Regional differences in beta-lactamase and ampicillin and cefprozil resistance in Haemophilus influenzae in the United States

This study tracks regional differences in resistance of Haemophilus influenzae to ampicillin and the second-generation cephalosporin cefprozil as well as the role of beta-lactamase. Isolates were gathered from 12 regions of the United States, and culture specimens were tested by standard methods for beta-lactamase production, serotype, and susceptibility to ampicillin and cefprozil. A total of 1240 isolates were obtained, for an average of 103 per region. Beta-lactamase was produced by 35.7% of the isolates, which were thus ampicillin resistant; 63.8% were beta-lactamase negative, ampicillin susceptible; 0.4% were betalactamase negative, ampicillin resistant; and 0.1% were beta-lactamase negative, ampicillin intermediate. Overall, 36.1% of the H. influenzae strains were not susceptible to ampicillin. Conversely, beta-lactamase production had a minimal effect on susceptibility to cefprozil, as only 4.3% of the strains were resistant to this drug. The number of beta-lactamase-positive strains of H. influenzae by serotype (type b, non-type b, nontypeable) was nearly identical. Beta-lactamase production by geographic region varied from 26.9% to 48.6%, indicating significant resistance in H. influenzae throughout the country. The in vitro activity of cefprozil against most ampicillin-resistant beta-lactamase-positive strains of H. influenzae suggests that an oral cephalosporin may be useful in the treatment of infections due to beta-lactamase-producing H. influenzae.