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Bacterial Infections
Community-associated Methicillin-resistant Staphylococcus aureus—
Overview of Our Current Understanding
a report by
Robert W Tolan, Jr, MD, FAAP, FIDSA
Chief, Division of Allergy, Immunology and Infectious Diseases, The Children’s Hospital at Saint Peter’s University Hospital, New Brunswick, and
Clinical Associate Professor of Pediatrics, Drexel University College of Medicine, Philadelphia
Methicillin-resistant Staphylococcus aureus (MRSA) has been recognized for and/or prior antibiotic use.
8,15,22
Finally, additional risk factors are prison or
more than 45 years.
1–5
Arising from antibiotic pressure in the healthcare daycare exposure, military service, sports activities (especially with sharing
environment, it typically enters communities as a result of the exposure of of equipment or towels), and belonging to certain groups, including Native
individuals to hospital settings, antibiotics, and other medical interventions.
3
Americans, Pacific Islanders, and men who have sex with men.
8,15,22
In the early 1990s, infections due to MRSA with an unusual drug However, significant numbers of patients lack any of the above-cited risk
susceptibility pattern began to appear,
6,7
and in the 21st century several factors.
6,15
Furthermore, interfamilial spread, particularly of SSTIs, cannot be
clones of this community-associated MRSA (CA-MRSA) with expanded predicted. Similarly, recurrence—while common—is unreliably associated
antibiotic sensitivity, virulence typical of methicillin-sensitive S. aureus with the presence of these risks. These issues make it challenging to
(MSSA),
4
and a particular propensity for causing skin and soft-tissue explain to patients and families what to expect when a person develops
infections (SSTIs)
8
have arisen in communities worldwide and begun to infection with CA-MRSA.
supplant both MSSA and other MRSA strains.
9–11
This review will focus on
CA-MRSA, with particular attention to recent advances in our Pathophysiology
understanding of the organism and the important ways in which it differs One of the most remarkable observations is the clonality of CA-MRSA
from healthcare-associated MRSA (HA-MRSA).
12,13
throughout the world.
9,10,17
Nearly all of the strains identified in the US
belong to the pulsed-field gel electrophoretic types USA300 and USA400.
Epidemiology Similarly, the genetic background of CA-MRSA seems to be continent-
Although retrospective reviews
5,6
suggest that MRSA sensitive to a variety of specific.
10
Never before have such distinct—nearly clonal—populations of an
antibiotics began to appear in the early 1990s, the rapid emergence of organism arisen and spread so rapidly to occupy a particular niche with such
CA-MRSA is an unprecedented phenomenon of the past five to 10 a wide geographical distribution.
9,10
years.
3,4,10,14
Since its appearance, the organism has become the primary
cause of SSTIs.
8,15,16
From its origins in the community, it has begun to S. aureus has five mobile antibiotic-resistance elements—staphylococcal
supplant HA-MRSA as the most common cause of staphylococcal infection cassette chromosome (SCC)mec types I–V—that carry the mecA gene
acquired by hospitalized patients.
6,16–22
Furthermore, the organism’s ubiquity conveying methicillin resistance.
5,22
Two of these convey resistance to a
and virulence
23
have resulted in an increase in the incidence of a variety of variety of antibiotics; consequently, they are large chromosomal elements
other staphylococcal infections.
6,15,16,24–26
whose presence may result in a less fit and less virulent organism. In
contrast, SCCmec type IV, found in essentially all CA-MRSA worldwide,
10
is
The main risk factors for CA-MRSA infection include age under two years a much smaller element that leaves the organism sensitive to most
(and particularly neonates in intensive care nurseries),
27
skin conditions or non-beta-lactam or -cephalosporin antibiotics (except erythromycin,
injury (including shaving), tattooing, illicit drug use, and other needle use.
24
generally) and also quite virulent.
22
This SCC is the first genetic element
Other common risk factors are nasal carriage of CA-MRSA and/or contact apparently shared by CA-MRSA strains worldwide. The second such
with an infected individual or a carrier, obesity, other comorbid conditions, element, carried by a bacteriophage, comprises the genes encoding the
Panton-Valentine leukocidin (PVL). Its role as one of the virulence factors of
CA-MRSA remains under study.
28
With the sequencing of the genome of a
Robert W Tolan, Jr, MD, FAAP, FIDSA, is Chief of the Division of
Allergy, Immunology and Infectious Diseases at The Children’s
clone of USA300,
29
another potential virulence factor that may help
Hospital at Saint Peter’s University Hospital, New Brunswick, promote the rapid global spread of CA-MRSA has been identified. Arginine
and Clinical Associate Professor of Pediatrics at Drexel University
catabolic mobile element (ACME), a novel mobile element likely derived
College of Medicine, Philadelphia. He is a Fellow of the
American Academy of Pediatrics (AAP) and the Infectious
from S. epidermidis, encodes an arginine deiminase system that may
Diseases Society of America (IDSA), and a member of the confer survival benefit in the human host.
20
The presence of other virulence
Pediatric Infectious Diseases Society (PIDS), the American Society
factors not universally identified in CA-MRSA
30
seems to depend primarily
of Microbiology (ASM), the American Medical Association
(AMA), the American Society of Tropical Medicine and Hygiene (ASTMH), and Physicians for Social
on the clone’s continent of origin.
10
Responsibility (PSR). Dr Tolan’s interests include cytomegalovirus, viral infections, antiviral
therapeutics, Internet-based medical publishing, Lyme infection, central nervous system infections,
PVL is thought to be associated with SSTIs.
15,31–33
Given the remarkable
endocarditis, toxic shock syndrome, influenza, and pertussis.
tendency of CA-MRSA to cause SSTIs and, for example, necrotizing
pneumonia,
34
necrotizing fasciitis,
35
and septicemia,
36
PVL was presumed to
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