Staphylococcal Infections

Organism

Gram+ cocci, grape-like clusters. Catalase+ (vs Strep). Facultative anaerobe. Hardy — survives drying/heat/high salt (7.5% NaCl = mannitol salt agar basis).

Split: Coagulase+ (S. aureus) vs CoNS (S. epidermidis, S. saprophyticus — less virulent, opportunists, prosthetic devices).

S. aureus Virulence Factors

Surface: Protein A (binds IgG Fc, blocks opsonization), Capsule (antiphagocytic), Coagulase (clots plasma, protective fibrin barrier).

Cytolytic toxins: Alpha-toxin (pore-forming), PVL (Panton-Valentine leukocidin — destroys leukocytes, severe skin/soft tissue infection + necrotizing pneumonia, MARKER of community-MRSA).

Superantigen toxins: TSST-1 (toxic shock syndrome, massive nonspecific T-cell activation), Enterotoxins (heat-stable, rapid food poisoning, superantigen mechanism).

Exfoliative toxins (A, B): cleave desmoglein-1 → SSSS (staphylococcal scalded skin syndrome, young children).

Enzymes: hyaluronidase, staphylokinase/fibrinolysin (spread, OPPOSITE of coagulase), lipases (sebum survival).

Clinical Manifestations

Localized skin/soft tissue (COMMONEST): folliculitis, furuncle/boil, carbuncle, impetigo (± S. pyogenes mixed), wound infection.

Toxin-mediated:

Food poisoning: rapid (2-6hr), VOMITING > diarrhea, preformed heat-stable enterotoxin (improperly stored food)
SSSS: widespread epidermal blistering/desquamation, SPARES mucous membranes (distinguishes from SJS/TEN)
TSS: fever+hypotension+diffuse macular rash+desquamation+multi-organ. Classic: tampon use. Also: any colonized wound/surgical site.

Invasive/deep: bacteremia, IE (LEADING overall cause, acute course — see IE topic), osteomyelitis/septic arthritis (COMMONEST organism — see Osteomyelitis topic), pneumonia (necrotizing, post-influenza), abscess (any site via seeding).

MRSA

mecA gene (SCCmec mobile element) → altered PBP2a → ↓affinity for ALL β-lactams (not just methicillin).

HA-MRSA: multidrug-resistant beyond β-lactams, device/bloodstream infection, hospitalized patients. CA-MRSA: often narrower resistance, frequently PVL+, aggressive skin/soft tissue infection ± severe necrotizing pneumonia, healthy individuals outside healthcare.

Laboratory Diagnosis

Microscopy: Gram+ cocci clusters. Culture: golden-yellow β-hemolytic colonies (blood agar, not universal). Mannitol salt agar (selective+differential — high salt + mannitol fermentation → yellow). Biochem: Catalase+ (vs Strep). Coagulase+ (vs CoNS) — slide test (rapid, bound coagulase/clumping factor) or tube test (definitive, free coagulase, up to 24hr, needed if slide negative/equivocal). DNase+ = further confirmation. MALDI-TOF/VITEK = standard definitive ID.

MRSA screening: Cefoxitin disk (reliable surrogate, > direct oxacillin testing). PCR mecA (or mecC) = rapid definitive.

Treatment

MSSA: anti-staph penicillins (cloxacillin, nafcillin) or 1st gen cephalosporin. MRSA: Vancomycin = mainstay serious infection. Alternatives: linezolid, daptomycin, clindamycin (if susceptible, D-test for inducible resistance). Abscess DRAINAGE = essential adjunct (antibiotics alone insufficient, poor pus penetration).

The Organism

Staphylococcus species are Gram-positive cocci arranged in irregular grape-like clusters (from cell division in multiple planes), catalase-positive (distinguishing the genus from streptococci at the first branch point of routine identification), facultatively anaerobic, and notably hardy — tolerating drying, heat, and high salt concentration (7.5% NaCl, the basis of mannitol salt agar’s selectivity) far better than most Gram-positive pathogens, which is exactly why staphylococci survive so well on skin, fomites, and hospital surfaces.

The genus splits clinically along the coagulase test: coagulase-positive staphylococci (essentially S. aureus) and coagulase-negative staphylococci (CoNS) — S. epidermidis, S. saprophyticus, and others, generally far less virulent but increasingly important as opportunists, particularly around prosthetic devices.

