Epidemiological and biological characteristics of methicillin-resistant staphylococcal infections in a Mexican hospital

Background. Methicillin-resistant Staphylococcus aureus (MRSA) has spread worldwide since 1960. However, there is little information concerning methicillin resistant coagulase-negative staphylococci (MRCNS) infections. Methods. In order to study the clinical and epidemiological characteristics of methicillin-resistant staphylococci (MRS) :infections and to determine the relationship between MRS and both synergistic hemolysis (SH) and slime production (SP), a laboratory-based survey and non-matched case-control study were carried out at a tertiary-care center in Mexico City. In regard to patients, from May 1991 to October 1992, 46 cases of MRS infection and 86 patients (controls) infected by methicillin-susceptible staphylococci (MSS) were included. Clinical and epidemiologic variables were analyzed. The isolates were identified and tested for antimicrobial susceptibility by standard methods. An MIC of oxacillin greater than or equal to 8 mu g/mL was defined as an MRS. Results. During the study, 94 nosocomial staphylococcal infections were diagnosed: S. aureus, 35 and CNS, 59; 43 (45.7%) by MRS (rate of MRS infections was 1.12 per 100 inpatients); 2 MRSA; 41 MRCNS, and only 19 were symptomatic. Three infections were community-acquired, including one MRSA and two MRCNS. After multivariate analysis, the significant risk factors were previous antimicrobial therapy (p = 0.013) and catheter-related (p = 0.009) and urinary-tract source (p = 0.0001). Forty-nine percent of MRS showed SH while only 15% of MSS (p < 0.001) showed SH, especially in 10/10 MR-S. hemolyticus. Additionally, 48% of MRCNS showed SP, as did 18% of MSCNS (p = 0.019), particularly in 15/20 MR-S. epidermidis. Of all MRS isolates, 38% showed a homogeneous phenotype, a trait associated with multi-drug resistance (p < 0.01) and SH (p < 0.001). Conclusions. CNS predominated as the cause of MRS infections in our setting. The homogenous phenotype was associated with SH and multi-drug resistance. (C) 1999 IMSS. Published by Elsevier Science Inc.