Microbiological aspects of enterococci isolated at King Edward VIII Hospital, Durban.

Abstract

The increasing frequency of enterococci as a major cause of nosocomial infections and the transmission of these organisms amongst hospital patients demands a greater awareness of the Enterococcus. Therapy of enterococcal infections is complicated by the pathogens continually changing resistance patterns to many broad-spectrum antibiotics. In addition, the ability of enterococci to cause serious invasive infections including endocarditis and septicaemia with associated high mortality rates; prompted this study which was aimed at identifying the biological properties of enterococci isolated from blood cultures of patients admitted at King Edward VIII hospital, Durban. Enterococci were identified to species level by the API 20 Strep system which identified 68% and a conventional biochemical system of Facklam and Collins which identified 100% of the isolates.The emergence of beta-Iactamase producing enterococci in other countries encouraged the testing of all isolates for this enzyme. All were beta-Iactamase negative. The reported false susceptibility for aminoglycosides and cephalosporins with blood enriched media encouraged the testing of these antibiotics with and without the supplementation of 5% lysed blood. The results showed that an average false susceptibility of 55 % occurred for gentamicin and 35% for tobramycin and netilmicin. The cephalosporins affected, cefotaxime and cefuroxime showed a false susceptibility of 28% and 17% respectively. The choice of treatment for serious enterococcal infections is a syllergistic combination of a beta-Iactam antibiotic plus an aminoglycoside for enterococci with intrinsic low-level resistance. The development of high-level aminoglycoside resistance, MIC 22000,ug/ml results in loss of synergism. This study showed that 26.4 % of enterococcal isolates displayed high level aminoglycoside resistance i.e. to gentamicin and streptomycin. Time-kill study showed reduced killing rate for these organisms for the beta-Iactams and glycopeptides with low-level gentamicin resistance. The results confirmed that a cell-wall active agent combined with gentamicin can be successfully used for enterococcal therapy if the organism has intrinsic low-level resistance to this amino glycoside. Pulsed-field gel electrophoresis (PFGE) carried out on a selected number of Enterococcus faecalis and Enterococcus faecium with high-level aminoglycoside resistance showed a variability in the restriction endonucelase digestion patterns. This suggests independent development of high-level gentamicin resistance and not clonal expression. The ease and reliability with which enterococcal isolates may be typed using this technique to compare different strains represent a significant advance.