment, but in the absense of tyrosine decarboxylase, which resulted in no dopamine production. They also found that increasing levels of tyrosine – the correct target of tyrosine decarboxylase – did not affect this production of dopamine.

Next, the researchers checked to see if carbidopa inhibits tyrosine decarboxylase.

What is carbidopa?

Carbidopa is a DOPA decarboxylase inhibitors, which does not cross the blood-brain-barrier – the protective membrane covering the entire brain that I mentioned above. This property is important as it prevents levodopa medication from being converted into dopamine elsewhere in the body.


The production of dopamine, using Levodopa. Source: Watcut

Outside the brain, there is a lot of DOPA decarboxylase in other organs of the body, and if this is not blocked then the levodopa will be converted into dopamine before it reaches the brain and the effect of Levodopa in the brain will be reduced. To this end, people with Parkinson’s are also given DOPA decarboxylase inhibitors like carbidopa which inhibits DOPA decarboxylase outside of the brain.

Interestingly, the researchers found that DOPA decarboxylase inhibitors had no impact on the bacteria produced tyrosine decarboxylase. That is to say, the tyrosine decarboxylase produced by the gut bacteria continued to convert levodopa into dopamine even in the presence of carbidopa.

Next the researchers analysed gut microbiota samples from people with Parkinson’s. The individuals were all being treated with different doses of levodopa/carbidopa medication (ranging from 300 up to 1100 mg levodopa per day). The researchers found that the amount of tyrosine decarboxylase in the samples were associated with both the amount of levodopa being administered AND the length of time each person had had Parkinsons.

Source: PMC

All of these results (and more) suggested to the researchers that decreasing efficacy of levodopa treatment observed in people with Parkinson’s might be explained by the overgrowth of small intestinal bacteria that can convert levodopa, even in the presence of carbidopa.

Source: PMC

And this is not the first time we have seen this kind of phenomenon – we have previously written about how helicobacter pylori infections of the gut can affect levodopa treatment (Click here to read that post).

The researchers also make one interesting note in the discussion of their research – some of the bacterial strains that produce the tyrosine decarboxylase are marketed as probiotics. Use of these dietary supplements may need to be investigated in the context of Parkinson’s – could these supplements be detrimental for levodopa treatment?

The researchers also question whether these bacteria could be used as biomarkers for subtyping people with Parkinson’s and tracking them over time. An interesting idea – one which the researchers are following up now.

Interesting. How else can the gut influence Parkinson’s?

There is a growing body of evidence suggesting that the gut and all of its constituent parts is having some kind of influential role on Parkinson’s. There is even some evidence to suggest that the condition may be starting in the gut (see the very first SoPD post to read more on that).

We have previously written about the connections between the gut and Parkinson’s (Click herehere and here for some examples of previous SoPD gut posts), and there are now studies suggesting that the gut could be influencing this debilitating condition, such as this study:


Title: Gut Microbiota Regulate Motor Deficits and Neuroinflammation in a Model of Parkinson’s Disease
Authors: Sampson TR, Debelius JW, Thron T, Janssen S, Shastri GG, Ilhan ZE, Challis C, Schretter CE, Rocha S, Gradinaru V, Chesselet MF, Keshavarzian A, Shannon KM, Krajmalnik-Brown R, Wittung-Stafshede P, Knight R, Mazmanian SK
Journal: Cell, 167 (6), 1469–1480
PMID: 27912057             (this report is OPEN ACCESS if you would like to read it)

The researchers (who have previously conducted a great deal of research on the microbiota of the gut and it’s interactions with the host) used mice that have been genetically engineered to produce abnormal amounts of alpha synuclein – the protein associated with Parkinson’s (Click here for more on this) and the clustering or aggregation of this protein is believed to be involved with the progression of the condition (Click here to read more about this).

The researchers tested these alpha synuclein producing mice and normal wild-type mice on some behavioural tasks and found that the alpha-synuclein producing mice performed worse.


A lab mouse. Source: USNews

The researchers then raised a new batch of alpha-synuclein producing mice in a ‘germ free environment’ and tested them on the same behavioural tasks. ‘Germ free environment’ means that the mice have no microorganisms living within them. So these mice have a very limited microbiota in their gastrointestinal tract.

And guess what happened:

The germ-free alpha-synuclein producing mice performed as well as on the behavioural task as the normal mice. There was no difference in the performance of the two sets of mice.

How could this be?

