There are many different bacteria with acid stress survival techniques, all bacteria that ever visit the stomach need to be able to cope with the low pH. Listeria monocytogenes is a bacteria that anyone whose every looked into pregnancy will know about, as although it does not often cause human dieases it can have potentially fatal effects on an unborn fetus. The bacteria are found in un-pasteurized cheeses and dairy produce, and apparently one of the reasons for this is that the food surrounding them helps to protect them against the acidity of the stomach.
There are various different ways that a bacterial cell can survive acid stress (which is distinct from surviving in constant acidic conditions). Regulatory systems for acid stress have been found, such as the two component EvgS/EvgA system in E. coli, which turn on a large number of acid-stress related genes when sensing low pH in the surrounding environment. In Bacillus cereus, which hides out in warm rice (and is the reason you have to re-heat takeaways to piping hot the next day at work) acid stress conditions were shown to induce a number of typical bacteria cellular responses - two component systems, alternative sigma factors (which lead to changes in the proteins being produced) and excess proteases and chaperones to get ensure more correctly folded proteins (harder in an acidic environment).
With the Listeria however, it was discovered that they were actually getting a lot of help from their surrounding food. Certain amino-acids, such as glutamate, are used by the bacteria to neutralize stomach acid and it turns out that cheese is quite high in glutamate. Also, a fair amount of acidity is used in the cheese making process, which helps the bacteria practice at their acid-resistance; essentially any bacteria in contaminated cheese will have already run an acidity gauntlet which stands it in good stead for the experience it will get in your stomach.
This work is also interesting as it shows that it's not just the amount of consumed bacteria that can lead to stomach poisoning, it's also dependent on the surrounding nutritional environment of your stomach. Eating Listeria in non-glutamate containing food is less likely to lead to an infection, unless you also like tomato juice (very high in glutamate) and drink that at the same time. As well as being interesting from a scientific view this is also very useful diet advice for people susceptible to certain bacterial infections. An understanding of how bacteria work within specific metabolic environments will allow better dietary advice about foods to keep away from.
There is no paper for this, so I'm working mostly off the press release, which can be read here. As I didn't attend the talk at the SGM conference, I'd welcome any corrections of any facts I may have either omitted or misinterpreted. Having read the abstract I'm kindof gutted I didn't get to this talk, it looks fascinating.
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