Table 5 Classification of MRSA Infections
MRSA Strain Description Treatment
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.
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.
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.Abbreviations: CA-MRSA, community-acquired MRSA; CO-MRSA, community-onset; MRSA HA-MRSA, hospital-
acquired MRSA; PVL, Panton-Valentine Leukocidin; SCC, staphylococcal cassette chromosome; TMP-SMX,
trimethoprim-sulfamethoxazole.
Source: Adapted from Refs. 66 and 67.
Table 7 Factors in the Selection of Antimicrobial Therapy for MSSA/MRSA Bacteremias
l Select an antibiotic with known clinical efficacy and a high degree of activityagainst the presumed or known
pathogen, e.g., VSE, VRE, MSSA, or MRSA.
l If needed,adjust dosageto achieve therapeutic concentrations in serum/tissue.
l Select a “low resistance” potential antibiotic, e.g., ertapenem, amikacin, minocycline, moxifloxacin,
levofloxacin, meropenem, tigecycline, and etc. Avoid “high resistance” potential antibiotics, e.g., imipenem,
ciprofloxacin, gentamicin, tobramycin, and minimize the use of those that select out on resistant organisms,
e.g., vancomycin, ceftazidime.
l Select an antibiotic with a favorable safety profileand a lowC. difficilepotential, e.g., daptomycin, tigecycline,
linezolid, Q/D, minocycline.
l Select an antibiotic that is relatively cost-effectivein the clinical context of bacteremia/endocarditis.
l If possible,select an oral antibiotic that is the same or equivalent to intravenous therapy for all/or part (IV?PO
switch) of the duration of antimicrobial therapy.aBactericidal preferred for therapy of ABE.
Abbreviations: IV, intravenous; MSSA, methicillin-sensitiveS. aureus; MRSA, methicillin-resistantS. aureus; Q/D,
quinupristin/dalfopristin; VRE, vancomycin-resistant enterococci; VSE, vancomycin-susceptible enterococci.
Source: Adapted from Ref. 69.
Table 6 Diagnostic Clinical Pathway: MSSA/MRSA ABE
l Differentiate S. aureus blood culture positivity(1/2–1/4) from 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 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 ABE
lEssential features
Continuous/high-grade MSSA/MRSA bacteremia
Cardiac vegetation on TTE/TEE
lNonessential features
Fever 1028 F (non-IVDAs)
MurmuraaWith early MSSA/MRSA, a murmur is not present. Later, a new murmur in ABE indicates a vegetation or valvular
destruction.
Abbreviations: MSSA, methicillin-sensitiveS. aureus; MRSA, methicillin-resistantS. aureus; TTE, transthoracic
echocardiography; TEE, transesophageal echocardiography; ABE, acute bacterial endocarditis.
Source: Ref. 68.
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