This is precisely what the researchers were wondering.

So they decided to have a look at the brains of the mice, where they found less aggregation of alpha synuclein protein in the brains of germ-free alpha-synuclein producing mice than their ‘germ-full’ alpha-synuclein producing mice.

This result suggested that the microbiota of the gut may be somehow involved with controlling the aggregation of alpha-synuclein in the brain. The researchers also noticed that the microglia – helper cells in the brain – of the germ-free alpha-synuclein producing mice looked different to their counterparts in the germ-full alpha-synuclein producing mice, indicating that in the absence of aggregating alpha synuclein the microglia were not becoming activated (a key feature in the Parkinsonian brain).

The researchers next began administering antibiotics to see if they could replicate the effects that they were seeing in the germ-free mice. Remarkably, alpha-synuclein producing mice injected with antibiotics exhibited very little dysfunction in the motor behaviour tasks, and they closely resembling mice born under germ-free conditions.


How antibiotics work. Source: MLB

Antibiotics kill bacteria via many different mechanisms (eg. disrupting the cell membrane or targeting protein synthesis; see image above), and they have previously demonstrated efficacy in models of Parkinson’s. We shall come back to this in a section below.

The researchers in the study next asked if the microbiota of people with Parkinson’s could affect the behaviour of their germ free mice. They took samples of gut bacteria from 6 people who were newly diagnosed (and treatment naive) with Parkinson’s and from 6 healthy age matched control samples. These samples were then injected into the guts of germ free mice… and guess what happened.

The germ-free mice injected with gut samples from Parkinsonian subjects performed worse on the behavioural tasks than those injected with samples from healthy subjects. This finding suggested that the gut microbiota of people with Parkinson’s has the potential to influence vulnerable mice.

Note the wording of that last sentence.

Importantly, the researchers noted that when they attempted this experiment in normal mice they observed no difference in the behaviour of the mice regardless of which gut samples were injected (Parkinsonian or healthy). This suggests that an abundance of alpha synuclein is required for the effect, and that the microbiota of the gut is exacerbating the effect.

Interesting. There are two potential treatment approaches here for Parkinson’s (microbiota transplant or antibiotics). Has any research been done on this?

Both are areas of investigation.

With regards to the microbiota transplant idea – there is a new clinical trial that has recently been initiated in Belgium to explore this idea in Parkinson’s (Click here to read more about this).

And with regards to antibiotics for Parkinson’s, there have been numerous reports demonstrating beneficial effects in preclinical models of the condition. Most recently this report:

Title: Repurposing doxycycline for synucleinopathies: remodelling of α-synuclein oligomers towards non-toxic parallel beta-sheet structured species.
Authors: González-Lizárraga F, Socías SB, Ávila CL, Torres-Bugeau CM, Barbosa LR, Binolfi A, Sepúlveda-Díaz JE, Del-Bel E, Fernandez CO, Papy-Garcia D, Itri R, Raisman-Vozari R, Chehín RN.
Journal: Sci Rep. 2017 Feb 3;7:41755.
PMID: 28155912                (This article is OPEN ACCESS if you would like to read it)

In this study, the researchers wanted to test the antibiotic doxycycline in disease-relevant models of Parkinson’s. The researchers found that doxycycline was able to inhibit the disease related clustering of alpha synuclein. In fact, by reshaping alpha synuclein into a less toxic version of the protein, doxycycline was able to enhance cell survival.

The investigators also conducted a ‘dosing’ experiment to determine the most effect dose and they found that taking doxycycline in sub-antibiotic doses (20–40 mg/day) would be enough to exert neuroprotection. They concluded their study by suggesting that these novel effects of doxycycline could be exploited in Parkinson’s disease by “repurposing an old safe drug”.

What does ‘sub-antibiotic doses’ mean?

These are doses at which the treatment no longer has an impact against bacteria.

Has there been a clinical trial for antibiotics in Parkinson’s?

Yes, there has, but that trial failed to demonstrate any beneficial effect and the antibiotic that was used – minocycline  – was associated with a higher drop out rate among the participants (Click here to read a report about that trial).

And, theoretically speaking, the use of antibiotics for Parkinson’s is a really bad idea.


If you look at the comments section under the González-Lizárraga research report, a cautionary message has been left by Prof Paul M. Tulkens of the Louvain Drug Research Institute in Belgium. He points out that:

“…using antibiotics at sub-therapeutic doses is the best way to trigger the emergence of resistance (supported by many in vitro and in vivo studies). Using an antibiotic for other indications than an infection caused by a susceptible bacteria is something that should be discouraged”

And he is correct.

