According to a recent study by Patricia Foster and Jill Layton, a protein - sigma 38, a protein that is expressed in response to stress - regulates in turn the expression of DNA Polymerase IV. DNA Polymerase IV has a significantly higher rate of error than DNA Polymerase I, II, or III. This means that mutations are much more likely to occur when DNA Polymerase IV is increasingly expressed. It is currently assumed that although an increased rate of mutations would lead to increased death due to deleterious mutations, it must be beneficial in some circumstances in its ability to produce better-adapted bacteria. An increased mutation rate might even be expected in extreme conditions. Incredibly, however, when E. coli bacteria were simply starved - a somewhat regular occurrence for bacteria - the expression of DNA Polymerase IV quadrupled. Clearly, adaptive mutation is not only a reality for some bacteria - it is also much more common that we originally thought.
http://newsinfo.iu.edu/news/page/normal/1160.html (for a review)
Foster, Patricia and Layton, Jill. "Error-prone DNA polymerase IV is controlled by the stress-response sigma factor, RpoS, in Escherichia coli," Molecular Microbiology, vol. 50, no. 2; pp. 549-561.