This type of specificity also is one reason that pathogens of one animal species usually do not infect other animals. If a pathogen cannot adhere, then it cannot colonize and infect. All pathogens must grow in order to cause disease and nutrients for such growth usually comes from the disruption of host cells. Many bacteria have mechanisms for avoiding our immune system once they have entered our bodies. More about such critters next week.
Many bacteria also exhibit phase variations once they are in the body.
One example is phase variation in fimbriae. Remember from lec. 4 and lec. 21 that fimbriae are involved in attachment and that they are antigenic and can elicit an immune response. A single bacterium can have several distinct types of fimbriae genes and recombination among such genes is common.