Serotypes, biotypes and antibiotic susceptibility of 126 clinical isolates of Haemophilus influenzae.

Serotypes, biotypes, and antibiotic susceptibility of 126 Haemophilus influenzae isolates were determined. Five of the 126 isolates were from blood and were encapsulated type b strains; those taken from other sites were not typable. There were 13% biotype I, 36% biotype II, 38% biotype III, 5% biotype IV, 4% biotype V, and 4% biotype VI isolates. Antibiotic susceptibility tests using the standard disk diffusion method showed the following resistance: ampicillin 51%, cefamandole 10%, cefuroxime 3%, chloramphenicol 28%, tetracycline 37% and sulfamethoxazole-trimethoprim 49%. None of the five type b isolates were resistant to cefotaxime, a third generation cephalosporin. The second generation cephalosporins, cefamandole and cefuroxime, showed a superior activity against H. influenzae isolates, compared to other antibiotics. Multiple drug resistance was found in 64 (51%) isolates. Four of the five type b isolates were resistant to multiple drugs. The multiple-resistance pattern most frequently observed was to ampicillin, chloramphenicol, tetracycline and sulfamethoxazole-trimethoprim. Most clinical isolates did not contain plasmids; therefore, the antibiotic resistance of these H. influenzae strains was probably chromosome-mediated.