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The Staphylococci
The staphylococci are gram-positive spherical cells, usually arranged
in grape-like irregular clusters. They grow readily on many types of
media and are active metabolically, fermenting carbohydrates and
producing pigments that vary from white to deep yellow. Some are
members of the normal flora of the skin and mucous membranes of
humans; others cause suppuration, abscess formation, a variety of
pyogenic infections, and even fatal septicemia. The pathogenic
staphylococci often hemolyze blood, coagulate plasma, and produce a
variety of extracellular enzymes and toxins.
The genus staphylococcus has at least 34 species. The three main
species of clinical importance are Staphylococcus aureus, Staphylococcus
epidermidis, and Staphylococcus saprophyticus. Staphylococcus aureus is
coagulase-positive, its a major pathogen for humans. Almost every
person will have some type of S aureus infection during a lifetime,
ranging in severity from food poisoning or minor skin infections to severe
life-threatening infections. The coagulase-negative staphylococci
(CoNs)are normal human flora and sometimes cause infection, often
associated with implanted appliances and devices, especially in very
young, old, and immune-compromised patients.
Morphology & Identification
Typical Organisms
Staphylococci are spherical cells about 1 μm in diameter arranged in
irregular clusters. Single cocci, pairs, tetrads, and chains are also seen in
liquid cultures. Young cocci stain strongly gram-positive; on aging, many
cells become gram-negative. Staphylococci are non-motile and do not
form spores.
Culture
Staphylococci grow readily on most bacteriologic media under aerobic
or micro-aerophilic conditions. They grow most rapidly at 37 °C but form
pigment best at room temperature (20–25 °C). Colonies on solid media
are round, smooth, raised, and glistening. S aureus usually forms gray to
deep golden yellow colonies. S epidermidis colonies usually are gray to
white on primary isolation. Various degrees of hemolysis are produced by
S aureus and occasionally by other species.
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Growth Characteristics
The staphylococci produce catalase, which differentiates them from the
streptococci. Staphylococci are variably sensitive to many antimicrobial
drugs. Resistance falls into several classes:
1. β-Lactamase production is common, is under plasmid control, and
makes the organisms resistant to many penicillins (penicillin G,
ampicillin, ticarcillin, piperacillin, and similar drugs).
2. Resistance to nafcillin (and to methicillin and oxacillin) is independent
of β-lactamase production.The mechanism of nafcillin resistance is
related to the lack or inaccessibility of certain penicillin-binding proteins
(PBPs) in the organisms.
3. S aureus is considered to be susceptible to vancomycin if the minimum
inhibitory concentration (MIC) is 4µg/mL; of intermediate susceptibility
if the MIC is 8–16 µg/mL; and resistant if the MIC is16µg/mL. Strains of
S aureus with intermediate susceptibility to vancomycin have been
isolated in Japan .These are often known as vancomycin-intermediate S
aureus, or "VISA.".
4. vancomycin-resistant S aureus (VRSA) strains were isolated from
patients in the United States.
5. Plasmid-mediated resistance to tetracyclines, erythromycins,
aminoglycosides, and other drugs is frequent in staphylococci.
6. "Tolerance" implies that staphylococci are inhibited by a drug but not
killed by it .
Antigenic Structure
Staphylococci contain antigenic polysaccharides and proteins as well
as other substances important in cell wall structure. Peptidoglycan is
important in the pathogenesis of infection: It elicits production of
interleukin-1 (endogenous pyrogen) and opsonic antibodies by
monocytes, and it can be a chemoattractant for polymorphonuclear
leukocytes, have endotoxin-like activity, and activate complement.
Protein A is a cell wall component of many S aureus strains that binds
to the Fc portion of IgG molecules except IgG3. protein A with attached
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IgG molecules directed against a specific bacterial antigen will
agglutinate bacteria that have that antigen ("coagglutination").
Some S aureus strains have capsules, which inhibit phagocytosis by
polymorphonuclear leukocytes unless specific antibodies are present.
Most strains of S aureus have coagulase, or clumping factor, on the cell
wall surface; coagulase binds nonenzymatically to fibrinogen, yielding
aggregation of the bacteria.
Enzymes & Toxins
Staphylococci can produce disease both through their ability to
multiply and spread widely in tissues and through their production of
many extracellular substances.
Catalase
Staphylococci produce catalase, which converts hydrogen peroxide
into water and oxygen. The catalase test differentiates the staphylococci,
which are positive, from the streptococci, which are negative.
