STEPTRACKERMICROBIOLOGY | GRAM-POSITIVE COCCI | HIGH-YIELD REVIEW

Staphylococci

Gram-positive cocci in clusters, catalase positive — S aureus does everything (toxins, abscesses, endocarditis), its coagulase-negative cousins live on plastic and in the bladder.

FOCUS ON
S aureus: coagulase ⁺, β-hemolytic, golden colonies, protein A. Two disease types — inflammatory (abscess, cellulitis, osteomyelitis, septic arthritis, endocarditis, pneumonia after influenza) and toxin-mediated (TSS, scalded skin, food poisoning). MRSA = altered PBP2a (mecA) → vancomycin, daptomycin, linezolid. S epidermidis: biofilm on catheters/prostheses, novobiocin S. S saprophyticus: UTI in sexually active women, novobiocin R.
ORGANISM · DISEASE
KEY FEATURES · VIRULENCE
CLINICAL PICTURE
CLUE · TREATMENT
S aureus — skin & soft tissue
Coagulase, protein A, hemolysins, Panton-Valentine leukocidin (CA-MRSA)
Impetigo (bullous), folliculitis, furuncles, carbuncles, abscesses, cellulitis
Drain abscess · TMP-SMX, doxycycline, clindamycin for CA-MRSA
S aureus — bone & joint
Hematogenous seeding of metaphysis
Most common cause of osteomyelitis and septic arthritis overall
Sickle cell → Salmonella too; IV drug use → Pseudomonas too
S aureus — endocarditis
Acute, destructive vegetations
IV drug users: tricuspid valve, septic pulmonary emboli; prosthetic valves
Most common cause of acute endocarditis
S aureus — pneumonia
Necrotizing, cavitating
After influenza, hospital/ventilator, CF
Empyema, abscess
Toxic shock syndrome
TSST-1 superantigen
Fever, hypotension, diffuse macular rash → desquamation of palms/soles, ≥ 3 organs
Tampons, nasal packing, surgical wounds · remove source, antibiotics, fluids
Scalded skin syndrome
Exfoliative toxin → desmoglein 1
Infants: fever, diffuse erythema, superficial skin peels, Nikolsky ⁺, mucosa spared
Granular-layer split (not TEN)
Staph food poisoning
Preformed, heat-stable enterotoxin
Vomiting, cramps 2–6 h after food (custard, mayonnaise, ham); no fever
Self-limited; reheating doesn't destroy toxin
MRSA
mecA → PBP2a with low β-lactam affinity
Hospital and community strains
Vancomycin, daptomycin (not lungs), linezolid, ceftaroline · VRSA rare
S epidermidis
coagulase −, novobiocin S
Skin flora; biofilm (slime) on plastic
Infections of catheters, prosthetic joints/valves, CSF shunts; common culture contaminant
Often methicillin-resistant → vancomycin
S saprophyticus
coagulase −, novobiocin R
Urease ⁺
Second most common cause of uncomplicated UTI in young, sexually active women
E coli is #1
S lugdunensis
Coagulase-negative but virulent
Aggressive native-valve endocarditis
Treat like S aureus

Mechanism — two ways S aureus makes you sick

InvasionCoagulase walls off, protein A hides
→
PyogenicAbscess, osteomyelitis, endocarditis
ToxinsSuperantigens, exfoliatin, enterotoxin
→
Toxin syndromesTSS, SSSS, food poisoning

Staphylococci compared

SPECIES
COAGULASE
NOVOBIOCIN
CLASSIC SITE
S aureus
+
S
Skin, bone, valve, lung
S epidermidis
−
Sensitive
Prosthetics, catheters
S saprophyticus
−
Resistant
Bladder (young women)

How it's tested

COMMON CONFUSIONStaph scalded skin (toxin, granular split, mucosa spared, children) vs TEN (drug, full-thickness, mucosa involved, adults).
THE TRAPFood poisoning within a few hours of eating = preformed toxin (S aureus, B cereus emetic) — antibiotics are useless.
YOUR APPROACHClusters + catalase ⁺ → coagulase → then setting (wound, bone, valve, catheter, bladder, toxin).

If you see… think…

Fever, sunburn-like rash, desquamating palms after tampon → TSS (TSST-1)
Infant with peeling skin, Nikolsky ⁺, mouth spared → scalded skin syndrome
Vomiting 3 h after potato salad, no fever → S aureus enterotoxin
Tricuspid endocarditis in IV drug user → S aureus
Cavitating pneumonia after influenza → S aureus
Most common cause of osteomyelitis → S aureus
Infected prosthetic hip months after surgery → S epidermidis
UTI in a sexually active 20-year-old, novobiocin resistant → S saprophyticus
Resistance via PBP2a → MRSA (mecA)
Recurrent boils in athletes, PVL toxin → CA-MRSA