We recklessly over use antibiotics all over the world at the moment and they are one of the few lines of defence that we have against the bacterial world. Long term use (which Parkinson’s would probably require) of an antibiotic at sub-therapeutic levels will only encourage the rise of antibiotic resistant bacteria.

The resistance of bacteria to antibiotics can occur spontaneously via several means (for example, through random genetic mutations during cell division). With the right mutation (inferring antibiotic resistance), an individual bacteria would then have a natural advantage over their friends and it would survive our attempts to kill it with antibiotics. Being resistant to antibiotic would leave that bacteria to wreak havoc upon us.

Its the purest form of natural selection.


How bacteria become resistant to antibiotics. Source: Reactgroup

And antibiotic resistant bacteria are fast becoming a major health issue for us, with the number of species of bacteria developing resistance increasing every year (Click here for a good review on factors contributing to the emergence of resistance, and click here for a review of the antibiotic resistant bacteria ‘crisis’).

But do not be upset on the Parkinson’s side of things. Prof Tulken adds that:

“If doxycycline really acts as the authors propose, the molecular targets are probably very different from those causing antibacterial activity. it should therefore be possible to dissociate these effect from the antibacterial effects and to get active compounds devoid of antibacterial activity This is where research must go to rather than in trying to use doxycycline itself.”

And he is correct again.

Rather than tempting disaster, we need to take the more prudent approach.

Ok, but what can we do about this?

Well, researchers are now exploring derivatives of antibacterial drugs that have neuroprotective effects to try and isolate compounds that maintain the neuroprotective properties, but lose the antibacterial behaviour. We will hopefully learn more about this in the near future.

So all we have to do now is wait for the microbiota clinical trial results or news about new non-antibiotic drugs?

Or we could start reprogramming the bacteria in the gut?

Que?!? What do you mean?

A Boston-based biotech firm recently reported something rather amazing, which probably deserved a lot more attention than it actually got.

The company is called Synlogic.

And they specialise in the genetic-engineering of bacteria. Specifically, the company genetically encourages bacteria to produce beneficial factors that can help with the treatment of dangerous and debilitating medical conditions.

The folks at Synlogics fully appreciate that bacteria in our gut represents an opportunity to regulate host metabolism. And this could be done in two ways:

  1. by engineering bacteria to produce beneficial nutrients, or
  2. by engineering bacteria to produce enzymes involved in the degradation of dietary products which may be toxic under specific circumstances.

Which approach has Synlogics taken?

Their most recent trick was to re-engineer bacteria in the gut to produce agents that can correct a mouse model of the metabolic condition of phenylketonuria.

What is phenylketonuria?

Phenylketonuria (or simply PKU) is a rare but serious inherited condition that results in an error of metabolism. Due to genetic variations in the PAH gene, the body fails to produce enough of an enzyme called phenylalanine hydroxylase.

What does phenylalanine hydroxylase do?

It breaks down an amino acid called phenylalanine, which subsequently builds up in the blood and brain. And this increase in levels can lead to brain damage.

PKU affects about 1 in 12,000 babies.

The current treatment for PKU is a diet low in foods that contain phenylalanine, plus some special supplements. This treatment diet should begin as soon as possible after birth, and it should be continued for at least the first 10 years of life, (if not continued for the entire life).

So how did the researchers at Synlogics treat Phenylketonuria?

They recently published this research report:

Title: Development of a synthetic live bacterial therapeutic for the human metabolic disease phenylketonuria
Authors: Isabella VM, Ha BN, Castillo MJ, Lubkowicz DJ, Rowe SE, Millet YA, Anderson CL, Li N, Fisher AB, West KA, Reeder PJ, Momin MM, Bergeron CG, Guilmain SE, Miller PF, Kurtz CB & Falb D
Journal: Nature Biotechnology, 2018 Oct;36(9):857-864.
PMID: 30102294

In this study, the researcher at Synlogic genetically engineered a specific strain of gut bacteria – E. coli Nissle – to produce enzymes that break down phenylalanine.

What is E. coli Nissle?

E. coli Nissle is a bacteria that does not colonize the human gut (it is not present in the gut one week after ingestion). Given this property, the synlogic researchers engineered E. coli Nissle to produce two enzymes that help to break down phenylalanine.