Coagulase and Clumping Factor
S aureus produces coagulase, an enzyme-like protein that clots
oxalated or citrated plasma. Coagulase binds to prothrombin; together
they become enzymatically active and initiate fibrin polymerization.
Coagulase may deposit fibrin on the surface of staphylococci, perhaps
altering their ingestion by phagocytic cells or their destruction within
such cells.
Clumping factor is a surface S aureus compound that is responsible
for adherence of the organisms to fibrinogen and fibrin.
Other Enzymes
Other enzymes produced by staphylococci include a hyaluronidase, or
spreading factor; a staphylokinase resulting in fibrinolysis but acting
much more slowly than streptokinase; proteinases; lipases; and β-
lactamase , DNase is an important in diagnosis of S. aureus.
Exotoxins
The α-toxin is a heterogeneous protein that acts on a broad spectrum of
eukaryotic cell membranes. The α-toxin is a potent hemolysin. The β-
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toxin degrades sphingomyelin and therefore is toxic for many kinds of
cells, including human red blood cells. The γ-toxin lyses red blood cells
from humans and animals.
Leukocidin
This toxin of S aureus has two components. It can kill white blood
cells of humans and rabbits.
Exfoliative Toxins
These epidermolytic toxins of S aureus are two distinct proteins of the
same molecular weight. Epidermolytic toxin A is a chromosomal gene
product and is heat-stable (resists boiling for 20 minutes). Epidermolytic
toxin B is plasmid-mediated and heat-labile. The epidermolytic toxins
causes the staphylococcal scalded skin syndrome. The toxins are
superantigens.
Toxic Shock Syndrome Toxin
Most S aureus strains isolated from patients with toxic shock syndrome
produce a toxin called toxic shock syndrome toxin-1 (TSST-1). TSST-1
is the prototypical superantigen . TSST-1 binds to MHC class II
molecules, yielding T cell stimulation, which promotes the protean
manifestations of the toxic shock syndrome. The toxin is associated with
fever, shock, and multisystem involvement, including a desquamative
skin rash.
Enterotoxins
There are multiple (A–E, G–I, K–M) enterotoxins. Approximately 50%
of S aureus strains can produce one or more of enterotoxins. Like TSST-
1, the enterotoxins are superantigens. The enterotoxins are heat-stable and
resistant to the action of gut enzymes.
Pathogenesis
Staphylococci, particularly S epidermidis, are members of the normal
flora of the human skin and respiratory and gastrointestinal tracts but can
causes diseases in immunecompromised patients such as bacteremia .
Nasal carriage of S aureus occurs in 20–50% of humans. Staphylococci
are also found regularly on clothing, bed linens, and other fomites in
human environments.
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Pathogenic, invasive S aureus produces coagulase and tends to
produce a yellow pigment and to be hemolytic. Nonpathogenic,
noninvasive staphylococci such as S epidermidis are coagulase-negative
and tend to be nonhemolytic. Such organisms rarely produce suppuration
but may infect orthopedic or cardiovascular prostheses or cause disease in
immunosuppressed persons. S saprophyticus is typically nonpigmented,
novobiocin-resistant, and nonhemolytic; it causes urinary tract infections
in young women.
Pathology
The prototype of a staphylococcal lesion is the furuncle or other
localized abscess. Groups of S aureus established in a hair follicle lead to
tissue necrosis (dermonecrotic factor). Coagulase is produced and
coagulates fibrin around the lesion and within the lymphatics, resulting in
formation of a wall that limits the process and is reinforced by the
accumulation of inflammatory cells and fibrous tissue. Within the center
of the lesion, liquefaction of the necrotic tissue occurs (enhanced by
delayed hypersensitivity) .
Focal suppuration (abscess) is typical of staphylococcal infection.
From any one focus, organisms may spread via the lymphatics and
bloodstream to other parts of the body. Suppuration within veins,
associated with thrombosis, is a common feature of such dissemination.
In osteomyelitis, the primary focus of S aureus growth is typically in a
terminal blood vessel of the metaphysis of a long bone, leading to
necrosis of bone and chronic suppuration. S aureus may cause
pneumonia, meningitis, empyema, endocarditis, or sepsis with
suppuration in any organ.
Staphylococci also cause disease through the elaboration of toxins,
without apparent invasive infection. Bullous exfoliation, the scalded skin
syndrome, is caused by the production of exfoliative toxins. Toxic shock
syndrome is associated with toxic shock syndrome toxin-1 (TSST-1).
