In vitro susceptibilities of Haemophilus influenzae isolates from a Veterans Administration Medical Center.

One hundred and one consecutively occurring clinical isolates of H. influenzae, from adult male patients, were tested for quantitative antimicrobial susceptibility (by broth microdilution) and classification properties. Almost all were susceptible to a variety of antimicrobials, except erythromycin and ampicillin; 14% were ampicillin resistant. Beta-lactamase was produced by all the ampicillin-resistant strains. Most isolates were biotype II (68%) or III (24%). Only 6% were serotype b, while 94% were from the lower respiratory tract. When isolates were grouped according to biotype, serotype, B-lactamase production and site of isolation, there were some significant differences in susceptibilities among the groups.