Download S. aureus

Staphylococci
Ass.Prof. Dr. Huda H. AL-Hassnawi
Staphylococci are Gram-positive cocci about 0.5 – 1.0 μm in diameter. They grow in
clusters, pairs and occasionally in short chains. The clusters arise because staphylococci
divide in two planes, catalase positive, oxidase negative, non-motile, non-spore
forming. Traditionally they were divided into two groups on the basis of their ability to
clot blood plasma (the coagulase reaction). The coagulase-positive staphylococci
constitute the most pathogenic species S. aureus. The coagulase-negative staphylococci
(CNS) are now known to comprise over 30, other species such as S. epidermidis and S.
saprophyticus. The CNS are common commensals of skin, although some species can
cause infections.
Staphylococcus aureus:Phenotypic characteristics of Staphylococcus aureus:Gram-positive, cluster-forming coccus, non-motile, non-spore forming facultative
anaerobe, fermentation of glucose produces mainly lactic acid, ferments mannitol
(distinguishes from S. epidermidis), catalase positive, coagulase positiv, golden yellow
colony on agar, normal flora of humans found on nasal passages, skin and mucous
membranes pathogen of humans, causes a wide range of suppurative infections, as well
as food poisoning and toxic shock syndrome.
Pathogenesis of S. aureus:S. aureus infections can spread through contact with pus from an infected wound,
skin-to-skin contact with an infected person by producing hyaluronidase that destroys
tissues, and contact with objects such as towels, sheets, clothing, or athletic equipment
used by an infected person. Deeply penetrating S. aureus infections can be severe.
Prosthetic joints put a person at particular risk of septic arthritis, and staphylococcal
endocarditis (infection of the heart valves) and pneumonia.
S. aureus expresses many potential virulence factors:
(1) surface proteins that promote colonization of host tissues:S. aureus cells express surface proteins that promote attachment to host proteins such
as laminin and fibronectin that form the extracellular matrix of epithelial and endothelial
surfaces. In addition, most strains express a fibrin/fibrinogen binding protein (clumping
factor) which promotes attachment to blood clots and traumatized tissue. Most strains
of S. aureus express both fibronectin and fibrinogen-binding proteins. In addition, an
adhesin that promotes attachment to collagen has been found in strains that cause
osteomyelitis and septic arthritis
(2)
invasins
that
promote
bacterial
spread
in
tissues
such
as
(leukocidin, staphylokinases, hyaluronidase), leukocidin a toxin that specifically acts on
polymorphonuclear leukocytes. Hyaluronidase breaks down hyaluronic acid and helps
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in spreading of Staphylococcus aureus. also staphylokinase which activates
plasminogen to form plasmin, which digest fibrin clots. This disrupts the fibrin
meshwork which can often form to keep an infection localized.
(3) surface factors that inhibit phagocytic engulfment (capsule, Protein A):
Capsular Polysaccharide :-The majority of clinical isolates of S. aureus express a
surface polysaccharide of either serotype 5 or 8. This has been called a microcapsule
because it can be visualized only by electron microscopy unlike the true capsules of
some bacteria which are readily visualized by light microscopy.
Protein A:- Protein A is a surface protein of S. aureus which binds IgG molecules by
their Fc region. In serum, the bacteria will bind IgG molecules in the wrong orientation
on their surface, which disrupts opsonization and phagocytosis.
(4) biochemical properties that enhance their survival in phagocytes
(carotenoids, catalase production).
(5) immunological disguises (Protein A, coagulase):Coagulase is an extracellular protein which binds to prothrombin in the host to form a
complex called staphylothrombin. The protease activity characteristic of thrombin is
activated in the complex, resulting in the conversion of fibrinogen to fibrin.
(6) membrane-damaging toxins that lyse eucaryotic cell membranes:1-alpha toxin (alpha-hemolysin):- The best characterized and most potent membranedamaging toxin of S. aureus is alpha toxin. In humans, platelets and monocytes are
particularly sensitive to alpha toxin. Susceptible cells have a specific receptor for alpha
toxin which allow low concentrations of toxin to bind, causing small pores through
which monovalent cations can pass. At higher concentrations, the toxin reacts nonspecifically with membrane lipids, causing larger pores through which divalent cations
and small molecules can pass.
