staphylococcus genus
TRANSCRIPT
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Staphylococcus genus
Bacteria in the genus Staphylococcus are pathogens of man and
other mammals.
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 nown to
comprise over !" other species.
#leven of these species can be isolated from humans as
commensals.
Classifcation
S. aureus (nares) and S. epidermidis (nares$ sin) are commoncommensals and also have the greatest pathogenic potential.
S. saprophyticus (sin$ occasionally) is also a common cause
of urinary tract infection.
S. haemolyticus, S. simulans, S. cohnii, S. warneri and S.
lugdunensis can also cause infections in man
Structure – defnition
Staphylococci are %ram-positive cocci about ".& - '." m in
diameter.
They grow in clusters$ pairs and occasionally in short chains.
The clusters arise because staphylococci divide in two planes.
The conguration of the cocci helps to distin-guish micrococci
and staphylococci from streptococci$ which usually grow in
chains.
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They grow in clusters$ pairs and occasionally in short chains.
Catalase Test
The catalase test is important in distinguishing streptococci
(catalase-negative) staphylo-cocci which are catalase
positive.
The test is performed by *ooding an agar slant or broth
culture with several drops of !+ hydrogen pero,ide.
Catalase-positive cultures bubble at once.
Isolation and Identifcation
The presence of staphylococci in a lesion might rst be
suspected after e,amination of a direct %ram stain.
owever$ small numbers of bacteria in blood preclude
microscopic e,amination and reuire culturing rst.
The organism is isolated by streaing material from the clinical
specimen (or from a blood culture) onto solid media such as
blood agar$ tryptic soy agar or heart infusion agar.
Specimens likely to be contaminated with other
microorganisms can be plated on mannitol salt agar
containing 7.5% sodium chloride, which allows the salts
tolerant staphylococci to grow
• S. aureus growth in manitol salt culture media
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/deally a %ram stain of the colony should be performed and
tests made for catalase and coagulase production$
allowing the coagulase-positive S. aureus to be iden-tied
uicly.
0nother very useful test for S. aureus is the production of
thermostable deoyribonuclease.
S. aureus can be conrmed by testing colonies for
agglutination with late particles coated with
immunoglobulin % and brinogen which bind protein 0 and the
clumping factor$ respectively$ on the bacterial cell surface.
These are available from commercial suppliers (e.g.$
Staphaurex ).
The most recent late, test (1astaure,) incorporates
monoclonal antibodies to serotype & and 2 capsular
polysaccharide in order to reduce the number of false
negatives.
Some recent clinical isolates of S. aureus lac production of
coagulase and3or clumping factor$ which can mae
identication di4cult.
The association of S. epidermidis (and to a lesser e,tent of
other coagulase-negative staphylococci) with nosocomial
in!ec"tions associated with ind#elling de$ices means that
isolation of these bacteria from blood is liely to be important
and not due to chance contamination$ particularly if
successi$e blood cultures are positi$e.
Nowadays$ identication of S. epidermidis and other species of
Staphylococcus is performed using commercial biotype
identication its$ such as 01/ Staph /dent$ 01/ Staph-Trac$
5ite %1/ Card and 6icroscan 1os Combo.
These comprise preformed strips containing test substrates.
Clinical ani!estations o! S aureus
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Skin infecitons
S. aureus is notorious for causing boils, !uruncles, styes,
impetigo and other supercial sin infections in humans.
/t may also cause more serious infections$ particularly inpersons debilitated by chronic illness$ traumatic in7ury$ burns
or immunosuppression.
Severe infections
These infections include pneumonia, deep abscesses,
osteomyelitis, endocarditis, phlebitis, mastitis and
meningitis, and are often associated with hospitali8ed patients
rather than healthy individuals in the community
S. aureus and S. epidermidis are common causes of infections
associated with indwelling devices such as 7oint prostheses$
cardiovascular devices and articial heart valves
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&athogenesis o! S. aureus In!ections
/n order to initiate infection the pathogen must gain access to
the host and attach to host cells or tissues.
S. aureus cells e,press on their surface proteins that promote
attachment to host proteins such as laminin and bronectin
that form part of the e,trace-llular matri, .
S aureus 'dheres to (ost &roteins
9ibronectin is present on epithelial and endo-thelial surfaces
as well as being a component of blood clots.
