This past week I’ve been spending quite a bit of time thinking about L. reuteri. One of the interesting things I’ve mentioned in the past and why I think that this particular strain is interesting out of all the other different species is because L. reuteri was a dominant species in the human gut in the middle twentieth century according to literature and samplings done during that time period. Present day this seems to not be the case. In fact, it seems to be quite rare.
One of the interesting things about L. reuteri is that the antimicrobial compound it produces, reuterin, is produced in the presence of glycerol. Glycerol as you know forms the backbone of triglycerides i.e. fat. In the lab, reuterin can be synthesized simply by culturing L. reuteri with glycerol. L. reuteri is unique in that it will thrive in 16:0 and 18:0 i.e palmitic and stearic acid growth medium.
It’s growth however, can be inhibited by excess consumption of polyunsaturated fat and in particular 18:2 or linoleic acid. While some strains have been shown to mutate in the presence of LA and evolve defense mechanisms to protect itself from LA, they don’t thrive as much as they do when LA is low. In other words LA is bactericidal/bacteriostatic. A bactericidal compound literally kills bacteria whereas a bacteriostatic compound simple inhibits growth. The effect that LA has on bacteria has been known since the early 20th century.If we take a look at these figures for sources of LA in the diet we can see that LA has dramatically increased mostly from increased soybean oil and poultry consumption.
If we take a look at these figures for sources of LA in the diet we can see that LA has dramatically increased mostly from increased soybean oil and poultry consumption.
That is a very interesting coincidence along with the falls in palmitic and stearic acid.
When bile is released it breaks down triglycerides into it’s basic components, free glycerol and 3 free fatty acids. The free glycerol would have provided substrates for L. reuteri to produce reuterin and the free fatty acids would have provided the approperiate energy substrates for L. reuteri to continue to thrive and remain dominant.
However, with declining saturated fat and meat intake and the increase in 18:2 since the middle part of the 20th century, this would have proved unhospitable to L. reuteri thus destroying any benefit to a symbiotic relationship. No more free lunch.While most bacterial strains can thrive with a variety of different energy substrates, they proliferate and dominate when they are provided with the right energy substrates that allow them to effectively compete. When you remove those things and simultaneously introduce a hostile environment they vacate.
Given the long evolutionary history we have seemed to have had with L. reuteri and the increasing amounts of carbohydrate and LA in the human diet and the decreasing prevalence of L. reuteri, I’d venture to say that this particular species is well suited to thrive in a diet low in carbohydrate and rich in fat. In other words it doesn’t need carbohydrate to thrive. I’ve often thought of the gut biomass as only being relevant in the context of diets that contain elevated amounts of ruffage. Clearly this is not always the case as L. reuteri likes glycerol, amino acids, and saturated fat, and doesn’t like LA, so as far as I’m concerned me and L. reuteri can be friends.
Given that L. reuteri thrives at physiological temperatures, culturing at 37C probably is fine.