I think that the potential for what you're intimating has been realized certainly in the hospital nosocomial environment, where we're using lots of anti-microbials on a constant basis, and those anti-microbials are used in patients who are immuno-compromised, in general. So that provides an opportunity for natural selection to occur. Whenever you put pressure on a microbial population--and microbial populations are a good example because they are large, there are millions and millions and millions--in any life system, any biological system, you're going to find, in large populations, an individual who is able to withstand whatever pressure is being placed on that population. So if it's an antibiotic, you're going to see antibiotic resistances develop. If it's pH resistance, you're going to see some organisms that are able to grow at pH 9, etc.
On April 29th, 2009. See this statement in context.