Chapter 115: Methicillin-Resistant Staphylococcus aureus (MRSA) Infection

THE ORGANISM

Staphylococci are aerobic or facultative anaerobic gram-positive cocci in clusters or, less often, chains, pairs, or tetrads. Staphylococcus aureus produces coagulase. Staphylococci that do not produce coagulase are reported as coagulase-negative staphylococci (CONS) and include S. epidermidis, S. lugdenensis, S. haemolyticus, and S. saprophyticus. Both S. aureus and certain species of CONS can cause serious infection in the newborn period. This chapter deals only with infections caused by S. aureus.

Staphylococcus aureus became resistant to penicillin shortly after penicillin was introduced in the 1940s by producing penicillinase, which inactivates the β-lactam ring in penicillin. Penicillinase-stable β-lactam antibiotics (methicillin, nafcillin, and oxacillin) were developed to overcome this resistance. However, in 1961, a strain of methicillin-resistant S. aureus (MRSA) was identified that was resistant to these newer β-lactam antibiotics.¹ Methicillin resistance occurs as a result of a modified drug target, penicillin-binding protein (PBP2a), encoded by the mecA gene² on a staphylococcal cassette chromosome (SCCmec types I–V).³ SCCmec types II–III typically carry multiple resistance determinants. SCCmec types IV–V carry fewer resistance determinants but may be associated with virulence factors such as Panton-Valentine leucocidin (PVL).

Initially, most strains of MRSA were health care or hospital associated (HA-MRSA) and were SCCmec types I–III. Now, MRSA can be acquired in the community (CA-MRSA) and colonizes or infects patients who have neither been hospitalized nor had recent access to health care. CA-MRSA is associated with SCCmec types IV and V. CA-MRSA tends to be susceptible to more antibiotics compared to HA-MRSA. However, resistance patterns are variable, depending on which strains are circulating in a community.

EPIDEMIOLOGY

Staphylococcus aureus has a propensity to colonize the anterior nares, and many studies evaluating colonization rely on nasal cultures.⁴ In infants, other body sites are likely to be colonized, including the throat, umbilicus, skin (groin and axillae), vagina, and rectum.^{5, 6, and 7} Approximately 25%–50% of the population is colonized with S. aureus, either methicillin-sensitive strains (MSSA) or MRSA.^{8, 9, and 10} Colonization with MSSA is more common than with MRSA. Children are more likely to be colonized than adults. Some patients are never colonized, some only intermittently, and some persistently. The rate of MRSA colonization in the United States is reported to be low (<2%)⁸ but varies depending on the population studied. For example, the following holds:

• Newborns: Fewer than 1% to 40% (colonization rates higher during neonatal intensive care unit [NICU] outbreaks)^{11, 12, 13, and 14}

• Healthy children less than 5 years old: Fewer than 1%–6.7%^{15, 16}

• Pregnant women: Fewer than 1%–10% (anogenital cultures)^{17, 18, 19, and 20}

  • – There are conflicting reports regarding the association of MRSA genital colonization and group B streptococcus genital colonization.²⁰

• Homeless in San Francisco: 2.8%

• Veterinarians: ...