ABSTRACT

Anyone can see that even members of the same species differ in their appearance. All dogs, for instance, are easily recognizable as dogs, but nobody is likely to confuse a Pekingese and a Dalmatian. Microorganisms are no less variable. Why do some strains of Escherichia coli cause serious disease, while most others live peacefully in your intestines without causing problems? Some bacteria have extracellular structures such as fi mbriae and capsules. Others of the same species do not. What explains this difference? How do organisms that were originally susceptible to a particular antimicrobial drug become resistant? Why do all living things, microbial or otherwise, tend to resemble their parents or siblings more than they do unrelated individuals?