A comparison of methicillin-resistant and methicillin-susceptible Staphylococcus aureus reveals no clinical and epidemiological but molecular differences

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Abstract

Most studies on Staphylococcus aureus have focused on the molecular epidemiology of methicillin-resistant S. aureus (MRSA) infections. In contrast, little information is available regarding the molecular epidemiology of currently circulating methicillin-susceptible S. aureus (MSSA) isolates in hospital settings, an epoch when the epidemiology of S. aureus has undergone significant changes. We conducted a cross-sectional study to compare the clinical, epidemiological, and genetic characteristics of MSSA and MRSA isolates at 3 tertiary-care hospitals in Medellín, Colombia, from February 2008 to June 2010. The infections were classified according to the Centers for Disease Control and Prevention (CDC) definitions. Genotypic analysis included spa typing, multilocus sequence typing (MLST) and staphylococcal cassette chromosome (mec) (SCCmec) typing. A total of 810 patients was enrolled. One hundred infections (12.3%) were classified as community-associated (31 CA-MSSA, 69 CA-MRSA), 379 (46.8%) as healthcare-associated community-onset (136 HACO-MSSA, 243 HACO-MRSA), and 331 (40.9%) as healthcare-associated hospital-onset (104 HAHO-MSSA, 227 HAHO-MRSA). Genotype analyses showed a higher diversity and a more varied spa type repertoire in MSSA than in MRSA strains. Most of the clinical-epidemiological characteristics and risk factors evaluated did not allow for discriminating MRSA- from MSSA-infected patients. The lack of equivalence among the genetic backgrounds of the major MSSA and MRSA clones would suggest that the MRSA clones are imported instead of arising from successful MSSA clones. This study emphasizes the importance of local surveillance to create public awareness on the changing S. aureus epidemiology.

Introduction

Staphylococcus aureus is a major human pathogen being responsible for a wide variety of diseases, ranging from superficial skin infections to life threatening conditions such as bacteremia, endocarditis, pneumonia, or toxic shock syndrome (Lowy, 1998). The epidemiology of S. aureus changed mainly due to the emergence of methicillin resistance in 1961. Methicillin-resistant S. aureus (MRSA) infections were increasingly common in hospitals worldwide and those at higher risk were hospitalized patients or residents of long-term care facilities. This type of infection was denominated healthcare-associated MRSA (HA-MRSA) (Deurenberg and Stobberingh, 2008). But, MRSA also emerged causing infection in the community (community-associated MRSA, CA-MRSA), affecting patients who had never been hospitalized nor had known risk factors for MRSA infection (Deleo et al., 2010, Deurenberg and Stobberingh, 2008). In the present decade, increasing evidence suggests that CA-MRSA strains are infiltrating healthcare settings in many countries all over the world (Popovich et al., 2008). In South America including Colombia, the USA300-related MRSA strains are now reported causing nosocomial infections (Jimenez et al., 2012, Reyes et al., 2009). In fact, CA-MRSA infection has become endemic and more prevalent than community-associated methicillin-susceptible S. aureus (CA-MSSA) infection (Frazee et al., 2005, King et al., 2006, Moran et al., 2005, Sattler et al., 2002). In South America, most studies have focused on the description of the molecular epidemiology of MRSA infections in hospitals and in the community. As a consequence, there is little information regarding the molecular characterization of MSSA infections in these settings (Van Dijk et al., 2002, Vivoni et al., 2006). Considering the changing epidemiology of S. aureus, this study was conducted to compare the clinical, epidemiological, and molecular features of currently circulating MSSA and MRSA strains in 3 hospitals of Medellín, Colombia. Understanding the epidemiology of MSSA and MRSA infection, particularly in developing countries where the knowledge of S. aureus transmission dynamics is probably limited, is fundamental for devising effective prevention and control strategies.

Section snippets

Institutional review board approval

The study protocol was approved by the Bioethics Committee for Human Research at Universidad de Antioquia (CBEIH-SIU) (approval No. 0841150). An informed consent to participate in the study was signed by participants, parents, and/or guardians.

Study population

An observational cross-sectional study was conducted from February 2008 to June 2010, at 3 tertiary care hospitals of varying sizes. Hospital A is a large 648-bed university hospital, hospital B is a 380-bed medium-size tertiary care center, and hospital

Results

A total of 903 S. aureus-positive cultures was obtained from patients admitted to the 3 hospitals from February 2008 to June 2010. Ninety-three patients were excluded because they did not meet the inclusion criteria; from these, 40 already had an evaluated isolate, 34 were considered colonization, 7 refused participation, and 14 had incomplete clinical information. Finally, 810 patients were enrolled, 515 (63.6%) (170 MSSA, 345 MRSA) were from hospital A, 235 from hospital B (29.0%) (78 MSSA,

Discussion

The present study constitutes a cross-sectional comparison of the epidemiological and clinical characteristics, and the genetic background of MSSA and MRSA strains isolated from patients with infections classified according to CDC definitions. It was carried out in a moment when the epidemiology and clinical manifestations of S. aureus had undergone important changes that affect medical practice and pose challenges in recognition, diagnosis, and treatment.

In Colombia, until this investigation,

Acknowledgements

This research was supported by the Departamento Administrativo de Ciencia, Tecnología e Innovación – COLCIENCIAS, Project: 1115-459-21442. Likewise, JNJ received a doctoral training grant from COLCIENCIAS.

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