6
Methicillin-Resistant Staphylococcus
aureus/Vancomycin-Resistant Enterococci
Colonization and Infection
in the Critical Care Unit
C. Glen Mayhall
Division of Infectious Diseases and Department of Healthcare Epidemiology, University of Texas Medical
Branch at Galveston, Galveston, Texas, U.S.A.INTRODUCTION
Methicillin-resistantStaphylococcus aureus(MRSA) and vancomycin-resistant enterococci (VRE)
are among the most common antibiotic-resistant nosocomial pathogens in health care in
general and in critical care units (CCUs) in particular. Although discovered shortly after its
introduction, resistance to methicillin was first reported in the United States in 1968 (1,2). Since
then, MRSA has spread throughout the world and has continued to spread in the United
States. In many health care facilities,50% ofS. aureusisolates are MRSA. In intensive care
units (ICUs), MRSA now makes up 60% ofS. aureusisolates (3).
As hospital-acquired methicillin-resistantS. aureus(HA-MRSA) continues to spread
within health care facilities, sites where health care is delivered face a new threat from
community-acquired methicillin-resistantS. aureus(CA-MRSA). These latter strains from the
community first appeared in the 1990s and now have been detected throughout the United
States and in many other countries throughout the world (4–12). Infections due to CA-MRSA
occur in patients with no risk factors or recent contact with health care facilities. They
commonly occur in healthy children and most commonly manifest as skin and soft tissue
infections (13–15). Most patients require treatment, and 23% to 29% have required hospital-
ization (14,15).
Over the past 10 years, CA-MRSA has continued to spread in the general population in
the United States and in other countries (16,17). The widespread dissemination of CA-MRSA in
the general population has been accompanied by an increasing prevalence of the pathogen in
hospitals and in other health care settings (18–21). Mathematical modeling indicates that CA-
MRSA will quickly replace the traditional HA-MRSA over the next few years (22). It is
anticipated that CA-MRSA may also be a more virulent pathogen for hospitalized patients.
Under these circumstances, patients in ICUs are going to be at even greater risk of infection
caused by more virulent pathogens. In the near future, infection control in ICUs will require
more resources and a much more intense application of preventive procedures and programs.
VRE are resistant gram-positive cocci that have appeared more recently in hospitals and
ICUs. VRE were first noted in November 1986 and reported in January 1988 (23). In July 1988,
VRE colonization of hematology patients was reported from Paris (24). In 1989, 0.3% of
enterococci (0.1% in ICUs) isolated from patients in hospitals participating in the National
Nosocomial Infection Surveillance (NNIS) system at the Centers for Disease Control and
Prevention (CDC) were resistant to vancomycin (25). In 1993, 7.9% of enterococci isolated in
NNIS system hospitals (13.6% in ICUs) were resistant to vancomycin. By 2003, 28.5% of
enterococci isolated in NNIS system hospital ICUs were resistant to vancomycin (26).
As normal flora, enterococci are not nearly as invasive as areS. aureus. Approximately
1 in 10 patients colonized with VRE develop infection (27), although this may vary with the
degree of immunosuppression of the patients (28,29). However, there is a growing body of
evidence that VRE are acquiring both genes that code for virulence and a putative
pathogenicity island, including theespgene (30,31). The most serious infections with VRE
are bacteremia, endocarditis, and meningitis. Urinary tract infections are less serious and