2-ß-toxin is a sphingomyelinase which damages membranes rich in this lipid. The
classical test for ß-toxin is lysis of sheep erythrocytes. The majority of human isolates
of S. aureus do not express ß-toxin. A lysogenic bacteriophage is known to encode the
toxin.
3-delta toxin is a very small peptide toxin produced by most strains of S. aureus. It is
also produced by S. epidermidis. The role of delta toxin in disease is unknown.
4- Panton and Valentine (PV) leukocidin: It causes dermonecrosis when injected
subcutaneously in rabbits. Furthermore, at a concentration below that causing
membrane damage, the toxin releases inflammatory mediators from human neutrophils,
leading to degranulation. So is associated with severe necrotizing pneumonia in
children.
(7) exotoxins that damage host tissues or otherwise provoke symptoms of disease:Superantigens: enterotoxins and toxic shock syndrome toxin :-
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S. aureus secretes two types of toxins with superantigen activity, enterotoxins, of which
there are six antigenic types (named SE-A, B, C, D, E and G), and toxic shock
syndrome toxin (TSST-1). Enterotoxins cause diarrhea and vomiting when ingested and
are responsible for staphylococcal food poisoning. TSST-1 is expressed systemically
and is the cause of toxic shock syndrome (TSS). When expressed systemically,
enterotoxins can also cause toxic shock syndrome. In fact, enterotoxins B and C cause
50% of non-menstrual cases of TSS. TSST-1 is weakly related to enterotoxins, but it
does not have emetic activity. TSST-1 is responsible for 75% of TSS, including all
menstrual cases. TSS can occur as a sequel to any staphylococcal infection if an
enterotoxin or TSST-1 is released systemically, and the host lacks appropriate
neutralizing antibodies. TSS is characterized by high fever , vomiting, diarrhea, sore
throat and muscle pain, within 48h it may progress to sever shock with evidence renal
and hepatic damage
Epidermolytic (exfoliative) toxin (ET)
EF toxins are implicated in the disease staphylococcal scalded-skin syndrome
(SSSS) or Ritter disease , which occurs most commonly in infants and young children
which results in widespread blistering with clear fluid that contains no bacteria or
leukocytes and loss of the epidermis. There are two antigenically distinct forms of the
toxin, ETA and ETB. The toxins have esterase and protease activity and apparently
target a protein which is involved in maintaining the integrity of the epidermis. . It also
may occur as epidemics in hospital nurseries
Figure (2):- Virulence determinants of Staphylococcus aureus
Clinical Manifestations of S. aureus
Staphylococcus aureus causes a variety of suppurative (pus-forming) infections and
toxinoses in humans. It causes superficial skin lesions such as boils, styes and
furuncules; more serious infections such as pneumonia, mastitis, phlebitis, meningitis,
and urinary tract infections; and deep-seated infections, such as osteomyelitis and
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endocarditis. S. aureus is a major cause of hospital acquired (nosocomial) infection of
surgical wounds and infections associated with indwelling medical devices. S. aureus
causes food poisoning by releasing enterotoxins into food, and toxic shock syndrome by
release of super-antigens into the blood stream, scalded skin syndrome, septicemia.
Collection of specimens
This depends on the area of the body affected. For example, those with a skin
infections or throat, nostrils and wound infections need to swabbed for pus and other
discharge with the bacteria. Swabs consist of a sterile absorbent cotton tipped sticks.
Those with a urinary tract infection need to provide a urine samples in sterile containers
and those with a generalized blood infection need to provide blood samples. Blood
samples are then transferred to blood culture bottle.
Identification of the bacteria
A small portion of the sample is swabbed onto a glass slide. This is then stained with
Gram stain or dyes like crystal violet and basic fuschin and viewed under the
microscope. S. aureus is Gram positive and stains blue or purple and appears as small
round cocci or short chains and most commonly as grape-like clusters. Since S. aureus
may be normally present on skin and mucous membranes, this test is not always
confirmatory. Coagulase positive is performed to distinguish it from coagulase negative
sp.
