Epidemiological and Biological Characteristics of Methicillin-Resistant Staphylococcal Infections in a Mexican Hospital

Abstract

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 ≥8 μg/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.