Q1 LONG ANSWER 10M ★ High-yield

Classify staphylococci. Describe the virulence factors, toxin-mediated diseases and laboratory diagnosis of Staphylococcus aureus.

Microbiology · 2nd Professional — Para-Clinical

Morphology and Classification

Gram-positive cocci arranged in irregular clusters resembling a bunch of grapes, because division occurs in several planes; non-motile, non-sporing, facultative anaerobes
Catalase positive — the single test that separates staphylococci from streptococci, which are catalase negative
Grow readily on ordinary media; produce a golden-yellow pigment on nutrient agar and beta-haemolysis on blood agar; tolerate high salt, which is exploited in mannitol salt agar

Species	Coagulase	Distinguishing features and clinical role
S. Aureus	Positive	Golden pigment; ferments mannitol; DNase and phosphatase positive; the major pathogen of the genus. Nasal carriage in the anterior nares in about 30% of healthy people, which is the reservoir for both endogenous infection and cross-infection
S. Epidermidis	Negative	White colonies; a skin commensal, and therefore the commonest contaminant of blood cultures. Its ability to form a biofilm makes it the leading cause of prosthetic and intravascular line infection, in which the device usually has to be removed
S. Saprophyticus	Negative	Novobiocin resistant (S. Epidermidis is sensitive); causes urinary tract infection in sexually active young women
S. Lugdunensis	Negative	Behaves aggressively like S. Aureus, causing destructive endocarditis

Virulence Factors

Factor	Action
Cell wall — protein A	Binds the Fc portion of IgG the wrong way round, so the antibody cannot engage phagocyte Fc receptors — antiphagocytic and antiopsonic. Exploited in the laboratory for co-agglutination tests
Capsule and slime (biofilm)	Antiphagocytic; the biofilm protects organisms on catheters, valves and prostheses from both antibiotics and immunity
Coagulase	Converts fibrinogen to fibrin, walling off the organism in a fibrin barrier — which is why staphylococcal infection tends to localise as an abscess, whereas streptococcal infection spreads
Catalase	Destroys hydrogen peroxide, allowing survival inside the phagocyte
Leucocidin (PANTON-valentine)	Kills leucocytes; associated with severe necrotising skin infection and necrotising pneumonia, particularly in community-acquired MRSA
Haemolysins (alpha, beta, gamma, delta)	Alpha toxin is a pore-forming cytolysin damaging many cell types
Spreading enzymes	Hyaluronidase, staphylokinase (fibrinolysin), lipase, DNase, proteases

Toxin-mediated Disease

Toxin	Disease	Key features
Enterotoxins (A–E)	Staphylococcal food poisoning	Heat-stable (survives boiling for 30 minutes) and preformed in the food, so the incubation period is very short — 2 to 6 hours — with violent vomiting predominating over diarrhoea, and recovery within a day. It is an intoxication, not an infection, so antibiotics are useless
TSST-1	Toxic shock syndrome	A superantigen that binds MHC class II and the T-cell receptor outside the groove, activating up to 20% of T cells and producing a cytokine storm. Fever, hypotension, a diffuse macular rash with later desquamation of palms and soles, and multi-organ involvement. Classically associated with high-absorbency tampons, and also with nasal packing and surgical wounds
Exfoliative (epidermolytic) toxin	Staphylococcal scalded skin syndrome (Ritter disease)	Splits the epidermis at the granular layer; widespread erythema and blistering with a positive Nikolsky sign, mainly in neonates and young children. The organism is at a distant focus, not in the skin lesions

CLINICAL PEARL

Coagulase explains the whole clinical personality of Staphylococcus aureus. By converting fibrinogen to fibrin it builds a wall around itself, so staphylococcal infection localises into an abscess that antibiotics penetrate poorly. Streptococci instead produce streptokinase, which dissolves fibrin and lets them spread as cellulitis. One enzyme, and the difference between a boil and erysipelas — and between an infection that needs drainage and one that does not.

