Biotypes of Haemophilus influenzae: relationship to clinical source of isolation, serotype, and antibiotic susceptibility.

A total of 94 clinical isolates of Haemophilus influenzae were studied to analyze the relationship of biotype to site of isolation, serotype, and pattern of antimicrobial susceptibility. Systemic infections were caused most commonly by biotype I, and the majority of these isolates possessed type b capsular polysaccharide. Other noncapsulated biotypes of H. influenzae, particularly biotype V, also were associated with invasive disease. Antimicrobial susceptibility testing was performed on all isolates by an agar dilution method against ampicillin, chloramphenicol, cefoxitin, rifampin, and rosoxacin, and all isolates were screened for beta-lactamase activity. Except for 15 isolates that produced beta-lactamase, no other substantial differences in antimicrobial susceptibilities among biotypes of H. influenzae were detected. Encapsulated strains of biotype I had the highest frequency of ampicillin resistance.