title: Antimicrobial activities and susceptibility testing considerations of ampicillin, cephalothin, cefaclor, and cefuroxime against invasive isolates of Haemophilus influenzae.
abstract: The activities of ampicillin, cefaclor, cephalothin, and cefuroxime against invasive clinical isolates of Haemophilus infleunzae were studied to determine the correlation between resistance and beta-lactamase production. Approaches to in vitro susceptibility testing of cephalosporins in the clinical laboratory were also assessed. Three hundred and eight isolates of H. influenzae were tested for ampicillin susceptibility, and those which required for inhibition greater than or equal to 1.0 micrograms/ml of ampicillin were tested for beta-lactamase production with a chromogenic cephalosporin. Twenty-two percent of isolates produced beta-lactamase and 85% were serotype b. All isolates considered resistant (MIC greater than or equal to 2.0 micrograms/ml) to ampicillin produced beta-lactamase. A single beta-lactamase-producing isolate was identified, but was inhibited by 1.0 microgram/ml of ampicillin, and a zone diameter of 20 mm was produced by disk diffusion testing. One hundred and ninety-seven isolates were tested for susceptibility to cefaclor, cefuroxime, and cephalothin. Chloramphenicol susceptibility testing by disk diffusion was also performed on these isolates. General agreement and interchangeable results were found among these three cephalosporins by both agar dilution and disk diffusion methods. We conclude that ampicillin-resistant H. influenzae not producing beta-lactamase is rare among these isolates of H. influenzae responsible for invasive disease. Susceptibility testing results of cephalothin, cefaclor, or cefuroxime appear to be interchangeable, although results of cephalothin testing would tend to underestimate the activities of cefaclor or cefuroxime.
authors: Pediatric Infectious Diseases, University of Oklahoma, Health Sciences Center, Oklahoma City.
authors: M M
authors: MM
authors: Shelton
authors: D F
authors: DF
authors: Welch
mesh_terms: D000667:Ampicillin / Q000494:pharmacology; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002433:Cefaclor / Q000494:pharmacology; D002444:Cefuroxime / Q000494:pharmacology; D002512:Cephalothin / Q000494:pharmacology; D006193:Haemophilus influenzae / Q000187:drug effects*; D006801:Humans; D008826:Microbial Sensitivity Tests; D010403:Penicillin Resistance
chemical_list: D000900:Anti-Bacterial Agents; D002433:Cefaclor; D000667:Ampicillin; D002444:Cefuroxime; D002512:Cephalothin