P594Intracellular killing of drug-resistant Staphylococcus aureus by different antibiotics

Objectives:  The aim of this study was to assess the carbapenem susceptibility of four nosocomial pathogens and to evaluate the reliability of the susceptibility results determined by E-test and disc diffusion (DD) methods.
Methods:  Escherichia coli ( n  = 73), Klebsiella pneumoniae ( n  = 60), Pseudomonas aeruginosa ( n  = 70) and Acinetobacter spp. ( n  = 70) isolated from nosocomial infections in 2002–2003 were included in the study. Thirty-five per cent of the strains were isolated from intensive care units. After determining antimicrobial susceptibility against imipenem and meropenem by DD (10  μ g; Oxoid, UK) and Etest (AB Biodisk, Solna, Sweden) methods, the results were categorised as susceptible (S), intermediate (I) and resistant (R) according to the NCCLS criteria. For statistical analyses, the intermediate group was included in the resistant category because of the low numbers of bacteria in the former group.
Results:  None of E. coli or K. pneumoniae strains were resistant to carbapenems, whereas, resistance reached up to 59.0% in Acinetobacter spp. and P. aeruginosa isolates. By either method, the pattern of the susceptibility of the four bacteria was not statistically significantly different for imipenem vs. meropenem. Total agreement of DD and E-test methods for susceptibility to imipenem was 95.7%, and 90.0% in Acinetobacter spp. and P. aeruginosa , respectively; and susceptibility to meropenem was 90.0% for both bacteria. However, the difference of the results obtained by either method was statistically significant for Acinetobacter spp.
Conclusion:  Study results suggest a high resistance rate for Acinetobacter spp. and P. aeruginosa strains against carbapenem antibiotics in our hospital. Further studies are needed to clarify whether E-test should be used to confirm meropenem resistance of Acinetobacter spp. determined by DD method.