Although the normal flora provides many health benefits, some of the microbes of the normal flora can cause serious infection and disease in the right circumstances. Microbes that can only cause infection when the host’s normal defenses are not fully intact are called opportunistic pathogens. Those microbes that can cause disease even with full defenses present are called primary pathogens. Most potential pathogens found in the normal flora are opportunistic pathogens.
Infections arising from a person’s own flora are considered endogenous. One way an endogenous infection can occur is for a bacterium which normally resides in one part of the body is introduced to another. An example is E. coli and other Enterobacteriaceae. Most Enterobacteriaceae can cause wound infections if they are introduced to broken skin, although this usually happens when someone is in a weakened state. Most urinary tract infections are caused by E. coli. E. coli can easily be introduced into the urethra when using the toilet or during intercourse (particularly for women). Once in the urethra, many strains of E. coli are able to adhere to the urinary tract with fimbriae and establish an infection. In rare cases, E. coli can even cause systemic infections such as meningitis or septicemia.
The viridans Streptococci normally found in the mouth can cause serious cardiovascular damage. If these bacteria are introduced into the bloodstream (usually by a dental procedure) they can settle and grow on damaged heart valves. These growths are called vegetations. The resulting infection is subacute bacterial endocarditis. In addition to flu-like symptoms (such as low-grade fever, fatigue, shortness of breath), characteristic symptoms of this disease are small, tender nodules on the fingers or toes, and tiny broken blood vessels on the whites of the eyes, the palate, inside the cheeks, on the chest, or on the fingers and toes. Once diagnosed, subacute bacterial endocarditis is usually easily treated.
Another way a microbe can cause endogenous infection is if the immune system is impaired or the normal flora is disrupted. Disruption of the normal flora, as mentioned above, can lead to infections with Candida or C. difficile.
Other examples of opportunistic pathogens of the normal flora which can cause endogenous infections include:
Pathogens from the normal flora can also infect other people. These are exogenous infections. Most opportunistic pathogens of the normal flora can also infect others. Many nosocomial (healthcare associated) infections can be acquired this way.
Occasionally, a person might carry a primary pathogen in their flora asymptomatically. Some pathogens such as Streptococcus pyogenes and Neisseria meningitidis can only grow in a human host. On occasion, people can be carriers of these and other pathogens without any sign of disease.