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/n addition$ most strains e,press a brinogen3
brin binding protein (the clumping factor) which promotes
attachment to blood clots and traumati8ed tissue.
6ost strains of S. aureus e,press bronectin and brinogen-binding proteins.
Collagen binding protein
The receptor which promotes attachment to collagen is
particularly associated with strains that cause osteomyelitis
and septic arthritis.
/nteraction with collagen may also be important in promoting
bacterial attachment to damaged tissue where the underlyinglayers have been e,posed.
)ole o! 'dherence in In!ections 'ssociated
#ith edical *e$ices
/nfections associated with indwelling medical devices ranging
from simple intravenous catheters to prosthetic 7oints and
replacement heart valves can
be caused by S. aureus and S. epidermidis
'$oidance o! (ost *e!enses
S. aureus e,presses a number of factors that have the
potential to interfere with host defense mechanisms.
Microcapsule
The ma7ority of clinical isolates of S. aureus e,press a surface
polysaccharide of either serotype & or 2.
This has been called a microcapsule because it can be
visuali8ed only by electron microscopy after antibody labeling$
unlie the copious capsules of other bacteria which are
visuali8ed by light microscopy.
Capsular &olysaccharide
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S. aureus isolated from infections e,presses high levels of
polysaccharide but rapidly loses it upon laboratory subculture.
The function of the capsule is not clear.
It may impede phagocytosis$ but in in vitro tests this was only
demonstrated in the absence of complement.
Conversely$ comparing wild-type and a capsule defective mutant
strain in an endocarditis model suggested that polysaccharide
e,pression actually impeded coloni8ation of damaged heart
valves$ perhaps by masing adhesins.
&rotein '
&rotein ' is a surface protein of S. aureus which binds
immunoglobulin + molecules by the c region.
/n serum$ bacteria will bind /g% molecules the wrong way
round by this non-immune mechanism.
In principle this #ill disrupt opsoni-ation and
phagocytosis.
/ndeed mutants of S. aureus lacing protein 0 are more
e4ciently phagocyto8ed in vitro$ and studies with mutants in
infection models suggest that protein 0 enhances virulence.
eukocidin
S. aureus can e,press a toin that specically acts on
polymorphonuclear leuocytes.
1hagocytosis is an important defense against staphylococcal
infection so leukocidin should be a virulence factor.
*amage to the (ost
S. aureus can e,press several di:erent types of protein
toins which are probably responsible for symptoms during
infections.
Some damage the membranes o! erythrocytes, causing
hemolysis; but it is unliely that hemolysis is relevant in vivo.
The leukocidin causes membrane damage to leukocytes
and is not hemolytic.
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Systemic release of alpha"toin causes septic shock,
while enterotoins and TSST"/ (to,ic scho- syndrome-
to,ine) cause toxic shock.
embrane *amaging Toins
0a1 alpha"toin
The best characteri8ed and most potent membrane-damaging
to,in of S. aureus is alpha-to,in.
Susceptible cells have a specic receptor for alpha-to,in which
allows low concentrations of to,in to bind$ causing small pores
through which monovalent cations can pass.
0t higher concentrations$ the to,in reacts non-specically with
membrane lipids$ causing larger pores through which divalent
cations and small molecules can pass.
owever$ it is doubtful if this is relevant under normal
physiological conditions.
'2 a"toin
/n humans$ platelets and monocytes are particularly sensitiveto alpha-to,in.
They carry high a4nity sites which allow to,in to bind at
concentrations that are physiologically relevant.
0 comple, series of secondary reactions ensue$ causing
release of cytoines which trigger production of in*ammatory
mediators.
These events cause the symptoms of septic shoc that occurduring severe infections caused by S aureus.
32 4"toin
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The delta-to,in is a very small peptide to,in produced by most
strains of S. aureus.
/t is also produced by S. epidermidis and S. lugdunensis.
The role of delta-to,in in disease is unnown (a:ects somecells integrity).
*2 gamma"toin and leukocidin
The gamma-to,in and the leuocidins are two-component
protein to,ins that
damage membranes of susceptible cells.
