Staphylococci
The most important gram-positive coccal pathogens in critical care are staphylococci and
group D enterococci. For clinical purposes, staphylococci may be subdivided into MSSA,
MRSA, orS. epidermidis, also known as CoNS.S. epidermidis(CoNS) are relatively avirulent
pathogens and are common colonizers of the nares/skin. Because of their lack of invasive
potential, CoNS are associated only with infections that are device related, i.e., prosthetic joint
infections, CVC infections, pacemaker/pacemaker generator/pacemaker-lead infections,
intracardiac prosthetic materials, prosthetic joints, or prosthetic heart valves.
S. aureus, either of the MSSA or MRSA variety, are common colonizers of the skin,
but have invasive capability and are highly virulent organisms if treated early and
with appropriate antibiotics, the clinical spectrum of infection and virulence potential and
outcomes are the same for MSSA and MRSA. Some clinicians have difficulty in determining
what is appropriate/effective antimicrobial therapy for MRSA infections if antibiotic selection
is based on in vitro susceptibility testing. Unlike MSSA, in vitro susceptibility testing for MRSA
does not correlate well with in vivo clinical effectiveness. MRSA isolates are often reported as
susceptible to fluoroquinolones or cephalosporins, but these agents are ineffective against
MRSA in vivo.The spectra of staphylococcal infections due to MSSA/MRSA are skin/soft
tissue infections, device-associated infections (as with CoNS vide supra), acute bacterial
endocarditis (ABE), and community-acquired pneumonia (CAP) only if superimposed on
underlying influenza/influenza-like illnesses (ILIs). An intravascular source of staphylococci
may metastatically spread to body sites, such as the CNS (brain abscess, meningitis), the
kidneys (renal abscess, perinephric abscess), or lungs (abscess), that are not normally infected
by staphylococci,. Staphylococci are not usual hepatobiliary, gastrointestinal, or urinary tract
pathogens (1).
Antimicrobial therapy of staphylococcal infections depends on the isolate susceptibility
(CoNS, MSSA, but not MRSA) and host factors (allergy history, renal/hepatic function, site of
infection, etc.). Duration of therapy depends on the type/site of infection ranging from one to
two weeks of therapy for skin/soft tissue infections to four to six weeks for ABE or
osteomyelitis. Clinically, in addition to appropriate/effective antimicrobial therapy, device-
associated infections due to CoNS or MSSA/MRSA usually require removal of the device for
cure (1).
Enterococcus faecalis(VSE) andEnterococcus faecium(VRE)
Clinically Relevant Microbiology of VSE/VRE
Group D streptococci are classified either as enterococcal group D streptococci or non-
enterococcal group D streptococci. Non-enterococcal group D streptococci are differentiated
from enterococcal group D streptococci microbiologically on the basis of penicillin suscep-
tibility, bile esculin hydrolysis, and growth in 0.9% sodium chloride. Non-enterococcal group
D streptococci are penicillin sensitive and do not ferment bile esculin or grow in 0.9% sodium
chloride, whereas group D enterococci are resistant to penicillin and do hydrolyze esculin and
grow in 0.9% sodium chloride. The most important non-enterococcal group D streptococci
encountered in clinical practice isS. gallolyticus(S. bovis). Clinically, the most important
enterococcal group D streptococci areE. faecalisandE. faecium.
Group D enterococci are also classified on the basis of their susceptibility to vancomycin.
Because nearly all strains ofE. faecalisare susceptible to vancomycin,E. faecalis, for practical
purposes, are termed vancomycin-sensitive enterococci (VSE). Similarly, nearly all isolates of
E. faeciumare resistant to vancomycin and for clinical purposes are termed vancomycin-
resistant enterococci (VRE). Presumptively, VRE may be differentiated from VSE isolates on
the basis of vancomycin susceptibility. Isolates that are vancomycin susceptible are invariably
ampicillin susceptible as well. Althoughin vivosusceptibility testing often reports VSE as
susceptible to penicillin, penicillin alone has no anti–group D enterococcal activity. Penicillin
combined with gentamicin has anti-VSE activity. Group D enterococci constitute a small part
of the normal gastrointestinal tract flora in the colon,75% of the bacteria are anaerobic, e.g.,
Bacteroides fragilis.The next most common organism making up the colonic microflora are
aerobic gram-negative bacilli (GNBs) that constitute20% of the colon’s flora. The remaining
498 Cunha