Enterococcus is intrinsically resistant to several antimicrobial drugs, with resistance to vancomycin encoded by several clusters of genes known as vancomycin resistance gene clusters (eg, vanA, vanB). The gene clusters generate resistance through multiple pathways which encode enzymes to determine the structure of peptidoglycan precursors [20,21]. Genetically diverse, hospital-associated VRE outbreaks have been associated with single clones, multiple clones, and changing molecular epidemiology over time [21]. While up to 25% of the VRE genome includes acquired elements, the majority of hospital-associated infections are traced to a few clonal complexes, which differ from community-associated VRE strains [22].
The evolution of these efficient mechanisms that promote drug resistance has made it extremely challenging to eradicate organisms such as MRSA and VRE. However, advances in recent years have furthered our understanding of the epidemiology, pathogenesis, and methods of prevention and containment.
RISK FACTORS FOR COLONIZATION AND INFECTION
MRSA
The risk factors underlying MRSA colonization and infection in the ICU setting can be categorized as either patient/host or environmental factors. A wide range of patient-level factors is associated with MRSA colonization upon admission. General principles regarding the transmission of MRSA in the community include close contact with colonized or infected individuals, breaks in the skin, crowded living conditions and poor hygiene. These factors, alone or in combination, are thought to underlie observed outbreaks among sports teams, military personnel, correction facilities, American Indian communities, and daycare centers [23–34].
Two recently published systematic reviews have summarized important patient-level factors associated with MRSA colonization at the time of hospital admission. Forster et al [35] examined 27 studies and identified previous admission to hospital, transfer from nursing home or long-term care facility, and previous antibiotic use as the top 3 factors associated with MRSA colonization. A similar review conducted by McKinnell and colleagues [36] found that prior hospitalization, nursing home contact, recent antibiotic use, and exposure to health care-associated pathogens (MRSA carriage, VRE carriage, or Clostridium difficile infection) were found to portend the highest risk. Specific comorbid conditions also conveyed an increased risk, including congestive heart failure, chronic wounds/bedsores, diabetes mellitus, pulmonary disease, immunosuppression, urinary catheter, and renal failure/dialysis. It is clear that health care contacts, especially recent hospitalization, residence in a long-term care facility, and antibiotic use, are significant risk factors for MRSA colonization [37–39].
In contrast to those already colonized with MRSA, some patients acquire MRSA during hospitalization. In these cases, transmission via hands of health care workers is likely the most common mechanism for spread of MRSA [6,40–42]. An understaffed ICU has also been cited as a potential risk factor for ICU MRSA transmission, perhaps due to sacrifices in hand hygiene practices by overextended staff [6]. Additional factors associated with increased risk of nosocomial MRSA acquisition include duration of antibiotic therapy, exposure to quinolone or macrolide antibiotics, length of hospital stay, enteral feeding, post-surgical status, insertion of central line or urinary catheter during admission, ICU admission, and proximity to another patient with MRSA infection or colonization [43–45]. A summary of risk factors for MRSA acquisition is shown in Table 1 .
