Infectious Diseases in Critical Care Medicine

METHICILLIN-SENSITIVE STAPHYLOCOCCUS AUREUS (MSSA) &
METHICILLIN-RESISTANT STAPHYLOCOCCUS AUREUS (MRSA)
Clinically Relevant Microbiology of MSSA/MRSA
Staphylococci are normal colonizers of the skin and may be classified on the basis of coagulase
production. The predominant CoNS of the skin isS. epidermidiswhereasS. aureusis the
predominant coagulase-positive staphylococcus.S. aureusmay be further classified on the
basis of susceptibility or to methicillin. Since methicillin is no longer used for in vitro
susceptibility testing, oxacillin is used in its place. Therefore, MSSA are reported as oxacillin-
susceptible. MRSA are reported as resistant to oxacillin.S. aureus(MSSA/MRSA) are common
colonizers of the nares/skin (19,20). Staphylococci are not part of the normal flora of the
mouth, GI tract, urine, or respiratory tract (1,21,22).
MRSA may be further subdivided on the basis of the site of origin or acquisition of the
infection. Strains of MRSA that originated in the hospital are termed hospital-acquired MRSA
(HA-MRSA). Strains of HA-MRSA which colonize/infect patients who are discharged to
the community and later return to the hospital with MRSA originally acquired in the hospital
have community-onsetMRSA (CO-MRSA). CO-MRSA infections are those that have anonset
in the community but originate in the hospital and are clinically and microbiologically
indistinguishable from HA-MRSA strains. In the past few years, a new strain ofS. aureus
emerged from the community without prior exposure to the hospital setting. These strains of
MRSA have been termed based on the location of acquisition as community-acquired MRSA
(CA-MRSA). In patients presenting with MRSA from the community, it is of critical
importance to differentiate those of communityonset(CO-MRSA) from thoseacquiredin the
community (CA-MRSA). CA-MRSA is genetically distinctive, i.e., HA-MRSA. CA-MRSA
strains have different staphylococcal chromosomal cassettes (SCC) than the HA-MRSA strains.
HA- and CO-MRSA genetically are characterized by SCCmecI, II, III, and elaborate several
S. aureustoxins. Another virulence factor for staphylococci is the presence of the Panton–Valentine
leukocidin gene that is rare in HA- and CO-MRSA. In contrast, CA-MRSA strains are characterized
genetically by the SCCmecIV and V genes and the PVL gene, which is common. CA-MRSA strains
that are PVL positive are highly virulent and present almost exclusively with severe pyodermas or
necrotizing soft tissue infections or as MRSA, CAP in patients with ILIs. CA-MRSA strains that are
PVL negative clinically resemble CO- and HA-MRSA strains in terms of their pathogenicity

Table 2 Anti-enterococcal Group D Streptococcal Antibiotics for Serious Systemic Infections

Preferred antibiotics Usual dosea Same/oral equivalent antibiotic

S. faecalis(VSE)
Ampicillin 2 g (IV) q4h Amoxicillin 500 mg (PO) q24h
Vancomycinþ 1 g (IV) q12hþ 600 mg (PO) q12h
gentamicin 120 mg (IV) q24h
(synergy dose)

None

Meropenem 1 g (IV) q8h Moxifloxacin 400 mg (PO) q24h

S. faecium(VRE)
Daptomycin 12 mg/kg (IV) q24hb Linezolid 600 mg (PO) q12h
Tigecycline 200 mg (IV) 1 dose, then
100 mg (IV) q24h

Minocycline 100 mg (PO) q12h or

linezolid 600 mg (PO) q12h

Linezolid 600 mg (IV/PO) q12h 600 mg (PO) 12h

Minocycline 100 mg (IV/PO) q12h 100 mg (PO) q12h

Quinupristin/dalfopristin 7.5 mg (IV) q8h Linezolid 600 mg (PO) q12h

Chloramphenicol 500 mg (IV/PO) q6h 500 mg (PO) q6h

aNormal renal function.
bIf repeat blood cultures are negative and no vegetation on TTE/TEE, treat enterococcal bacteremia for two

weeks. Treat native valve enterococcal SBE for 4 weeks in patients with symptoms of<3 months and for 6 weeks
in patients with symptoms of>3 months.
Abbreviation: SBE, subacute bacterial endocarditis.
Source: Adapted from Ref. 1.

Antimicrobial Therapy of VRE and MRSA in Critical Care 501