Overview of Staphylococcus aureus
Staphylococcus aureus is gram-positive cocci and the In the Greek word, ‘Staphyle’ means a bunch of grapes, and ‘kokkos’ means berry.
It is spherical cocci, around 1 μm in diameter, & arranged in grape-like clusters so-called staphylococcus. It is a facultative anaerobe, non-motile, non-sporing, and occasionally capsulated organism. Catalase and coagulase test are positive in Staphylococcus aureus.
Virulence factors of Staphylococcus aureus
Cell wall-associated factors:
It consists of a thick peptidoglycan layer, teichoic acid, protein-A, and cell surface adhesins (e.g. clumping factor).
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- Peptidoglycan: It confers rigidity to the cell wall and maintains the shape. It induces an inflammatory response and also has endotoxin-like activity.
- Teichoic acid: It helps in the adhesion of cocci to mucosal surfaces and inhibits opsonization.
- Protein A: Anti-complementary, chemotactic, mitogenic, inhibition of opsonization and induction of platelet damage. It mediates co-agglutination reaction.
- Cell surface adhesins: Clumping factor/bound coagulase, Fibronectin binding Adhesin, and Collagen-binding Adhesin.
Toxins
Membrane active toxin is
- Hemolysins: α, β, γ, and δ hemolysins. Membrane damaging toxins act on the RBCs and show their lethal, dermonecrotic, and leukocidal activity.
- Leukocidins/Panton-Valentine toxin: It has two components F (Fast) and S (slow) based on their migration to the carboxymethyl cellulose column. It shows synergistically with γ-hemolysin to damage leukocytes, RBCs, and macrophages.
Epidermolytic/Exfoliative Toxin (ET)
ET is responsible for staphylococcal scalded skin syndrome (SSSS). There two proteins-ET-A (chromosomal, heat-stable) and ET-B (plasmid coded, heat-labile).
It more commonly occurs in newborns and children. There are various symptoms like localized tender blisters and bullae, etc.
Enterotoxin
The enterotoxin is responsible for staphylococcal food poisoning and some cases of pseudomembranous colitis following the use of broad-spectrum antibiotics.
The toxin stimulates the vagus nerve and the vomiting center also stimulates the intestinal peristaltic activity. The symptoms are nausea, vomiting, occasionally diarrhea, hypotension, and dehydration.
Enterotoxins have many serotypes (A- E, G- I, R-T, and V). Type A is causing food poisoning and serotypes F causing toxic shock syndrome.
The detection of enterotoxin in food by ELISA or latex agglutination test or enterotoxin gene detected by multiplex PCR. The treatment for the enterotoxin-causing disorder is correcting fluid and electrolyte imbalance.
Toxic shock syndrome toxin (TSST)
TSST is responsible for toxic shock syndrome. There are two subtypes such as TSST-1 and TSST-2. TSST-1 is a staphylococcal enterotoxin.
Enterotoxin F or pyrogenic exotoxin C is the most common form or type of TSST-1. Enterotoxin-B or C may also be associated with TSST-1.
TSST-1 is a superantigen it stimulates the T-cells non-specifically (by binding to the Vβ region of the T-cell receptor) causing excessive production of cytokines which leads to fatal multisystem disease.
There are various clinical features are present such as fever, hypotension, mucosal hyperemia, vomiting, diarrhea, confusion, myalgia, etc.
TSST is more severe if anti-TSST antibodies fail to appear. The diagnosis of toxic shock syndrome by Latex agglutination test and enzyme immunoassay.
Enzymes
- Coagulase
- Heat stable thermonucleases
- Dnase
- Staphylokinase (fibrinolysin) breaks down fibrin clots and may facilitate the spread of infection.
- Hyaluronidase breaks down the connective tissues.
- Lipases and phospholipases break down the lipids.
Pathogenesis of Staphylococcus aureus
- Colonization occurs on body surfaces (anterior nares, axilla, and perineal skin).
- Introduction into the tissue as a result of minor abrasions or instrumentation.
- Invasion into the tissues by elaborating enzymes such as serine proteases, thermonucleases, hyaluronidases, and lipases.
- Evasion of defense mechanisms by antiphagocytic activity, inhibition of leukocyte migration, and intracellular survival inside the endothelial cells.
- Metastatic spread by hematogenous spread.
Laboratory diagnosis of Staphylococcus aureus
Sample collection
Depends on the nature of the lesion, they are;
| Infection | Specimen |
| Suppurative lessons | Pus and wound swab |
| Respiratory infections | Sputum |
| Pyrexia of unknown origin (PUO), bacteremia | Blood |
| UTI | Mid stream urine |
| Food poisoning | Feces, vomitus, and food |
| Carriers | Nasal and perianal swab |
Microscopy: Gram stain: gram-positive cocci in clusters with pus cells.
Culture characteristics
Incubated at 37◦C aerobically.
- Nutrient agar: 1-3 mm in size, circular, smooth, convex, opaque, and easily emulsifiable golden yellow pigments colonies.
- Blood agar: similar characteristics of colonies of Nutrient agar and with a narrow zone of beta hemolysis.
- MacConkey agar: small pink colonies due to lactose fermentation.
- Liquid medium: peptone water.
- Selective media: mannitol salt agar, salt milk agar, and Ludlam’s medium.
Biochemical test
- Catalase test and coagulase test: positive
- DNase Test: positive
- Phosphatase test: positive
- Heat stable Thermo nuclease test-positive
- Mannitol sugar is fermented
- Black colored colonies on potassium tellurite agar
- Gelatin liquefaction-positive
- Protein A detection
Typing methods
- Phenotypic methods: bacteriophage typing and antibiogram typing.
- Genotypic methods such as PCR-RFLP (Restricted fragment length polymorphism).
Antimicrobial susceptibility test
Done on Mueller Hinton agar.
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