347 β-LACTAMASE ACTIVITY & ANTIMICROBIAL SUSCEPTIBILITY OF HEMOPHILUS INFLUENZAE AND PARAINFLUENZAE IN MICH

Abstract

The incidence of β-lactamase activity of 1000 Hemophilus strains collected Sept 76-Jan 78 from pediatric and adult patients throughout Michigan (utilizing an iodometric assay, AAC 7:265:1975) was 8/52 (15%) for type b, 29/318 (9%) for non-type b H. influenzae and 31/630 (5%) for H. parainfluenzae. From Sept 76-July 77 H. influenzae type b isolates were recovered from 16 children with meningitis; none were positive for β-lactamase activity. Subsequently 4/7 CSF isolates were positive for β-lactamase activity. The in vitro susceptibilities of 30 β-lactamase (+) Hemophilus strains to ampicillin, chloramphenicol, cefamandole, cefachlor, tetracycline and trimethoprim-sulfamethoxazole were evaluated using a microtiter broth dilution method. 70% of strains were resistant to [amp] of ≥10 μg/ml (MIC range = 4-128 μg/ml), while all were inhibited by ≥1 μg/ml of chloramphenicol (MIC=0.25-1 μg/ml). Strains were highly susceptible to cefamandole (90% inhibited by ≥0.5 μg/ml), although only 25% of strains were inhibited by ≥1 μg/ml of cefachlor (MIC-1-16 μg/ml). All isolates were inhibited by [tetracycline] of ≥1 μg/ml. MICs of TMP-SMZ ranged from .0037/.148-.125/2.37 μg/ml with 75% of strains susceptible to ≥0.03/.59 μg/ml. The incidence of β-lactamase (+) Hemophilus strains in Michigan appears to be increasing. These strains are highly susceptible in vitro to chloramphenicol, cefamandole and TMP-SMZ.