Clinical Spectrum of MSSA/MRSA Infection
MSSA/MRSA skin/soft tissue infections.As mentioned previously, staphylococcal infections
originate from trauma or procedures done through the skin. Hence, staphylococci are the most
common pathogen implicated in skin/soft tissue infections and are important pathogens in
CVC-associated infections (1,11,12). Staphylococcal abscesses may complicate any invasive
procedure done penetrating the skin.
MSSA/MRSA Bacteremia/ABE.MSSA/MRSA are the most common causative organisms
responsible for nosocomial ABE. S. aureus ABE is also the most frequent pathogen in
intravenous drug abusers (IVDAs) who have right-sided ABE. Nonnosocomial MSSA/MRSA
ABE may complicate prolonged high-grade/continuous bacteremia from a distant source, i.e.,
a staphylococcal abscess, a CVC-related infection. The most common nosocomial ABE are
associated with CVCs (temporary or semipermanent), invasive cardiac procedures, i.e., radio
frequency ablation or implanted devices, i.e., defibrillator/pacemaker-lead/generator-associated
infections (10,12,21). Staphylococcal ABE is not a complication of cardiac catheterization and is
an extremely rare complication following coronary stent placement. Right-sided ABE may be
differentiated clinically from left-sided ABE by the presence or absence of pulmonary
involvement (10). Patients with right-sided ABE have a clinical presentation similar to those
with left-sided ABE except that septic pulmonary emboli invariably complicate right-sided
staphylococcal ABE. The presence of bilateral cavitary infiltrates some of which may be wedge-
shaped/pleural-based with temperatures 1028 F is diagnostic of septic pulmonary emboli in a
patient with right-sided ABE. Bilateral septic pulmonary emboli may be differentiated from
bland pulmonary emboli by fever, i.e., septic pulmonary emboli are associated with
temperatures 1028 F, whereas with bland pulmonary emboli, fevers are 1028 F (1,10). Also,
with bland pulmonary emboli, there are one or very few lesions, whereas in septic pulmonary
emboli, there are multiple lesions that rapidly cavitate. Whereas pulmonary infarcts may
cavitate, later and without fever> 1028 F, they should not be easily confused with the massive
bilateral multiple acutely cavitating lesions of septic pulmonary emboli from right-sided
MSSA/MRSA ABE (10,11,24–26).
Unlike the relatively avirulent pathogens, i.e., viridans streptococci that cause SBE,
MSSA/MRSA are capable of attacking normal native heart valves and do not require
preexisting valvular damage to initiate the infectious process. Therefore, non-IVDAs in
patients with ABE present with fever 1028 F with a continuous high-grade MSSA/MRSA
bacteremia that may not be accompanied by a murmur. The presence of a murmur indicates
valvular dysfunction. If a patient with ABE presents early there will be no cardiac murmur.
However, subsequently, the patient will develop a new/changing murmur typical of ABE
(10,25,26). In contrast, patients with SBE present with a cardiac murmur that remains
unchanged during the subacute course of SBE. Whereas CoNS are the most common
pathogens associated with prosthetic valve endocarditis (PVE), MSSA/MRSA may also cause
PVE (10).
A common problem faced by clinicians in critical care is to assess the clinical significance
of positive blood cultures, particularly those containing gram-positive cocci. Preliminary blood
culture results are usually presented as gram-positive cocci in clusters growing in blood
culture bottles. Since CoNS and MSSA/MRSA all appear the same on Gram stain, the clinician
must await speciation to be sure which staphylococcal species the initial report represents.
However, the clinician may fairly accurately predict the clinical significance of the isolate
based on the degree of blood culture positivity (1).
Clinicians must differentiate between positive blood cultures contaminated during the
venipuncture/blood culture processing from true bacteremias. Gram-positive cocci in 1/4–2/4
blood cultures most frequently are indicative of skin contamination during venipuncture
(11,25). Blood cultures should be obtained from peripheral veins and unless there is no
alternative should not be drawn from arterial lines or peripheral/central venous lines.
Straphyococcal bacteremias are likely with high blood culture positivity, i.e., 3/4–4/4 positive
blood cultures. If a patient with high degree of blood culture positivity is later identified
as CoNS then the clinician should search for a device-associated source. Most commonly,
CoNS bacteremias, when not blood culture contaminants, are associated with CVC
Antimicrobial Therapy of VRE and MRSA in Critical Care 503