Staphylococcus aureus

Virulence Factors

S. aureus owes its pathogenicity to a large armamentarium of surface and secreted factors:

Surface components: protein A (binds the Fc portion of IgG, blocking opsonization and phagocytosis — a genuinely elegant immune-evasion trick), capsule (further antiphagocytic), and coagulase (clots plasma, forming a protective fibrin barrier around the organism — the basis of the coagulase test itself).
Cytolytic/membrane-damaging toxins: alpha-toxin (pore-forming, damages a wide range of host cells), Panton-Valentine leukocidin (PVL, destroys leukocytes, strongly associated with severe skin/soft-tissue infection and necrotizing pneumonia, and a marker of community-associated MRSA strains specifically).
Superantigen toxins: toxic shock syndrome toxin-1 (TSST-1, causing toxic shock syndrome by massive nonspecific T-cell activation and cytokine storm) and staphylococcal enterotoxins (heat-stable, causing rapid-onset food poisoning via the same superantigen mechanism, and also implicated in a subset of toxic shock cases).
Exfoliative toxins (A and B): cleave desmoglein-1 in the epidermis, causing staphylococcal scalded skin syndrome (SSSS) in young children/infants, whose immature renal clearance lets toxin accumulate.
Enzymes: hyaluronidase, staphylokinase (fibrinolysin, aiding tissue spread — functionally the opposite of coagulase), and lipases (aiding survival in sebum-rich skin sites).

Clinical Manifestations

Localized skin/soft tissue infection: folliculitis, furuncles (boils) and carbuncles (coalesced, deeper furuncles), impetigo (also caused by S. pyogenes, often mixed), and wound infection — these are by far the commonest S. aureus presentations.

Toxin-mediated disease (illness from toxin, without necessarily needing ongoing bacterial invasion): staphylococcal food poisoning (rapid onset, 2–6 hours, severe vomiting predominating over diarrhoea, from preformed heat-stable enterotoxin in improperly stored food — cream pastries, salads, cooked meat left at room temperature); staphylococcal scalded skin syndrome (widespread epidermal blistering/desquamation from exfoliative toxin, sparing mucous membranes — a useful distinguishing feature from Stevens-Johnson syndrome/TEN); toxic shock syndrome (fever, hypotension, diffuse macular rash with later desquamation, multi-organ involvement — classically linked to tampon use but also seen with any S. aureus-colonized wound or surgical site).

Invasive/deep infection: bacteraemia (often from a skin/soft-tissue or device-related source), infective endocarditis (see Infective Endocarditis topic — S. aureus is the leading overall cause and typically runs an acute course), osteomyelitis and septic arthritis (see Osteomyelitis topic — S. aureus is the single most common causative organism), pneumonia (often necrotizing, notably as a severe post-influenza secondary bacterial pneumonia), and abscess formation at essentially any site once bacteraemic seeding occurs.

Methicillin Resistance (MRSA)

Methicillin resistance arises from the chromosomally-encoded mecA gene (carried on the mobile genetic element SCCmec), which codes for an altered penicillin-binding protein, PBP2a, with drastically reduced affinity for essentially every β-lactam — meaning MRSA is resistant to the entire β-lactam class, not just methicillin itself. Two epidemiologically distinct MRSA populations exist: healthcare-associated MRSA (HA-MRSA), typically multidrug-resistant beyond just β-lactams, associated with invasive device-related and bloodstream infection in hospitalized patients; and community-associated MRSA (CA-MRSA), often narrower-spectrum resistant but frequently PVL-positive, causing aggressive skin/soft-tissue infection and occasionally severe necrotizing pneumonia in otherwise healthy individuals outside a healthcare setting.

Laboratory Diagnosis

Microscopy: Gram-positive cocci in clusters.

Culture: golden-yellow, β-haemolytic colonies on blood agar (a useful macroscopic clue, though not universal — some strains are non-pigmented or non-haemolytic); mannitol salt agar (selective and differential — high salt suppresses most competing flora, and mannitol fermentation by S. aureus turns the phenol red indicator yellow).

Biochemical/rapid identification: catalase-positive (separates Staphylococcus from Streptococcus); coagulase-positive (separates S. aureus from CoNS) — either the rapid slide test (detects bound coagulase/clumping factor) or the definitive tube test (detects free coagulase, taking up to 24 hours, needed when the slide test is negative or equivocal); DNase-positive is a further confirmatory test. Automated systems (MALDI-TOF, VITEK) are now standard for definitive species identification in most laboratories.

MRSA screening: cefoxitin disk testing is used as a reliable phenotypic surrogate for oxacillin/methicillin resistance (more reliable than testing oxacillin directly, per the Antimicrobial Susceptibility Testing topic), with PCR-based detection of mecA (or the newer mecC) available for rapid, definitive confirmation.

Treatment

MSSA (methicillin-susceptible): anti-staphylococcal penicillins (cloxacillin, nafcillin) or first-generation cephalosporins. MRSA: vancomycin remains the mainstay for serious infection; linezolid, daptomycin, and clindamycin (when susceptible, with the D-test used to rule out inducible resistance — see Antimicrobial Susceptibility Testing) serve as alternatives, particularly for skin/soft-tissue infection or when vancomycin is contraindicated/ineffective. Abscess drainage remains an essential adjunct wherever a drainable collection is present — antibiotics alone rarely suffice for a true abscess, since drug penetration into pus is poor.