As noted in prior posts, almost anything has the potential to act as an evolutionary selection factor. [Yes, Virginia, there really is evolution.] A recent assessment that traces the history of a virulent strain of pneumonia-causing bacteria demonstrates that both antibiotics and vaccines can act as selection factors.
An international group of researchers did a genetic analysis of 240 samples of the drug-resistant strain Streptococcus pneumoniae taken from humans in 22 countries between 1984 and 2008. This strain was first identified in a Barcelona hospital in 1984, though the assessment has concluded it likely first came to be around 1970. This time-frame is important because the organism emerged, so to speak, into a world in which penicillin was frequently used; since this strain was not killed by penicillin, it spread quickly. [I know, what a surprise. Not unlike what is seen with the overuse of antibiotics in food animals.]
The study also set forth some of the "tricks" used by the organism to develop drug resistance. Since the organism emerged, it has changed one of its DNA letters every 15 weeks [no this is not a misprint]. The rate is rapid, but not that different from what is seen with MRSA, the antibiotic resistant Staphylococcus aureus. [For the sake of full disclosure, I (and one of my partners) was involved in the last several years in MRSA related administrative and litigation matters for an agency of California State Government.] Not surprisingly, the Streptococcus pneumoniae was also involved in DNA swaps and recombinations with other bacteria. I say "not surprisingly" because bacteria engage in such activities all the time. It turns out that the swaps and recombinations may be more important in terms of antibiotic resistance than the DNA letter changes. Each DNA swap brought about 72 single letter changes, on average, and sometimes introduced entirely new genes, or versions of genes.
One of the strategies that bacteria use to evade the body's immune system is to wrap themselves in a "sugar" coating called a polysaccharide capsule. The strain that is used by our buddy, Streptococcus pneumoniae, is designated serotype 23F (to distinguish it from slightly different capsules used by some other bacteria). The capsule was one target of a vaccine called PCV7, introduced in 2000.
Unfortunately, by the time the vaccine was in use a small number of Streptococcus pneumoniae had swapped DNA with other bacteria and changed their capsule to serotype 19A. Given the sampling, it is estimated that the swap occurred in the U.S. in 1996 and independently in Spain in 1998.
So, guess what? The vaccine drastically reduced the number of bacteria with the 23F capsule, which left the "environment" to those bacteria with the 19A capsule; they thus moved in, so to speak, and became predominate, at least for the moment.
Once again, it is demonstrated that all sorts of interesting events can act as selection factors on the evolution and predominance of organisms.
Reports on this tale can be found at http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(09)61204-6/abstract and http://www.sciencemag.org/content/331/6016/430.abstract.