Colonizing opportunistic pathogens (COPs): The beasts in all of us
Colonizing opportunistic pathogens (COPs) are microbes that asymptomatically colonize the human body and, when the conditions are right, can cause infections. Their ability to persist indefinitely and to be transmitted without detection gives COPs a unique epidemiology that warrants special consideration. There are examples of COPs among bacteria, fungi (e.g., Candida albicans), protozoa (e.g., Blastocystis), and viruses (e.g., Rhinovirus), but bacterial COPs are of particular relevance because of their major contribution to today’s antibiotic resistance crisis. The COPs include a long list of notorious bacteria that live double lives as passive stowaways and virulent foes. Some of the best-known COPs include Staphylococcus aureus, extraintestinal pathogenic Escherichia coli (ExPEC), Klebsiella pneumoniae, and Streptococcus pneumoniae (Table 1). Their capacity for benign coexistence with humans belies their alter egos that exact a heavy burden of human disease. For example, in the United States, ExPEC bloodstream infections kill as many as 40,000 people annually, but, ExPEC are also benign colonizers in the gastrointestinal tract. Host factors, including age, sex, health status, anatomy, and behavior, all play profound roles in infection susceptibility and severity. In particular, immunocompromised individuals are at excess risk for infections caused by diverse bacteria, including COPs and even commensals. Yet, health status is not the sole determinant of infection by COPs. For example, healthy women more frequently suffer from urinary tract infections than men because of anatomical differences, including shorter urethrae. Likewise, healthy children more commonly suffer from acute otitis media than adults due to their shorter, flatter eustachian tubes.