Classical &anton and alentine 0&1
leukocidin
The classical 1anton and 5alentine (15) leuocidin is distinct
from the leuoto,in e,pressed by the gamma-to,in locus.
/t has potent leukotoicity and$ in contrast to gamma-to,in$
is non-hemolytic.
=nly a small fraction of S. aureus isolates (>+ in one survey)
e,press the 15 leuocidin$ whereas ?"+ of those isolated from
severe dermonecrotic lesions e,press this to,in.
This suggests that & leukocidin is an important factor in
necroti-ing skin in!ections.
&"leukocidin causes dermonecrosis when in7ected
subcutaneously in rabbits.
/n small amount$ the to,in releases in*ammatory mediators
from human neutrophils$ leading to degranulation.
This could account for the histology of dermonecrotic
infections (vasodilation$ inltration and central necrosis).
Superantigens2 enterotoins and toic
shock syndrome toin
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S. aureus can e,press two di:erent types of to,in with
superantigen activity$ enterotoins$ of which there are si,
serotypes (0$ B$ C$ @$ # and %) and toic shock syndrome
toin (TSST-').
6nterotoins cause diarrhea and vomiting when ingested
and are responsible for staphylococcal !ood poisoning.
Ahen e,pressed systemically$ enteroto,ins can cause toic
shock syndrome (TSS) - indeed enteroto,ins B and C cause
&"+ of non-menstrual TSS.
TSST"/
TSST-' is very wealy related to enteroto,ins and does nothave emetic activity.
TSST-' is responsible for &+ of TSS$ including all menstrual
cases.
TSS can occur as a seuel to any staphylo-coccal infection if
an enteroto,in or TSST-' is released systemically and the host
lacs appropriate neutrali8ing antibodies.
)isk !actors
history of using super-absorbent tampons (described in early
'?2")
surgical wounds
a local infection in the sin or deep tissue
history of using the diaphragm or contraceptive sponge
history of childbirth or abortion
TSS
Tampon-associated TSS is not a true infection$ being caused
by growth of S. aureus in a tampon and absorption of the to,in
into the blood stream.
TSS came to prominence with the introduction of super-
absorbent tampons; and although the number of such cases
has decreased dramatically$ they still occur despitewithdrawal of certain types of tampons from the maret.
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Superantigens
Superantigens stimulate T cells non-specically without
normal antigenic recognition (ne,t gure).
Dp to one in ve T cells may be activated$ whereas only ' in
'"$""" are stimulated during antigen presentation.
Cytoines are released in large amounts$ causing the
symptoms of TSS. Superantigens bind directly to class // ma7or
histocompatibility comple,es (6C) of antigen-presenting
cells outside the conventional antigen-binding grove.
Superantigens and the non-specic stimulation of T
cells
Symptoms o! TSS in$ol$e many systems and may
resemble other in!ections. Ahile each person may
e,perience symptoms di:erently$ the following are the most
common symptoms of Staphylococcal TSS$ according to the
Centers for @isease Control and 1revention (C@C)
!e$er higher than !2.?EC or '">."E9
chills$ malaise
headache$ fatigue
rash (red and *at that covers most of the areas of the body)
shedding of the sin in large sheets especially over the palms
and soles (This is seen one to two wees after the onset of
symptoms.)
lo# blood pressure
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vomiting
diarrhea
muscle pain
increased blood *ow to mouth$ eyes$ and vagina maing them
appear red
decreased urine output and sediment in urine
decreased liver function
bruising due to low blood platelet count
disorientation and confusion
6pidermolytic 0e!oliati$e1 toin 06T1
This to,in causes the scalded skin syndrome in neonates$
with widespread blistering and loss of the epidermis.
There are two antigenically distinct forms of the to,in$ #T0 and
#TB.
There is evidence that these to,ins have protease activity.
ther 6tracellular &roteins Coagulase
Coagulase is not an en8yme.
/t is an extracellular protein which binds to prothrombin in thehost to form a comple, called staphylothrombin.
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The protease activity characteristic of thrombin is activated in
the comple,$ resulting in the conversion of brinogen to brin.
This is the basis of the tube coagulase test$ in which a clot is
formed in plasma after incubation with the S. aureus broth-
culture supernatant.
Coagulase
Coagulase is a traditional marer for identifying S. aureus in
the clinical microbiology laboratory.
owever$ there is no evidence that it is a virulence
factor .