Diseases and Laboratory Diagnosis

Skin and soft tissue — folliculitis, furuncle (boil), carbuncle, impetigo, cellulitis, wound and burn infection, breast abscess, styes
Deep and systemic — osteomyelitis (the commonest cause), septic arthritis, pneumonia (often following influenza, and necrotising with Panton-Valentine leucocidin), empyema, acute endocarditis (right-sided tricuspid disease in intravenous drug users), bacteraemia and metastatic abscesses, and device-related infection
Toxin-mediated — food poisoning, toxic shock syndrome, scalded skin syndrome
Laboratory diagnosis — Gram stain of pus showing gram-positive cocci in clusters with pus cells; culture on blood agar and mannitol salt agar; catalase to separate from streptococci, then coagulase (slide test detects bound clumping factor and is a screening test; the tube test detects free coagulase and is confirmatory); DNase and mannitol fermentation; and antibiotic susceptibility including a cefoxitin disc for MRSA
In food poisoning the organism or toxin is sought in the food rather than the patient, since the illness is an intoxication

Differentiation Within the Genus

Test	S. Aureus	S. Epidermidis	S. Saprophyticus
Coagulase	Positive	Negative	Negative
Pigment	Golden	White	White
Mannitol fermentation	Positive	Negative	Variable
DNase and phosphatase	Positive	Negative	Negative
Novobiocin	Sensitive	Sensitive	Resistant — the defining test
Haemolysis	Beta	None	None
Chief clinical role	Abscess, osteomyelitis, endocarditis, toxin disease	Prosthetic and line infection; commonest blood culture contaminant	Urinary infection in young women

A coagulase-negative staphylococcus in one blood culture bottle is usually contamination; growth in several bottles from separate sites, or in a patient with a prosthesis or line, is far more likely to be real

Treatment and Applied Aspects

Drainage is the treatment for an abscess; antibiotics penetrate pus poorly and source control outranks drug choice
Antibiotic choice — cloxacillin or flucloxacillin for methicillin-sensitive strains (plain penicillin is useless, since over 90% produce penicillinase); vancomycin, linezolid, daptomycin, clindamycin or co-trimoxazole for MRSA
Nasal carriage is the reservoir; decolonisation with mupirocin to the anterior nares and chlorhexidine washes reduces surgical site infection in carriers
Staphylococcal bacteraemia is never trivial — it demands a search for endocarditis and metastatic foci, removal of any line, repeat blood cultures, and a prolonged course; treating it as a simple bacteraemia leads to relapse
Food poisoning is prevented by food handling — excluding staff with skin lesions, refrigeration, and avoiding prolonged holding of cooked food at room temperature, which is a common practice at Indian social gatherings
Hand hygiene is what limits spread in hospital; staphylococci survive well on skin, clothing and surfaces, and the hands of staff are the principal vehicle
Recurrent boils suggest carriage or an underlying problem — check for diabetes, and consider decolonisation of the patient and close contacts, since the reservoir is usually the patient's own nose
Suspect toxic shock in an unexplained febrile illness with rash and hypotension, and look for a retained tampon, nasal packing or an occult wound; the focus may be trivial while the illness is life-threatening
Separate scalded skin syndrome from toxic epidermal necrolysis; the split is at the granular layer in the first and at the dermo-epidermal junction in the second, which biopsy resolves and which changes both treatment and prognosis
Never dismiss S. Aureus in urine; unlike coliforms it rarely ascends, so its presence often indicates haematogenous seeding from an unsuspected bacteraemia
Osteomyelitis and septic arthritis need prolonged treatment and often surgical washout; short courses guided by clinical improvement alone lead to chronic infection and sequestrum formation
Echocardiography is indicated in staphylococcal bacteraemia, and a transoesophageal study where the transthoracic view is inadequate or suspicion remains high
Food poisoning needs no antibiotic — the toxin is already formed and the organism may not even be present; treatment is rehydration and the investigation is of the food
Exclude food handlers with boils or septic lesions; a staphylococcal skin lesion on the hands of someone preparing food is the classical route to an outbreak
Coagulase-negative staphylococci are not harmless in a patient with a prosthetic valve, joint or shunt, where they are the leading pathogen and the device usually has to be removed

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