FFinalistSouthampton · 2027
Infections
AKT · Infections/Key pathogens & antimicrobials

Staphylococcus aureus (incl. MRSA)

Gram-positive cocci in clusters, coagulase-positive, catalase-positive

Overview

The commonest cause of skin/soft-tissue infection, abscess, and — crucially for the exam — the pathogen to reach for in several classic contexts. Coagulase-POSITIVE (vs coag-negative S. epidermidis). Nasal carriage in ~30%. MRSA (methicillin-resistant) needs a glycopeptide (vancomycin/teicoplanin) — β-lactams (flucloxacillin, co-amoxiclav) DO NOT cover it. Toxin-mediated disease: toxic shock syndrome (TSST-1), scalded skin, food poisoning (rapid, preformed enterotoxin).

Recognise

  • Skin/soft tissue: impetigo (incl. BULLOUS impetigo), folliculitis, boils/carbuncles, cellulitis, abscess (pus)
  • Deep/systemic: abscess, osteomyelitis, septic arthritis, INFECTIVE ENDOCARDITIS (esp. IV drug users → tricuspid), device/line infection, secondary bacterial pneumonia (post-influenza, cavitating)
  • Toxin-mediated: toxic shock syndrome (tampon/packing), staphylococcal scalded skin (children), rapid-onset food poisoning (1–6 h)

Red flags

  • S. aureus bacteraemia → always seek a source (echo for endocarditis, MRI spine for discitis) — never dismiss as a contaminant
  • MRSA or PVL-positive strains (necrotising) → glycopeptide + source control

Differentials & how to tell them apart

Coagulase-negative staph (S. epidermidis)coagulase-NEGATIVE; prosthetic-device/line infection, or a blood-culture contaminant
Streptococcus pyogenesGram-positive cocci in CHAINS; erysipelas, non-bullous impetigo, scarlet fever, nec fasc — β-haemolytic
PseudomonasGram-negative rod; burns, hot-tub folliculitis, blue-green pus

Investigations

Culture pus/blood/swabs (Gram stain: clusters); coagulase test (positive); MRSA screen (nasal/axilla/groin swab) on admission; if bacteraemic hunt the source — echocardiography, MRI, imaging.

Management

Flucloxacillin for MSSA; vancomycin/teicoplanin for MRSA; drain any abscess and seek the source in bacteraemia

  1. 1Identify the syndrome and drain any collection. MSSA → flucloxacillin; if MRSA is grown or suspected → vancomycin/teicoplanin.Gate: MRSA is NOT covered by flucloxacillin/co-amoxiclav — a stated MRSA infection needs a glycopeptide (vancomycin). Screen and decolonise carriers.
  2. 2In S. aureus bacteraemia, actively seek a deep focus (endocarditis on echo, discitis on MRI) and remove infected devices/lines; add clindamycin for toxin-mediated disease (TSS, nec fasc).
Flucloxacillin (MSSA) — first-line for methicillin-SENSITIVE S. aureus skin/soft-tissue/bacteraemia
Vancomycin or teicoplanin (MRSA) — glycopeptide — the answer whenever MRSA is stated; β-lactams do not cover MRSA
Incision & drainage — abscess = source control; antibiotics alone fail undrained pus
Clindamycin / linezolid — toxin-suppressing (nec fasc/TSS) or alternative; add clindamycin in toxin-mediated disease

Key points

Coagulase-POSITIVE Gram-positive clusters. Exam contexts: IVDU endocarditis → S. aureus (tricuspid); bullous impetigo → S. aureus; post-influenza cavitating pneumonia → S. aureus; abscess → drain it. The single highest-yield fork: if the stem says MRSA, the answer is VANCOMYCIN, not a β-lactam.

Monitor & prognosis

Response, repeat blood cultures to document clearance in bacteraemia, source-control adequacy.

Good for localised disease; bacteraemia/endocarditis carry significant mortality if a focus is missed.

Source: NICE NG141 (cellulitis); UKHSA MRSA; BSAC