Most normal, healthy body surfaces, such as the skin, large intestine, outer openings of the urogenital tract, and oral cavity, provide numerous habitats for a virtual “garden” of microorganisms. These normal colonists or residents, called the microbiota, consist mostly of harmless or beneficial bacteria, but a small number of them can potentially be pathogens. We defer a more detailed discussion of this topic to chapter 13 and later chapters. Here we focus on the general effects of drugs on this population.
If a broad-spectrum antimicrobial drug is administered to treat infection, it will destroy microbes regardless of their roles in maintaining a balance, affecting not only the targeted infectious agent but also many others in sites far removed from the original infection (figure 1). When this therapy destroys beneficial resident species, resistant microbes that were once in small numbers begin to overgrow and cause disease. This complication is called a superinfection.
Fig1. The role of antimicrobials in disrupting microbiota and causing superinfections. (a) A primary infection in the throat is treated with an oral antibiotic. (b) The drug is carried to the intestine and is absorbed into the circulation. (c) The primary infection is cured, but drug-resistant pathogens have survived and create an intestinal superinfection.
Some common examples demonstrate how a disturbance in the microbiota leads to replacement pathogens and superinfection. A broad-spectrum cephalosporin used to treat a urinary tract infection by E. coli will cure the infection, but it will also destroy the lactobacilli in the vagina that normally maintain a protective acidic environment there. The drug has no effect, however, on Candida albicans, a yeast that also resides in normal vaginas. Released from the inhibitory environment provided by lactobacilli, the yeasts proliferate and cause symptoms. Candida can cause similar superinfections of the oropharynx (thrush) and the large intestine.
Oral therapy with tetracyclines, clindamycin, and broad- spectrum penicillins and cephalosporins is associated with a serious and potentially fatal condition known as antibiotic-associated colitis (commonly referred to as C-diff disease). This condition is due to the overgrowth in the bowel of Clostridioides difficile, an endospore-forming bacterium that is resistant to the antibiotic.
It invades the intestinal lining and releases toxins that induce diarrhea, fever, and abdominal pain.
Refer to table 1 for a general summary of major drug groups and their side effects.
Table1. Major Adverse Toxic Reactions to Common Drug Groups
