- staphylococci are gram-positive cocci
- Grape-like clusters
- More than 45 species are known
- Staphylococcus aureus is the most important human pathogen, commonly causing localised suprative lesions in humans.
- Coagulase-positive staphylococcus:
- Coagulase-negative staphylococci
- Staphylococcus aureus
- 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.
- 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
- This is mediated through the production of B-lactamase or pencillinase, which inactivates pencillin by splitting the beta lactum ring.
- These are inducible enzymes which are plasmid mediated.
- Transmitted by transduction or conjugation.
- 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
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.
- 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.
- Cell-associated polymers
- Polysaccharide peptidoglycon: activates the complement and induces release of inflammatory cytokinesis.
- 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.
- Capsular polysaccharide:inhibits opsonisation.
- Cell surface proteins
- Protein -A: Chemotactic,antiphagocytic,anti-complimentary effects. Also induces platelet damage and hypersensitivity.
- Clumping factor :bound coagulase ,which is responsible for the slide coagulase test.
- Staphylococci possess protein receptors which facilitates staphylococci adhesion to host cells and tissues.
- This is an enzyme which brings clotting of human or rabbit plasma.
- Binds to prothrombin and converts fibrinogen to fibrin.
- Lipid hydrolases:
- These help to infect the skin and subcutaneous tissues.
- Breaks down the connective tissue.
- Fibrinolysin and proteases helps in the initiation and spread of infection.
- A heat stable DNase helps in the identification of organism.
- Cytolytic toxins are membrane-active substances,consisting of four hemolysins and a leucocidin.
- Alpha hemolysin:
- It is the most important among the four
- It is a protein inactivated at 70°C,but reactivated paradoxically at100°C
- 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.
- It lysed rabbit erythrocytes but less active against sheep and human red cells.
- It is also leucocidal, cytotoxic,dermonecrotic, neurotoxic.
- It is toxic to macrophages,lysosomes,muscle tissues and circulatory system.
- Beta hemolysin:
- It is a sphingomyelinase, hemolytic for sheep,but not for human or rabbit cells
- 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.
- It is also a two component toxin,like gamma lysin.
- Such bi-component membrane active toxins have been grouped as synergohymenotropic toxins.
- This is responsible for the manifestations of staphylococcal food poisoning like
- 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.