Clinical Findings
A localized staphylococcal infection appears as a "pimple," hair follicle
infection, or abscess. There is usually an intense, localized, painful
inflammatory reaction that undergoes central suppuration and heals
quickly when the pus is drained.
S aureus infection can also result from direct contamination of a
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wound, eg, postoperative staphylococcal wound infection or infection
following trauma .
If S aureus disseminates and bacteremia ensues, endocarditis, acute
hematogenous osteomyelitis, meningitis, or pulmonary infection can
result. The clinical presentations resemble those seen with other
bloodstream infections. Secondary localization within an organ or system
is accompanied by the symptoms and signs of organ dysfunction and
intense focal suppuration.
Food poisoning due to staphylococcal enterotoxin is characterized by a
short incubation period (1–8 hours); violent nausea, vomiting, and
diarrhea; and rapid convalescence. There is no fever.
Toxic shock syndrome is manifested by an abrupt onset of high fever,
vomiting, diarrhea, myalgias, rash, and hypotension with cardiac and
renal failure in the most severe cases.
Diagnostic Laboratory Tests
Specimens
Surface swab, pus, blood, tracheal aspirate, or spinal fluid for culture,
depending upon the localization of the process.
Smears
Typical staphylococci are seen in stained smears of pus or sputum. It is
not possible to distinguish saprophytic (S epidermidis) from pathogenic
(S aureus) organisms on smears.
Culture
Specimens cultivated on blood agar plates give rise to typical colonies
in 18 hours at 37 °C, but hemolysis and pigment production may not
occur until several days later and are optimal at room temperature. S
aureus but not other staphylococci ferment mannitol. Specimens
contaminated with a mixed flora can be cultured on media containing
7.5% NaCl; the salt inhibits most other normal flora but not S aureus.
Mannitol salt agar is used to screen for nasal carriers of S aureus. The
isolates are
catalase and coagulase
positive when the members of S.
aureus , S. intermedius and S.delphini were isolated. All isolates of S.
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aureus give a positive results for DNase test.
Serologic and Typing Tests
Serologic tests for diagnosis of S aureus infections have little practical
value.
Molecular typing techniques have been used to document the spread of
epidemic disease-producing clones of S aureus and PCR can be using in
detection of MRSA and VRSA .
Treatment
In the skin infections scrupulous local antisepsis is important to control
recurrent furunculosis. Serious multiple skin infections (acne,
furunculosis) occur most often in adolescents. Tetracyclines are used for
long-term treatment.
Abscesses and other closed suppurating lesions are treated by drainage,
which is essential, and antimicrobial therapy. Many antimicrobial drugs
have some effect against staphylococci in vitro.
Acute hematogenous osteomyelitis responds well to antimicrobial
drugs. In chronic and recurrent osteomyelitis, surgical drainage and
removal of dead bone is accompanied by long-term administration of
appropriate drugs.
Bacteremia, endocarditis, pneumonia, and other severe infections due
to S aureus require prolonged intravenous therapy with a β-lactamase-
resistant penicillin. Vancomycin is often reserved for use with nafcillin-
resistant staphylococci.
S epidermidis infections are difficult to cure because they occur in
prosthetic devices where the bacteria can sequester themselves in a
biofilm. S epidermidis is more often resistant to antimicrobial drugs than
is S aureus; approximately 75% of S epidermidis strains are nafcillin-
resistant.
Epidemiology & Control
Staphylococci are ubiquitous human parasites. The chief sources of
infection are shedding human lesions, fomites contaminated from such
lesions, and the human respiratory tract and skin. Contact spread of
infection has assumed added importance in hospitals, where a large
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proportion of the staff and patients carry antibiotic-resistant staphylococci
in the nose or on the skin. Although cleanliness, hygiene, and aseptic
management of lesions can control the spread of staphylococci from
lesions.
In hospitals, the areas at highest risk for severe staphylococcal
infections are the newborn nursery, intensive care units, operating rooms,
and cancer chemotherapy wards. Massive introduction of "epidemic"
pathogenic S aureus into these areas may lead to serious clinical disease.
Personnel with active S aureus lesions and carriers may have to be
excluded from these areas. In such individuals, the application of topical
antiseptics to nasal or perineal carriage sites may diminish shedding of
dangerous organisms.