Increasing incidence of ampicillin resistance in Hemophilus influenzae.

The incidence of ampicillin resistance in Hemophilus influenzae and its medical significance have not been extensively determined. During 1975-1977, we tested for ampicillin resistance 489 consecutive middle-ear isolates of HI obtained from children in Huntsville, Alabama, and 719 consecutive laboratory isolates of HI from Children's Hospital, Boston. The annual incidence of Amp resistance rose progressively in each survey, from initial values of 1.4 to 5.3% in 1975, to 14 to 16% in 1977 (P less than 0.05), a mean annual rate of increase approximately twofold. Resistance was equally prevalent among type b and non-b isolates and among nasally carried and disease-associated isolates (from blood, CSF, middle ears). Patients harboring AmpR isolates were much more likely to have had recent exposure to beta-lactam antibiotics (P less than 0.01).