influenza pneumonia may be due to MSSA, CO/CA-MRSA. The virulence of MSSA/MRSA
CAP is the same if the MRSA strain is CA-MRSA (PVL) (21,22). Excluding CA-MRSA (PVL+)
strains, the virulence of MSSA, HA-MRSA, CO-MRSA, and CA-MRSA (PVL) strains is the
same (36–42).
Antimicrobial Therapy of MSSA/MRSA
Outcomes of MSSA/MRSA (PVL) strains are the same if treated appropriately and early. The
therapy of MRSA depends on the nature/severity and location of the infection. Selection of an
anti-MRSA antibiotic should be based on clinical experience and not in vitro susceptibility
testing (21,23). MRSA is an organism where in vitro susceptibility does not necessarily
correlate with in vivo effectiveness (21,23). In the 1970s when MRSA first became widespread
in the United States, there was no experience in treating this organism. Patients infected with
MRSA were treated according to susceptibility testing often using betalactam antibiotics to
which MRSA was reportedly susceptible. Over time, clinicians noted the discrepancy between
susceptibility testing results and clinical outcomes, which led to the realization that only
certain antibiotics were effective against MRSA regardless of in vitro susceptibility testing (23).
It has been shown over time that the antibiotics with demonstrated clinical efficacy against
MRSA infections are limited to vancomycin, minocycline, quinupristin/dalfopristin, linezolid,
daptomycin, tigecycline, and ceftibiprole (18,43–61). Other antibiotics have invariably been
effective clinically against MRSA, i.e., TMP-SMX and doxycycline. Other antibiotics should not
be used in spite of susceptibility testing, i.e., quinolones and cephalosporins (1,21). If a
tetracycline is selected to treat CA-MRSA, use minocycline, not doxycycline.
As mentioned previously, CA-MRSA has different susceptibilities than HA/CO-MRSA.
HA-MRSA is susceptible to TMP-SMX, doxycycline, and chloramphenicol whereas HA/CO-
MRSA strains are not. Since nearly all strains presenting to the hospital from the community
are CO-MRSA rather than CA-MRSA, it is prudent to treat all MRSA as HA-MRSA or CA-
MRSA. HA/CO-MRSA antibiotics will also be effective against CA-MRSA (PVLþ/PVL
strains) as well (Table 5) (1,22).
There are only two clinically effective anti-MRSA antibiotics available as oral
formulations, i.e., minocycline and linezolid (1,16,21,26,43). All of the other clinically effective
anti-MRSA antibiotics are only available parenterally (1,21). As with other infectious diseases,
the preferred treatment for MRSA abscesses is surgical drainage. Similarly, MRSA line
infections should be treated primarily by removal of CVC lines. Unless there is associated ABE,
antimicrobial therapy for MRSA CVC line infections is ordinarily two weeks (1,21).
Complicated skin/soft tissue infections are usually treated with an IV/PO anti-MRSA
antibiotic for one to two weeks (1,21). MRSA PVE is treated with valve removal and
antimicrobial therapy. Native valve MRSA ABE is treated for four to six weeks of IV/PO
Table 4 Diagnostic Clinical Pathway: MSSA/MRSA Bacteremias/ABE
l Differentiate S. aureus blood culture positivity(1/2–1/4) from MSSA/MRSA bacteremia (3/4–4/4 positive blood
cultures)
l WithS. aureusbacteremia,differentiate low intensity/intermittent bacteremia(1/2–2/4 positive blood cultures)
from continuous/high intensity bacteremia(3/4–4/4 positive blood cultures)
l ABE isnota complication oflow intensity/intermittent S. aureus bacteremia.TTE/TEE is unnecessary, but will
verify no vegetations
l If continuous/high-grade MSSA/MRSA bacteremia, obtain a TTE or TEE to rule out or document cardiac
vegetation and confirm diagnosis of ABE
l Diagnostic criteria for MSSA/MRSA ABE8 Essential features
Continuous/high-grade MSSA/MRSA bacteremia
Cardiac vegetation on TTE/TEE
8 Nonessential features
Fever 1028 F (non-IVDAs)
Heart murmurAntimicrobial Therapy of VRE and MRSA in Critical Care 505