Bacteria newly recognized as nosocomial pathogens.

Bacteria recently recognized as nosocomial pathogens generally fall into three categories: those that grow slowly, those that are fastidious in their nutritional or atmospheric requirements and those that resemble commensals. Each characteristic has contributed to the delay in perceiving their importance. Mycobacterium chelonei and Myco. fortuitum--which grow slowly, although characterized as "rapid-growing" mycobacteria--cause sternal osteomyelitis, pericarditis and endocarditis after cardiac surgery as well as other wound infections after many types of surgery. Myco. chelonei-like organisms have been found to cause "sterile" peritonitis in patients receiving long-term peritoneal dialysis. Legionella pneumophila and L. micdadei are fastidious bacteria that were more difficult to detect because they stain poorly with the Gram method. They cause pneumonia and lung abscess, especially in immunocompromised people. Clostridium difficile is an anaerobe that causes toxin-mediated pseudomembranous colitis in persons given antibiotics that inhibit competing gut bacteria. Chylamydia trachomatis, an intracellular organism that has not been grown in vitro, causes pneumonia and conjunctivitis in young infants who acquire the organism from their mothers at birth. Group JK bacteria cause septicemia in patients whose immune responses have been suppressed and must be distinguished from "diphtheroid" contaminants in blood cultures. Clinicians, microbiologists and epidemiologists must be alert to the characteristics of these organisms that make them easily overlooked and should also anticipate the existence of other bacteria not yet identified.