Infectious Diseases in Critical Care Medicine

and clinical presentation. In addition to PVL toxin, PVL-positive HA-MRSA strains also
produce other toxins that are virulence factors (21,22).
CA-MRSA strains also are susceptible to antibiotics that are usually ineffective against
CO- or HA-MRSA strains. CA-MRSA strains are usually susceptible to clindamycin,
trimethoprim-sulfamethoxazole (TMP-SMX), or doxycycline. These antibiotics are not
uniformly effective against CO- or HA-MRSA strains. Clinicians must be careful not to
assume that all patients with MRSA being admittedfromthe community have CA-MRSA
strains. Unless the patient presents with severe pyodermas/necrotizing soft tissue infections
or necrotizing MRSA CAP with influenza, all patients comingfromthe community should
be considered as CO-MRSA until proven otherwise. Therapeutically, this is important since
the antibiotics that are effective against HA- and CO-MRSA strains, i.e., vancomycin,
quinupristin/dalfopristin, minocycline, linezolid, daptomycin, or tigecycline are reliably
effective against all MRSA strains including HA-MRSA. Therefore, patients severely ill with
MRSA infections comingfromthe community should be treated as HA- or CO-MRSA because
these antibiotics are effective againstallMRSA strains. Conversely, it is not prudent to assume
that all MRSA strainsfromthe community are CA-MRSA because nearly all excluding those
mentioned above are of the CO-MRSA variety and will not respond to empiric treatment with
clindamycin, TMP-SMX, or doxycycline (21–23).

Epidemiology of MSSA/MRSA
Staphylococci colonize the skin/nares. Unlike colonization with VRE, colonization with MRSA
is episodic and not continuous. Unlike VRE, MSSA/MRSA has more inherent invasive
potential/virulence. Because MSSA/MRSA commonly colonize the skin, it is predictable that
nearly all staphylococcal infections originate from the skin and are the result of breaching the
integrity of the skin as a protective antimicrobial barrier. Unlike aerobic GNBs, staphylocci
may be transmitted from person to person. Staphylococci do not colonize the urine, but urine
cultures may be contaminated by staphylococci from the skin of distal urethra during urine
specimen collection. HA-and CO-MRSA occur in all age groups and are related to either skin
trauma or invasive procedures that traverse the skin. In contrast, CA-MRSA occurs primarily
in young adults in the community who experience skin abrasion/trauma. In some cases, CA-
MRSA may also complicate influenza pneumonia (Table 3) (1,22).

Table 3 Classification of MRSA Infections

MRSA strain Description Treatment

. Hospital-acquired MRSA
(HA-MRSA)

These strains originate within the

hospital environment and have

SCCmecI, II, III genes

Pan resistant to most antibiotics.

Only vancomycin, quinupristin/

dalfopristin, minocycline, linezolid,

tigecycline, and daptomycin are

reliably effective

. Community-onset MRSA
(CO-MRSA)

These strains originate from the

hospital environment but later

present from the community. They

too have SCCmecI, II, III genes

(CO-MRSA = HA-MRSA)

Since CO-MRSA strains are in

actuality HA-MRSA strains that

present from the community, they

should be treated as HA-MRSA

. Community-acquired
MRSA (CA-MRSA)

Onlycommunity MRSA infections

presenting with severe

pyomyositis or severe/necrotizing

community-acquired pneumonia

(with influenza) should be

considered as CA-MRSA PVL-

positive strains (SCCmecIV, V

genes).All other MRSA infections

presenting from the community

should be regarded as CO-MRSA

CA-MRSA are pauci-resistant, i.e.,

susceptible to clindamycin, TMP-

SMX, and doxycycline. Antibiotics

used to treat CO-MRSA/

HA-MRSA are effective against

CA-MRSA, butnotvice versa.

Therefore, all MRSA strains

can be treated as CO-MRSA/

HA-MRSA

Abbreviation: PVL, Panton–Valentine leukocidin.
Source: Adapted from Refs. 21 and 22.

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