Staphylococcus Genus

Embed Size (px)

Citation preview

  • 8/18/2019 Staphylococcus Genus

    1/19

    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.

  • 8/18/2019 Staphylococcus Genus

    2/19

     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

  • 8/18/2019 Staphylococcus Genus

    3/19

    /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

  • 8/18/2019 Staphylococcus Genus

    4/19

    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

  • 8/18/2019 Staphylococcus Genus

    5/19

    &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.

  • 8/18/2019 Staphylococcus Genus

    6/19

    /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

  • 8/18/2019 Staphylococcus Genus

    7/19

    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.

  • 8/18/2019 Staphylococcus Genus

    8/19

    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

  • 8/18/2019 Staphylococcus Genus

    9/19

     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

  • 8/18/2019 Staphylococcus Genus

    10/19

    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.

  • 8/18/2019 Staphylococcus Genus

    11/19

    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

  • 8/18/2019 Staphylococcus Genus

    12/19

    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.

  • 8/18/2019 Staphylococcus Genus

    13/19

     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.

  • 8/18/2019 Staphylococcus Genus

    14/19

    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

  • 8/18/2019 Staphylococcus Genus

    15/19

  • 8/18/2019 Staphylococcus Genus

    16/19

     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

  • 8/18/2019 Staphylococcus Genus

    17/19

    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

  • 8/18/2019 Staphylococcus Genus

    18/19

  • 8/18/2019 Staphylococcus Genus

    19/19