They called this new bacteria SYNB1618.

Experiments in both a mouse model of PKU and in primates fed a diet high in phenylalanine suggested that oral treatment with SYNB1618 significantly reduced levels of phenylalanine, supporting the idea that systemic proteins could be treated from within the gut (the SYNB1618 bacteria suck up phenylalanine and break it down internally).

Source: bio5c

In addition, the biproducts of this process are excreted via urine, which can be used as a biomarker in the clinical trial setting.

And that leads me nicely into the craziest part of the Synlogic research: SYNB1618 is aready being evaluated in clinical trials.

The company has already completed a Phase I/IIa single- and multiple-ascending doses clinical trial of SYNB1618 in healthy participants (Click here to read more about that trial and click here to read the press release about the results). They have reported that the initial formulation of SYNB1618 is a liquid, which they are hoping to reformulate and develop capsule or sachet versions.

Wow! That’s amazing. What about Parkinson’s? Can we re-engineer gut bacteria to produce Levodopa?

Most likely, but we probably would not want to.

Why not?

High levels of levodopa may cause excessive nausea due to too much conversion into dopamine in the gut. In addition, levodopa dose needs to be carefully managed. Bacterial production of levodopa could be rather chaotic, which could potentially lead to other complications, such as dyskinesias. Levodopa is probably best administered either in carefully managed tablet or gel (Duodopa) form.

But it could be interesting for researchers to explore other ideas involving genetically engineered gut bacteria.

So what does it all mean?

Yes, summing up – apologies for the length of this post, but I thought the nature of the content was really interesting.

Based on what we currently know and the way things are going research-wise, it is a pretty safe bet to say that the future holds amazing potential. The ability of a biotech company in Boston to correct a developmental disorder that affects the brain, by simply re-engineering gut bacteria is one example of this potential.

And using gut bacteria to help treat medical conditions could have important implications for Parkinson’s as we have discovered that the microorganisms in our intestinal system are having an effect on the treatments we are using to manage this condition.

Be assured that there is a lot more gut-related Parkinson’s research coming in 2019.


EDITOR’S NOTE:  Synlogic is a publicly traded company. That said, the material presented on this page should under no circumstances be considered financial advice. Any actions taken by the reader based on reading this material is the sole responsibility of the reader. Synlogic has not requested that this material be produced, nor has the author had any contact with the company or any associated parties. This post has been produced for educational purposes only.

In addition, the information provided by the SoPD website is for information and educational purposes only. Under no circumstances should it ever be considered medical or actionable advice. It is provided by a research scientist, not medical practitioners. Any actions taken – based on what has been read on the website – are the sole responsibility of the reader. Any actions being contemplated by readers should firstly be discussed with a qualified healthcare professional who is aware of your medical history. Please speak with your medical physician before attempting any change in an existing treatment regime.

The banner for today’s post was sourced from Youtube.

34 thoughts on “Bac to the future

  1. Thanks for another interesting blog.

    You write: “Bacterial production of levodopa could be rather chaotic, which could potentially lead to other complications, such as dyskinesias. Levodopa is probably best administered either in carefully managed tablet or gel (Duodopa) form.”

    But chaos is what we have now. The problem with the present situation (mainly orally taken pills) is that, while there is good control of the amount of levodopa taken (e.g. three 100mg tablets per day), there is little control over the amount absorbed. If you had a way of getting reliable effects from successive doses, this would give major therapeutic advantages. Having better control of levodopa levels, you could increase your dose with less risk of dyskinesia, leading to longer “on” time.


    1. Ah John, you caught me!
      It felt pretty cheap as I wrote it and your comment only hammered that home. I spent the days before publishing this post trying to think of a clever way this technology could be used for PD, but dim witted as I am I could not. The beauty of the SYNB1618/PKU approach is that everything is done in inside the SYNB1618 bacteria in the gut. The only excreted parts are the waste products. The bacteria appears to leave very little footprint and presummably does not disrupt the microbiota of the gut. My worry is that Parkinson’s would require the excretion of compounds (like Levodopa or a Gcase/GBA activator), which would beg the question why not just do it in a more controlled fashion with small molecules.
      I agree with you though. What we have at present is chaotic (peaks and troughs), but I’m not sure bacteria would be superior to a continuous treatment system like duodopa.
      Thanks for calling me out.
      Kind regards,


  2. Thankyou thankyou! Your blogs continue to offer hope to us Parkies. But this one has struck a cord with me.

    You report “…the amount of tyrosine decarboxylase in the samples were associated with both the amount of levodopa being administered AND the length of time each person had had Parkinsons. All of these results (and more) suggested to the researchers that decreasing efficacy of levodopa treatment observed in people with Parkinson’s might be explained by the overgrowth of small intestinal bacteria that can convert levodopa, even in the presence of carbidopa.”