Staphylokinase
6any strains of S. aureus e,press a plasminogen acti$ator
called staphylokinase.
The genetic determinant is associated with lysogenic
bacteriophages.
0 comple, formed between staphyloinase and plasminogen
activates plasmin-lie proteolytic activity which causes
dissolution o! fbrin clots.
The mechanism is identical to streptoinase$ which is used in
medicine to treat patients su:ering from coronary thrombosis.
0s with coagulase there is no evidence that staphyloi-nase is
a virulence factor$ although it seems reasonable to imagine
that locali8ed brinolysis might aid in bacterial spreading.
6n-ymes S. aureus can e,press proteases, a lipase, a
deoyribonuclease 0*8ase1 and a !atty acid modi!ying
en-yme (906#).
The rst three probably pro$ide nutrients for the bacteria$
and it is unliely that they have anything but a minor role in
pathogenesis.
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owever$ the '6 en-yme may be important in
abscesses$ where it could modify anti-bacterial lipids and
prolong bacterial survival.
The thermostable *8ase is an important diagnostic test
!or identifcation of S. aureus.
Coagulase 8egati$e Staphylococci
Staphylococci other than S. aureus can cause infections in
man.
S epidermidis is the most important coagulase-negative
staphylococcus (CNS) species and is the ma7or cause of
in!ections associated #ith prosthetic de$ices andcatheters.
CNS also cause peritonitis in patients receiving continuous
ambulatory peritoneal dialysis and subacute endocarditis in
those with prosthetic valves.
These infections are not usually nosocomially acuired.
=ther species such as S. haemolyticus, S. warneri, S. hominis,
S. capitis, S. intermedius, S. schleiferi and S. simulans are
infreuent pathogens.
S. lugdunesis is a newly recogni8ed species.
/t is probably more pathogenic than are other CNS species$
with cases of subacute endocar"ditis and other infections
being reported.
/t is liely that the incidence of infections caused by these
organisms is underestimated because of di4culties in
identication.
@iagnosis of CNS infections is di4cult.
/nfections are often indolent and chronic with few obvious
symptoms.
This is due to the smaller array of virulence factors and to,ins
compared to those in the case of S. aureus.
S. epidermidis is a skin commensal and is one of the mostcommon contaminants of samples sent to the diagnostic
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S. aureusPenicillin[1950s]Penicillin-resistant
S. aureus
Methicillin[1970s]
Methicillin-resistant
S.
aureus(MRSA
)
Vancomycin-resistantenterococci (VRE)
Vancomycin
[1990s][1997]
Vancomycinintermeiate-resistant S. aureus
(V!SA)
Vancom
ycin-
resistant S.aur eus
"e# Resistant $acteria
M%tations
S%sce&ti'le $acteriaResistant Bacteria
Resistance Gene Transfer
/n addition$ S. aureus e,presses resistance to antiseptics and
disinfectants$ such as uaternary ammonium compounds$
which may aid its survival in the hospital environment.
#volution of @rug FesistanS. Aureus
#mergence of 0ntimicrobial Fesistance
Selection for 0ntimicrobial-Fesistant Strains
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Resistant Strains
Rare
Resistant StrainsDominant
Antimicro'ial Exposure
)esistance o! Staphylococci to 'ntimicrobial
*rugs
Since the beginning of the antibiotic era S. aureus has
responded to the introduction of new drugs by rapidly
acuiring resistance by a variety of genetic mechanisms
including
(') acuisition of etrachromosomal plasmids or
additional genetic information in the chromosome via
transposons or other types of @N0 insertion and
(>) by mutations in chromosomal genes
6any plasmid-encoded determinants have recently become
inserted into the chromosome at a site associated with the
methicillin resistance determinant.
There may be an advantage to the organism having resistance
determinants in the chromosome because they will be more
stable.
There are essentially four mechanisms of resistance toantibiotics in bacteria
(') en8ymatic inactivation of the drug$
(>) alterations to the drug target to prevent binding$
(!) accelerated drug eGu, to prevent to,ic concentrations
accumulating in the cell$ and
(H) a by-pass mechanism whereby an alternative drug-
resistant version of the target is e,pressed
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