Correlations between consumption of antibiotics and methicillin resistance in coagulase negative staphylococci

The correlation between antibiotic consumption, expressed in defined daily doses (DDD), and antibiotic resistance rates was studied, using 976 isolates of coagulase negative staphylococci (CNS) from human pathological material. Data from four hospitals, including 14 participating departments, were analysed for this purpose. Susceptibility tests were performed according to Dutch national standards, except for methicillin, which for the majority of isolates was tested according to adapted NCCLS standards. Resistance to methicillin was most frequent in Staphylococcus epidermidis (29%) and S. haemolyticus (16%). Among the departments, thoracic surgery (29-47%), surgical intensive care (68%) and neonatology (32%) scored highest. Significant correlations were found between percentages of methicillin resistance in CNS and consumption (DDD/month/bed) of (flu)cloxacillin (P0.008), of cephalosporins (P0.01) and of gentamicin (P0.005). (Flu)cloxacillin was used mainly prophylactically, cephalosporins and gentamicin therapeutically. Results were similar for S. epidermidis (n = 639) alone. There was no significant correlation between consumption and resistance to trimethoprim, erythromycin (P0.08) or gentamicin (P0.09). Analysis of data from individual patients showed significant differences in proportions of methicillin resistance rates in CNS, between use and non-use of penicillinase resistant beta-lactams or gentamicin. It is concluded that clinical use of both (flu)cloxacillin and cephalosporins selects for methicillin resistant CNS.