Ampicillin-resistant Haemophilus influenzae. 1. Incidence, mechanism, and detection.

Ampicillin resistance in Haemophilus influenzae, an organism once thought to be universally susceptible to ampicillin, is increasing. It varies from one institution or community to another, and rates of 6.6% to 48% have been reported. The vast majority of resistant strains produce beta-lactamase, an enzyme that hydrolyzes the beta-lactam ring of ampicillin and other susceptible beta-lactam antibiotics. The beta-lactamase production is mediated by a gene contained on a plasmid (piece of extrachromosomal DNA). It is important for physicians and microbiologists to be aware that an infection such as meningitis or otitis media could be caused by ampicillin-resistant strain of H influenzae. Knowledge of the incidence of resistance for the institution or community is particularly pertinent in selection of empiric therapy.