    Clearly, this can be part of the explanation as to why levodopa becomes less effective over time. Can it also explain why levodopa is not very effective at the outset for some of us? And what is the effect of the levodopa outside of the brain (organs, muscles)? Might it account for some side-effects? For those of us with concerns, is it worth having stool samples tested?

    What are your thoughts on nicotinic acid as a TDC inhibitor?

    Do COMT inhibitors have a significant role in this internal battle to get levodopa to the brain?


    1. Hi Jim,
      Thanks for the interesting comment/questions. I really like your point about this result explaining why levodopa may not be very effective at the outset for some folks. It’s a good observation, one worthy further research (I’m hoping the researchers who conducted the study may be looking into it).
      Regarding peripheral effects of levodopa, this is largely taken care of by the inhibitory effect of carbidopa (the levodopa does not get converted). I have not seen any data to suggest that levodopa is doing anything negative. There have been VERY rare cases of liver damage (, but given the wide scale use of levodopa, these cases are exceedingly rare.
      On the nicotinic acid question, someone asked me offline a similar sort of question regarding nicotine/cigarettes and their effect on the microbiota of the gut. The study discussed in this post suggested that use of TDC inhibitors may help solve the problem (and I guess the researchers are currently testing this), so increasing nicotinic acid may not be necessary. Nicotinic acid could still be investigated in this context in the lab though I suppose. And I’m not sure if anyone has investigated COMT inhibitors in this context (happy to be corrected on this matter).
      I hope this helps.
      Kind regards,


  3. Is low dose doxycycline really that bad in terms of antibiotic resistance? It’s only one antibiotic. There are others. As long we don’t do it with all antibiotics? Isn’t doxycycline also used as a malarial preventative medicine. So lots of people taking in long term at quite low doses anyway?


    1. Hi Chris,
      Thanks for the interesting comment.
      You are right that doxy has been used as a preventative treatment of malaria, but the point there is that daily doses of doxy have been shown (across multiple and independent studies) to be highly effective at killing the parasite which is the causal agent in malaria ( In the case of Parkinson’s, the causal agent is unknown, thus the logic for using low dose doxy in this context is less clear. Having said that I have heard that there are efforts to get a clinical trial off the ground to test this effectiveness of doxy in PD, but the failure of minocycline – a similar tetracycline antibiotic – to show any effect in PD will probably make this new trial a hard sell.
      I have also been searching for a clear definition of what is meant by sub-antibiotic doses and it seems to vary between sources and (disturbingly) regions (
      On the philosophical matter of using low dose doxy, the question for me is not “is low dose really that bad? We have other antibiotics”, the real question is “should mankind really be taking the risk?”. 80% of us would not be here if not for antibiotics, and it is only a matter of time before we lose that advantage. Reckless use of these compounds will only speeds up that clock. Apologies for the dramatics and I am happy to be corrected on my logic here if I’ve got this wrong, but when the experts in the field – like Prof Tulkens – are saying we are playing with fire, I am inclined to listen.
      Perhaps if I was diagnosed tomorrow, my attitude might change on this matter. But for now I have to walk the responsible line.
      Kind regards,


  4. The linked diagram from Wikipedia (Biosynthetic pathways for catecholamines and trace amines in the human brain) shows AADC as the enzyme involved in both the conversion of L-Tyrosine to p-Tyramine, and the conversion of L-DOPA to Dopamine.

    Your diagrams represent the situation outside of the brain, and show AADC as two separate enzymes (TDC and DDC).

    Have I understood that correctly, that there is a difference between how things work inside the brain compared to how they work outside the brain? Or is there an error in one of the diagrams?


    1. I think I’ve found the answer. I’ve learnt from your Semanticscholar reference that the term AADC is collective. That is, both TDC and DDC are AADCs.


