Staphylococcus

  • staphylococci are gram-positive cocci
  • Grape-like clusters
  • Ubiquitous
  • More than 45 species are known
  • Staphylococcus aureus is the most important human pathogen, commonly causing localised suprative lesions in humans.
  • Coagulase-positive staphylococcus:

Staphylococcus aureus

S.intermedius

S.hyicus

  • Coagulase-negative staphylococci

S.epidermis

S.lugdenensis

S.haemolyticus

S.hominis

S.saprophyticus.

 

 

  • Staphylococcus aureus

Morphology:

  • Spherical cocci
  • Approximately 1um in diameter
  • Arranged characteristically in grape-like clusters.
  • Cluster formation is due to cell division occurring in three planes ,with daughter cells tending to remain in close proximity.
  • Non-motile and non-sporing.
  • Few strains possess microscopically visible capsules.
  • Stain readily with aniline dyes.
  • Uniformly gram-positive.

 

Resistance:

  • Staphylococci are among the more resistant non-sporing bacteria.
  • Uniformly resistant to lysozymes
  • Sensitive to lysostaphin-a mixture of enzymes produced by a particular strain of S.epidermidis.

Types of resistance

  • Beta-lactamase-mediated:
  1. This is mediated through the production of B-lactamase or pencillinase, which inactivates pencillin by splitting the beta lactum ring.
  2. These are inducible enzymes which are plasmid mediated.
  3. Transmitted by transduction or conjugation.
  4. Pencillinase producing strains remain sensitive to pencillinase-resistant pencillins such as methicillin and cloxacillin.

 

Methicillin-resistant staphylococcus aureus (MRSA)

  • Alterations in the pencillin binding site PBP2a and changes in bacterial surface receptors reduces binding affinity for beta-lactum antibiotics to cells
  • This mechanism imparts resistance to beta lactam antibiotics and has been named MRSA.
  • This showed resistance for pencillinase resistant pencillins like methicillin and oxacillin.
  • This mechanism is regulated by a set of chromosomal genes called staphylococcal cassette chromosomal MEC genes (SCC MEC).
  • Based on the type of these genes the MRSA strains are divided as

Hospital-acquired MRSA

Community-acquired MRSA

 

Vancomycin resistance (VRSA)

  • The resistance have started to emerge in some parts of world
  • They respond to linezolid.

 

Vancomycin intermediate resistance (VISA):

  • These donot carry any resistance genes,but have a thickened cellwall.
  • These have been isolated from patients who are on prolonged vancomycin treatment.

Pathogenecity and virulence

  • Staphylococci produces two types of diseases.
  • Infectious
  • In infectious ,the cocci gain access to damaged skin , mucosal or tissue sites,colonize by adhering to cells,evade host multiply and cause tissue damage.
  • In intoxications,the disease is caused by the bacterial toxins produced.

Virulence factors:

 

Cell-associated factors

  • Cell-associated polymers
  1. Polysaccharide peptidoglycon: activates the complement and induces release of inflammatory cytokinesis.
  2. Teichoic acid: an antigenic component of the cellwall, facilities adhesion of the cocci to the host cell surface and protects them from complement-mediated opsonisation.
  3. Capsular polysaccharide:inhibits opsonisation.
  • Cell surface proteins
  1. Protein -A: Chemotactic,antiphagocytic,anti-complimentary effects. Also induces platelet damage and hypersensitivity.
  2. Clumping factor :bound coagulase ,which is responsible for the slide coagulase test.
  3. Staphylococci possess protein receptors which facilitates staphylococci adhesion to host cells and tissues.

 

Extracellular enzymes:

  • Coagulase:
  1. This is an enzyme which brings clotting of human or rabbit plasma.
  2. Binds to prothrombin and converts fibrinogen to fibrin.
  • Lipid hydrolases:
  • These help to infect the skin and subcutaneous tissues.

 

  • Hyaluronidase:
  1. Breaks down the connective tissue.
  2. Fibrinolysin and proteases helps in the initiation and spread of infection.
  • Nuclease:
  • A heat stable DNase helps in the identification of organism.

 

Toxins:

  • Cytolytic toxins are membrane-active substances,consisting of four hemolysins and a leucocidin.
  • Alpha hemolysin:
    1. It is the most important among the four
    2. It is a protein inactivated at 70°C,but reactivated paradoxically at100°C
    3. This is because at 60-70°C the toxin combines with a heat liable inhibitor which is denatured at 100°C, leaving the toxin free.
    4. It lysed rabbit erythrocytes but less active against sheep and human red cells.
    5. It is also leucocidal, cytotoxic,dermonecrotic, neurotoxic.
    6. It is toxic to macrophages,lysosomes,muscle tissues and circulatory system.
  • Beta hemolysin:
  1. It is a sphingomyelinase, hemolytic for sheep,but not for human or rabbit cells
  2. Exhibits a hot-cold phenomenon,the hemolysis being initiated at 37°C,but becoming evident only after chilling.
  • Gamma hemolysin: composed of two separate proteins
  • Delta hemolysin:has a detergent like effect on the cell membranes of erythrocytes, leucocytes, macrophages and platelets.

 

Panton-valetine leucocidal(PVL):

  • It is also a two component toxin,like gamma lysin.
  • Such bi-component membrane active toxins have been grouped as synergohymenotropic toxins.

 

Enterotoxin:

  • This is responsible for the manifestations of staphylococcal food poisoning like
  1. Nausea
  2. Vomiting
  3. Diarrhea 2-6 hrs after consuming food contaminated by the toxin.
  • The toxin is heat stable resisting 100°C for 10-40min depending on the concentration of the toxin and nature of the medium.
  • Eight antigenic types of enterotoxin are known. A,B,C(1-3),D,E and H.
  • The toxin is believed to act on the autonomic nervous system , rather than gastrointestinal mucosa.
  • The toxin also exhibits pyrogenic, mitogenic, hypotensive,thrombocytopenic and cytotoxic effects.

 

Toxic shock syndrome toxin(TSST)

  • It is potentially fatal multisystem disease presenting with fever, hypotension, myalgia, vomiting, diarrhea,mucosal hyperemia and an erythematous rash which desquamates subsequently.

 

Exfoliative (epidermolytic) toxin:

Responsible for staphylococcal scalded skin syndrome (SSSS),an exfoliative skin disease in which outer layer of the epidermis gets separated from the underlying tissues.

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