Susceptibility of respiratory strains of Staphylococcus aureus to fifteen antibiotics: results of a collaborative surveillance study (1992-1993)

Abstract

As part of the Alexander Project during 1992 and 1993, 690 Staphylococcus aureus strains isolated from community-acquired lower respiratory tract infections by clinical microbiology centres located in Europe and the USA were analysed by a co-ordinating laboratory that determined minimal inhibitory concentrations of 15 antimicrobial agents using a standardised microdilution technique. The prevalence of penicillin-susceptible microorganisms in this collection of pathogens was significantly higher in Europe (21.2%) than it was in the USA (12.1%). Most isolates (72.5%), however, were strains that had acquired the ability to synthesise a β-lactamase but which were sensitive to methicilhn. The incidence of methicillin-resistance (9.1 % overall) was highly variable depending on geographic location and year of isolation. Analysis of MIC50,, MIG90, MIC range and modal MIC of the 15 antibiotics assayed disclosed no major differences between the data sets obtained during the 2-year survey. Except for methicillin-resistant S. aureus, the activity of all the β-lactamase tested, with the exclusion of penicillin, amoxycillin and cefixime (that were completely inactive), was satisfactory. The effect of β-lactamase synthesis was inhibited by the combination of amoxycillin with clavulanate, and by cefuroxime and ceftriaxone. Cefaclor was slightly less effective. Erythromycin, clarithromycin and azithromycin showed identical cross-resistance rates (around 10%). Resistance to the macrolides was more frequent in the USA than in Europe and was the sole trait found to increase during the survey. Doxycycline, chloramphenicol, co-trimoxazole and the two fluoroquinolones tested (ofloxacin and ciprofloxacin) were remarkably effective (resistance lower than 1%). Only doxycycline and, to a lesser extent, co-trimoxazole were partially active against methicillin-resistant strains.