    2. Hi Jeffreyn,
      Thanks for the comment. I’m not sure I fully understand the question here, but you are correct that Aromatic L-amino acid decarboxylase (or AADC; also known as DOPA decarboxylase (or DDC)) is the enzyme involved in both the conversion of L-Tyrosine to p-Tyramine, and the conversion of L-DOPA to Dopamine. Bacterial Tyrosine decarboxylase (or TDC), which is an enzyme that converts tyrosine into tyramine, is a member of the “aromatic amino acid” family, but the gene for the TDC enzyme has not been found in flies or any higher animals (and certainly not in humans). For clarity’s sake in humans, AADC is generally only used to refer to DOPA decarboxylase.
      As far as I’m aware, there are no differences between how things work in and outside the brain (happy to be corrected on this). In the study discussed in this post, it was simply found that bacterial TDC can also convert levodopa to dopamine. The Semanticscholar diagram demonstrates the tyrosine into tyramine conversion, while DDC exhibits the Levodopa into dopamine conversion.
      I hope this answers your question.
      Kind regards,


      1. Yes, this answers my question. Thanks.

        What I hadn’t grasped earlier is that TDC is a bacterial decarboxylase but not a human decarboxylase.


  5. “All of these results (and more) suggested to the researchers that decreasing efficacy of levodopa treatment observed in people with Parkinson’s might be explained by the overgrowth of small intestinal bacteria that can convert levodopa, even in the presence of carbidopa.”

    If I’ve understood correctly, the “suggestion” is that the decreasing efficacy of levodopa treatment is caused by decreasing amounts of levodopa entering the blood stream. Should be easy to confirm, right?


    1. Re-reading this now, I realise that my question wasn’t very clear.

      What I meant to ask was whether it has already been established that the decreasing efficacy of levodopa (over time) is associated with decreasing amounts of levodopa entering the bloodstream.

      (and if this has not yet been established then it would seem to be fairly easy to confirm via blood tests)


      1. Hi Jeffreyn,
        Longitudinal evaluations of levodopa treatment have been previously evaluated (for example ), but a more up to date study – following up on the data presented in the study discussed in this post – would be relatively straight forward to conduct. I’m guessing that the researchers are already investigating this.
        Kind regards,


  6. From the research paper (van Kessel et al.):
    “… a better equilibration of levodopa treatment between patients could potentially be achieved by co-administration of an effective TDC inhibitor that targets both human and bacterial decarboxylases.”

    It seems to me that this should solve the problem. But it may create a new problem if TDC has a valid role to play outside of the brain.


    1. Hi Jeffreyn,
      Yes, you are correct. And as I understand things, the researchers behind this study are following up with investigations of TDC inhibition. It will be interesting to see what they find. Hopefully, as you say, as long as TDC inhibitors can be safely used it will allow reduced doses of levodopa to be used. Fingers crossed. Independent replication of this current research is required before we start thinking seriously about these sorts of ideas though.
      Kind regards,


      1. I see now that the wording of the text I quoted from the research paper contributed to my earlier mis-understanding.

        Perhaps a clearer wording would be:
        “… a better equilibration of levodopa treatment between patients could potentially be achieved by co-administration of an effective TDC inhibitor. This would enable the targeting of both human (DDC) and bacterial (TDC) decarboxylases.”


  7. Thanks jeffreyn for trying to clarify matters, but I’m still confused. Is tyrosine decarboxylase one thing, i.e. a particular molecule, or is it a class of similar acting enzymes? When mention is made of human TDC or bacterial TDC does “human” or “bacterial” refer to the source of the TDC (i.e. the same thing, but from different places) or to different things? Is the TDC that converts tyrosine to tyramine the same thing as that which it is proposed converts levodopa to dopamine?

    I think of TDC (provided it biosynthesises levodopa to dopamine) as “negative” levodopa. This has all sorts of clinical possibilities. See:


    1. Hi John,

      While we wait for Simon to give an authoritative response, I’ll throw in my 2c worth.

      Q1: TDC is a particular molecule.
      Q2: It’s the same thing, but from different places.
      Q3: It’s the same thing.

      My (current) understanding is that humans produce DDC, bacteria produce TDC, and mosquitos (and perhaps some other insects) produce both (the diagram sourced from Semanticscholar comes from a paper dealing with a particular type of mosquito).

      Rather than thinking of TDC as negative levodopa, it might be simpler to think of it as carbidopa-resistant DDC.



