Antibiotic susceptibilities of Haemophilus influenzae in central Scotland

OBJECTIVE: To ascertain the incidence of antibiotic resistance in Haemophilus influenzae in central Scotland and the beta-lactamases produced by these isolates. METHODS: A total of 213 H. influenzae isolates from four medical centers in Scotland [Aberdeen (n=58), Edinburgh (n=55), Glasgow (n=64) and Dundee (n=36)] were tested for susceptibility to a range of antimicrobials including beta-lactams, beta-lactam/beta-lactamase-inhibitor combinations, and a representative 4-quinolone, antifolate and macrolide. Susceptibility testing of the beta-lactam/beta-lactamase-inhibitor combination amoxicillin plus clavulanic acid was conducted at both 2:1 and 4:1 ratios and with clavulanic acid fixed at a concentration of 2 mg/L. Each strain was further investigated for the presence of beta-lactamase activity. RESULTS: Although the incidence of resistance to amoxicillin was 15%, in the presence of clavulanic acid, this resistance was reduced to 4.2%, 5.6% and 4.2% with the 2:1 ratio, 4:1 ratio and 2 mg/L fixed concentration, respectively. Sixteen percent of the isolates demonstrated immediate beta-lactamase production. Isoelectric focusing showed that 77.4%, 16.1% and 6.5% of the beta-lactamase-positive strains were found to contain TEM-1, VAT-1 and both TEM-1 and VAT-1 beta-lactamases, respectively. A further 29% of the strains were recognized as being beta-lactamase-positive after prolonged incubation with nitrocephin. CONCLUSIONS: This study suggests that current testing for beta-lactamases may underestimate the prevalence of beta-lactamase production in H. influenzae.