Treatment
Staphylococcus aureus causes a variety of manifestations and diseases. The
treatment of choice for S. aureus infection is penicillin. In most countries, S. aureus
strains have developed a resistance to penicillin due to production of an enzyme by the
bacteria called penicillinase. The first line therapy is penicillinase-resistant penicillins
like oxacillin, flucloxacillin, nafcillin, cefazolin etc.. Therapy is often given in
combination with aminoglycosides like gentamicin for more serious infections. The
duration of treatment depends on the site of infection and on severity. Hospital acquired
infection is often caused by antibiotic resistant strains (e.g. MRSA) and can only be
treated with vancomycin or an alternative.
Methicillin resistant Staphylococcus aureus (MRSA):MRSA are strains of the Staphylococcus aureus that are resistant to the action of
methicillin and related beta-lactam antibiotics (e.g. penicillin, oxacillin, amoxacillin).
MRSA have evolved resistance not only to beta-lactam antibiotics, but to several classes
of antibiotics. Some MRSA are resistant to all except one or two antibiotics, including
vancomycin. Reports of VRSA (Vancomycin-Resistant S. aureus) or VRSA are
troublesome in the ongoing battle against Staphylococcus infections. So Choice of
antibiotics
for MRSA
includes
Linezolid,
Vancomycin,
Clindamycin,
Quinupristin/dalfopristin, tetracyclines, Doxycycline, Minocycline etc. are chosen.
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Staphylococcus epidermidis
Staphylococcus epidermidis is a gram-positive, coagulase-negative cocci that is a
part of our normal flora. Consequently, it is a true opportunistic pathogen, as it requires
a major breach in the host’s innate defenses. It is one of the leading pathogens of
nosocomial infections, particularly associated with foreign body infections. Those most
susceptible to infection are intravenous drug users, newborns, elderly, and those using
catheters or other artificial appliances. The organism produces slime layers, which
forms a hydrophobic biofilm. This film is adhesive to hydrophobic biopolymers of
prosthetics, creating diseases such as endocarditis. The biofilm of S. epidermidis
consists of clusters of cells that are embedded in extracellular slime substance that is up
to 160 micrometers thick, exceeding 50 cells
Disease
Infections are associated with intravascular devices (prosthetic heart valves, shunts,
etc.) but also commonly occur in prosthetic joints, catheters, and large wounds. Catheter
infections along with catheter-induced UTIs lead to serious inflammation and pus
secretion. In these instances, urination is extremely painful. Septicemia and endocarditis
are also diseases associated with S. epidermidis. Their symptoms run the gamut from
fever, headache, and fatigue to anorexia and dyspnea. Septicemia is especially prevalent
resulting from neonatal infections, particularly in very low birth weights. Endocarditis is
an infection of the heart valves and parts of the inside lining of the heart muscle.
Treatment
The drug of choice is often vancomycin, to which rifampin or aminoglycoside can be
added
Staphylococcus saprophyticus
is a gram positive and coagulase-negative species of Staphylococcus bacteria. S.
saprophyticus is often implicated in urinary tract infections
Symptoms of infection:S. saprophyticus is implicated in 10-20% of urinary tract infections (UTIs). In
females between the ages of about 17-27 it is the second most common causative agent
of acute UTIs, after Escherichia coli.
Some of the symptoms of infection by these bacteria are burning sensation when
passing urine, the urge to urinate more often than usual, a 'dripping effect' after
urination, weak bladder, a bloated feeling with sharp razor pains in the lower abdomen
around the bladder and ovary areas, and razor-like pains during sexual intercourse.
Antibiotic susceptibility:S. saprophyticus is resistant to the antibiotic novobiocin, a characteristic that is used
in laboratory identification to distinguish it from S. epidermidis. Quinolones are
commonly used in treatment of S. saprophyticus urinary tract infections.
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