Antimicrobial resistance in Haemophilus isolates: a Minnesota experience and literature review

Annual ampicillin susceptibility rates for Haemophilus influenzae isolates at the St. Paul-Ramsey Medical Center gradually decreased from 100% in 1974 to 83.3% in 1980 and then remained stable at 88.90%. Penicillin susceptibility rates were similar to those for ampicillin. Ampicillin rates were source dependent: eye 95%, respiratory 90%, miscellaneous sources 82%, and blood and CSF 80%. Rates for Haemophilus parainfluenzae varied and showed no trend. H. parainfluenzae isolates were distinctly less susceptible to penicillin (70%) than to ampicillin (96%). H. influenzae isolates were highly susceptible to chloramphenicol (99.6%) and tetracycline (97.5%), with the latter also showing source dependency. Characterization of isolates for colony morphology and hemolysis showed no clinical relevancy. Ampicillin and penicillin MICs were determined for 128 clinical isolates saved in stock culture during 1978-1983. All 19 resistant isolates (MIC greater than or equal to 4 micrograms/mL) were resistant to both penicillin and ampicillin and produced beta-lactamase. Eight had penicillin MICs of 1 or 2 micrograms/mL and three had ampicillin MICs of 1 or 2 micrograms/mL. The significance of isolates with MICs of 1-2 micrograms/mL is discussed in relation to our findings and a review of the literature.