Thinking With Your Gut Bacteria

Is your personality really yours – or is it your bacteria’s?

What prompts this thought is a new paper that studied mate preference in fruit flies. It turns out that the gut bacteria in fruit flies influence their mate preference. [1]

Image source: Wikimedia commons.

It’s been known for 20 years that fruit flies raised on one type of food prefer to mate with fruit flies raised on that same food. Now, researchers have proven that the preference is dictated by gut bacteria. When fruit flies are given antibiotics, they forget their mate preference and will mate with fruit flies that eat different diets. [2]

Mating preference seems to be dominated by a single species:  Lactobacillus plantarum. This species is common in probiotic supplements and in fermented vegetables. So if you find yourself developing an attraction to starch-fed fruit flies, your probiotic could be to blame.

Personality-Altering Pathogens

Personality-altering pathogens may not be that rare. In the book we mention that Toxoplasma gondii, a protozoal parasite, infects 20% to 60% of the population in most countries, forms cysts throughout the body including the brain, and makes its victims behave recklessly:

  • Rats infected with T. gondii lose their fear of cats. [3]
  • Humans infected with T. gondii are 6 times more likely to get in traffic accidents. [4]


If people seem to be behaving increasingly oddly lately, perhaps it’s not your misanthropism. It might be a bug going around.


[1] “Gut Bugs Affect Mating,” The Scientist, Nov 2, 2010,

[2] Sharon G et al. Commensal bacteria play a role in mating preference of Drosophila melanogaster. PNAS,, 2010.

[3] Webster JP et al. Parasites as causative agents of human affective disorders? The impact of anti-psychotic, mood-stabilizer and anti-parasite medication on Toxoplasma gondii’s ability to alter host behaviour. Proc Biol Sci. 2006 Apr 22;273(1589):1023-30.

[4] Flegr J et al. Increased incidence of traffic accidents in Toxoplasma-infected military drivers and protective effect RhD molecule revealed by a large-scale prospective cohort study. BMC Infect Dis. 2009 May 26;9:72.

Leave a comment ?


  1. Did you catch this Ted Talk about the “brain” in the gut by Heribert Watzke?

    I’ve been working towards getting a combination of vector borne infections out of my system for awhile now. Clearing a lot of junk out of my diet seemed to help tremendously with the more cognitive/”mental” symptoms. The crazy roller coaster of emotions stopped being so severe around the same time I managed to straighten out my digestive issues.

  2. Hi Natalie,

    No, I hadn’t seen that, thanks for the link! He has a nice Richard Wrangham + Leslie Aiello introduction. I knew we had an independent nervous system there but didn’t know it was the same size as a cat brain.

    I’m glad you’re doing better!

    Best, Paul

  3. Paul, careful you may be called upon to provide expert testimony in tragic divorce cases where the affection of one’s spouse is stolen away by a deliberate gut bacteria change in the other. :-{

  4. erp,

    Maybe that’s why they usually do fecal transplants from the spouse!

  5. Radio Lab had a fascinating & entertaining episode on parasites.

  6. Paul,

    This is fascinating, no wonder I have been feeling so off since struggling with all of these stomach issues…

    wondering your thoughts on taking planatarum probiotics–I did once and swelled up like a balloon. on a side note, your thoughts on taking plant-based digestive enzymes, HCL supplementation, etc..for us folks with undiagnosed digestive issues like IBS and food intolerances? The enzymes seem to help but its unclear and still struggling with constipation.


  7. Hi Kayla,

    Have you read our bowel disease series? On the categories list click “Bowel Disease” and look for Parts 1, 2, 3, and 4.

    Re constipation, I think nutritional deficiencies and hypothyroidism often cause that. I would optimize vitamin D and thyroid hormone levels (see our thyroid posts) and also supplement selenium, vitamin C, NAC or glutathione. Hypothyroidism is very common in people with gut diseases.

    Food intolerances indicate a leaky gut. Could be caused by infections or by food toxins e.g. wheat. Our first post in the recent wheat series was about leaky gut.

    There’s a lot of steps that can be taken. Diet and nutrition will help and probably cure it eventually, but can be slow. Fixing the gut bacteria helps a lot. My next post will be an update on fecal transplants as a way to do that, see the last bowel disease post for the original installment.

  8. Thanks Michelle! I wish I had time to listen to it.

  9. Hi
    I received the book in the mail today, thank you!

    I also had a question. My (84 yo) grandma is in the hospital, she had surgery and now has pneumonia. Her blood glucose is in the 160s, and they’ve been giving her insulin which isn’t working. Any reason why this could be?

  10. Hi Abby,

    What are they feeding her? Is she diabetic?

    Insulin is usually pretty good at lowering blood glucose. Hopefully they’re not feeding her carbs. High blood glucose is a good predictor of mortality in the hospital.

    I’ll pray for her.

  11. I’m trying to figure out what they’re feeding her. I won’t know till tomorrow probably.
    She has no diabetes or any blood sugar problems. Other than osteopenia, shes been pretty healthy.

  12. That is a sad thought. To think, we are only our gut bacteria.

    I don’t doubt it, but I guess it would be nicer if it weren’t true. After all, what really makes us human then? Our gut bacteria?

    Crazy to think, maybe our sense of humor will change? Our interests? Brings up many many questions.

  13. Really nice Edge talk with R. Sapolsky on toxo:

    Thanks for the great blog btw!

  14. Thanks Pieter! Great interview!

  15. Paul wrote:

    Thanks Michelle! I wish I had time to listen to it.

    Hi Paul, IKWYM. I get in my podcasts on my drive to work, and sometimes before falling asleep.


  16. Hi Michelle,

    I would love to listen to it, I’m sure it has good material I can use, but the first 5 minutes were some sort of comedy schtick and I’m very busy, so I put it off.

    I am grateful for the link. Hopefully others enjoyed it! And I’ve saved the link for later.

  17. Perfect Health Diet » The Danger of Hospital Food - pingback on November 8, 2010 at 8:42 pm
  18. Could you expand on treatment for toxoplasmosos t gondii. I am following your diet but also supplement with amino acids such as
    Gaba aND dlpa from reading the mood cure by Julia Ross. Your book had a paragraph about fighting the infection but low serotonin was a side effect. So could using these supplements be making it worse?

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