      1. Hi Jeffryn,
        Nothing authoritative here – this is new terriory for me too (thank heavens for google and pubmed!). And your analogy works as well (a carbidopa-resistant DDC).
        Kind regards,


    2. Hi John,
      I will do my best to confuse things further:
      1. There are two TDC genes in bacteria (TDC1 and TDC2) and it is not clear to me in the study discussed above which one is being investigated. It will depend entirely on which TDC the E. coli Nissle bacteria produce.
      2. There is no ‘human’ TDC. We do not have it in our DNA. Humans do not produce TDC. The enzyme is considered non-human. Bacteria, on the other hand, produce TDC. So any reference to TDC is strictly bacterial in origin.
      3. The bacterial TDC in our gut – which converts tyrosine to tyramine – is the same TDC that is converting levodopa to dopamine in the study.
      And yes, you are correct: it could be considered a “negative” levodopa.
      I hope this helps.
      Kind regards,


      1. So then, in (gut) bacteria, the TDC1 gene codes for the TDC1 enzyme, and the TDC2 gene codes for the TDC2 enzyme. The researchers have found that one of these two enzymes is able to convert levodopa to dopamine. We don’t know if the second one can also do that, and if so, to what extent.

        Have I understood that correctly?


      2. Hi Jeffreyn,
        That is correct. It will be interesting in an independent replication of this research to clarify which version of TDC is having the effect (or perhaps both do it – we shall see).
        Kind regards,


  8. It is worth looking at the Supplementary material provided for the paper by van Kessel et al, especially Tables 4 and 5. Table 4 provides the raw data. Table 5 provides the correlation between the TDC values and the other variables.

    I’ve taken the data from Table 4 and focused on some other correlations. (My apologies about the formatting.)
    A B C D
    tdc x10^-9 UPDRS Levodopa mg Duration yr
    155 35 300 2
    430 30 1100 11
    65 36 500 8
    90 13 300 4
    401 26 400 14
    24 35 300 6
    623 21 1000 22
    87 27 800 6
    200 19 600 15
    275 40 700 10

    correlation= -0.1941662463 -0.0393810676 -0.3029839186
    x= A C D
    y= B B B
    As I understand the model, low TDC levels are good, being linked to high levels of levodopa being absorbed. UPDRS is good for low values. Therefore, I would have expected a positive correlation between TDC and UPDRS. It is in fact -0.194. This is not statistically significant in itself. But taken together with the correlations which fitted the model, e.g. TDC and Duration, this may end up overall being non-significant.

    Perhaps the cohort is not representative. I would have expected a positive correlation between Duration and UPDRS. It is in fact -0.3.


    1. Hi John,
      Thanks to the interesting comment as usual.
      Supplemental table 4 indicates that only 10 PD subjects were used in the analysis (not a large cohort) and while the disease duration has a wide range (2-22 years), the Hoehn and Yahr Staging was similar across the cohort (all stage 2-2.5). I’d really like to see the results replicated in a much larger, more diverse cohort, which actually could be done rather quickly considering the number of fecal analysis studies that have/are being conducted on PD at the moment.
      Kind regards,


  9. Someone in my Parkinson’s support group who takes levodopa reports that after taking antibiotics (for reasons not related to Parkinson’s) she has more severe dyskinesias than usual. Could this simply be because the antibiotics kill some bacteria that produce tyrosine decarboxylase so that after taking antibiotics an unusually high amount of levodopa reaches the brain and triggers the dyskinesias? If so, I would like to tell her, because I would find it reassuring to imagine that in this case it’s not the antibiotics or levodopa that are the bad guys but some bacteria that steal levodopa, and that without those bacteria the brain would still get by with less levodopa.


    1. Hi zz,
      Thanks for the interesting comment. That is an intriguing situation. Anecdotal perhaps, but certainly worth noting.
      Thanks for sharing.
      Kind regards,


  10. Hi Simon,

    I have a number of questions inspired by a post at the Health Unlocked – Parkinson’s Movement Forum:

    Assuming that the TDC-related research results can be confirmed, is it possible that a PwP with a lot of TDC-producing bacteria in their gut could have a lot of their levodopa medication entering the bloodstream as dopamine, rather than entering the brain as levodopa?

    If so:

    (1) Could a consequence of this be that the PwP could experience symptoms of dopamine overdose?

    (2) Is it biologically possible for some of the excess dopamine in the bloodstream to make its way to the adrenal gland, and to be converted there to noradrenaline and then to adrenaline? Could a consequence of this be that the PwP could experience symptoms of adrenaline overdose?

    Warm Regards,


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