Microbes have been living inside vertebrates for a long time now. They and their hosts are constantly interacting. I think it is not clear however on how many scales they interact. For sure the host has things an acidic stomach and bile secretions (which is like a detergent) to keep the microbe populations from growing outta control. The innate immune system also has plenty of peptides that have antimicrobial properties. For every host combatant action, there
must be a subsequent microbe response for the microbe to survive (e.g. E. coli can export detergent-like bile molecules to survive in intestine). But is it known if bacteria can respond to general non-combatant host properties like the presence of hormones circulating about?
For sure if you have a big boast in adrenaline the bacteria populations will be stimulated, but this could occur simply as a side effect to the host's response to the hormone. I want to know if bacteria can directly sense any hormones. What about leptin? Work in Gordon lab at WashU suggests that low leptin might send a signal to the microbiota to become more efficient at extracting calories from food. I'd like to know if this or any other hormonal signals are directly sensed by the microbial community.
Could try running microarrays of microbes in the presence/absence of different hormones.
(see Nature News & Views article "Obesity and gut flora", Bajzer M and Seeley RJ and the two articles from the Gordon lab in the same issue "An obesity-associated gut microbiome with increased capacity for energy harvest" and "Human gut microbes associated with obesity").