082098 Staphylococcus aureus Infections - Goodsamim.com

MEDICAL PROGRESS
is for those without HIV infection. 90 In cases of endocarditis
that are not related to drug use, the disease
is often left-sided, the patients are older, the
mortality rate is high (20 to 44 percent), and the
disease usually involves previously damaged cardiac
valves. 61,81,88 In one review, the incidence of both
embolic and neurologic complications of left-sided
S. aureus endocarditis was 50 percent. 88
S. aureus is one of the most common pathogens
in nosocomial and prosthetic-valve endocarditis, and
intravascular catheters are the most frequent source
of bacterial inoculation. The mortality rate for nosocomial
endocarditis, regardless of the pathogen, is
40 to 56 percent, and the rate is even higher when
the pathogen is S. aureus. 91 In many of these cases,
the diagnosis is obscured by other conditions or the
administration of antibiotics. Prosthetic-valve endocarditis,
especially in the early postoperative period,
is often fulminant and is characterized by the formation
of myocardial abscesses and the development of
valvular insufficiency. Fang et al. 92 noted a 43 percent
incidence of endocarditis in patients with prosthetic
valves who had nosocomial bacteremia. The
most common pathogen was S. aureus.
Metastatic Infections
S. aureus has a tendency to spread to particular
sites, including the bones, joints, kidneys, and
lungs. 78,82,88 Suppurative collections at these sites
serve as potential foci for recurrent infections. 78 Patients
with persistent fever despite appropriate therapy
should be evaluated for the presence of suppurative
collections.
Sepsis
A minority of bacteremic or local infections
progress to sepsis. Risk factors for sepsis include advanced
age, immunosuppression, chemotherapy, and
invasive procedures. The presentation of staphylococcal
sepsis is similar to that of gram-negative sepsis,
with fever, hypotension, tachycardia, and tachypnea.
S. aureus is one of the most common gram-positive
pathogens in cases of sepsis. 66 Severe cases progress
to multiorgan dysfunction, disseminated intravascular
coagulation, lactic acidosis, and death. 66 In both
gram-positive and gram-negative sepsis, the levels of
circulating tumor necrosis factor a, interleukin-1,
and interleukin-6 are predictive of the outcome. 93
Toxic Shock Syndrome
Staphylococcal toxic shock syndrome came to
prominence in 1980–1981, when numerous cases
were associated with the introduction of superabsorbent
tampons for use during menstruation. The disease
is characterized by a fulminant onset, often in
previously healthy persons. The diagnosis is based
on clinical findings that include high fever, erythematous
rash with subsequent desquamation, hypotension,
and multiorgan damage. Alternative diagnoses,
including Rocky Mountain spotted fever, streptococcal
scarlet fever, and leptospirosis, must be ruled
out. The toxic shock syndrome often develops from
a site of colonization rather than infection. 94
Although toxic shock syndrome toxin 1 accounts
for more than 90 percent of cases of the syndrome
that are associated with menstruation, other enterotoxins
account for 50 percent of cases unrelated to
menstruation. Nonmenstrual cases have increased
and now account for approximately one third of all
cases. These nonmenstrual cases have been associated
with localized infections, surgery, or insect bites.
Patients with nonmenstrual toxic shock syndrome
have a higher mortality rate than those with menstrual
toxic shock syndrome. 76
MECHANISMS OF RESISTANCE
TO ANTIMICROBIAL AGENTS
Penicillin is inactivated by b-lactamase, a serine
protease that hydrolyzes the b-lactam ring. Less than
5 percent of isolates remain sensitive to penicillin.
Resistance to methicillin confers resistance to all
penicillinase-resistant penicillins and cephalosporins.
This high level of resistance requires the presence of
the mec gene that encodes penicillin-binding protein
2a. 95 The mec genes probably originated from a different
species of staphylococci. 96 Although many
methicillin-resistant strains appear to be descendants
of a limited number of clones, some appear to be
multiclonal in origin, suggesting the horizontal transfer
of mec DNA. 96-98 Other staphylococcal genes, including
bla (for b-lactamase) and fem (for factors
essential for methicillin resistance), affect the expression
of resistance. The expression of resistance to
methicillin is often heterogeneous, and the percentage
of a bacterial population that expresses the
resistance phenotype varies according to the environmental
conditions. Antimicrobial-sensitivity testing
has been modified to enhance the detection of
the resistance phenotype. 95
There has been increasing concern about the possible
emergence of vancomycin-resistant S. aureus
strains. Resistance to vancomycin has been reported
in clinical isolates of S. haemolyticus, 99 a coagulase-negative
species. The enterococcal plasmid-bearing gene
for resistance to vancomycin has been transferred by
conjugation to S. aureus in vitro. 100 Four recent case
reports (one from Japan and three from the United
States) have documented the isolation of clinical
strains with intermediate sensitivity to vancomycin
(minimal inhibitory concentration, 8 mg per milliliter).
39,101 The mechanism of resistance in these isolates
is not known but is not due to the van genes
present in enterococci. Both increased cell-wall synthesis
and alterations in the cell wall that prevent vancomycin
from reaching sites of cell-wall synthesis have
been suggested as mechanisms. 39,102 Screening for
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