AKT · Infections/Key pathogens & antimicrobials

Streptococcal infections

Gram-positive cocci in chains, catalase-negative

Overview

Streptococci are Gram-positive cocci in CHAINS (catalase-negative — distinguishing them from staph). The exam tests species-by-context: S. pyogenes (Group A, β-haemolytic) — tonsillitis, scarlet fever, erysipelas/impetigo, necrotising fasciitis, and post-infectious rheumatic fever & glomerulonephritis; S. pneumoniae (α-haemolytic, diplococci) — commonest CAP, otitis media, bacterial meningitis in adults; S. viridans — subacute endocarditis on abnormal valves after dental work; S. bovis/gallolyticus — endocarditis associated with COLORECTAL CANCER (investigate the bowel); S. agalactiae (Group B) — neonatal sepsis/meningitis.

Recognise

S. pyogenes (GAS): sore throat + Centor features, sandpaper scarlet-fever rash + strawberry tongue, erysipelas, nec fasc; sequelae = rheumatic fever, post-strep GN
S. pneumoniae: lobar CAP (rusty sputum), otitis media, sinusitis, the commonest bacterial meningitis in adults; asplenic patients at high risk
S. viridans → subacute IE (dental); S. bovis → IE + look for colorectal cancer; Group B strep → neonatal early/late sepsis

Red flags

Necrotising fasciitis / streptococcal toxic shock (GAS) → surgical debridement + benzylpenicillin + clindamycin
Pneumococcal meningitis/sepsis → do not delay benzylpenicillin/ceftriaxone; asplenia raises the stakes

Differentials & how to tell them apart

Staphylococcus aureusGram-positive cocci in CLUSTERS, coagulase-positive; impetigo (bullous), abscess, IVDU endocarditis

Viral pharyngitislow Centor score, no exudate/fever — antibiotics not indicated

Enterococcusbowel/biliary/urinary source; IE after GI/GU procedures; often resistant

Investigations

Culture (throat swab, blood, sputum, CSF); Gram stain (chains/diplococci); ASO titre (recent GAS); haemolysis pattern (β vs α); if S. bovis grown → colonoscopy.

Management

Penicillin (streptococci are usually sensitive); add clindamycin for toxin-mediated GAS; colonoscopy if S. bovis

1Identify species by context/haemolysis. Penicillin/amoxicillin is first-line for most streptococcal disease; escalate to ceftriaxone for meningitis.Gate: Necrotising fasciitis or streptococcal toxic shock is a SURGICAL emergency — debridement + benzylpenicillin + clindamycin, not antibiotics alone.
2Chase sequelae/associations: ASO + follow-up for rheumatic fever/post-strep GN after GAS; colonoscopy after S. bovis endocarditis; look for the abnormal valve in S. viridans IE.

Phenoxymethylpenicillin / benzylpenicillin — GAS pharyngitis, erysipelas; streptococci are usually penicillin-sensitive

Amoxicillin / benzylpenicillin — pneumococcal CAP; ceftriaxone for meningitis

Benzylpenicillin + clindamycin — necrotising fasciitis / streptococcal TSS (clindamycin suppresses toxin)

Investigate the bowel — S. bovis/gallolyticus endocarditis → colonoscopy for colorectal cancer

Key points

Gram-positive cocci in CHAINS (catalase-negative). Species-by-context is the exam: S. pyogenes (GAS) rash/nec-fasc/rheumatic fever; S. pneumoniae CAP/meningitis; S. viridans dental → subacute IE; S. BOVIS endocarditis → hunt COLORECTAL CANCER; Group B strep → neonate. Chains = strep, clusters = staph.

Monitor & prognosis

Clinical response; ASO/complement for post-strep sequelae; colonoscopy result for S. bovis.

Excellent for uncomplicated infection; nec fasc/TSS and untreated sequelae are the serious outcomes.

Source: NICE CKS; UKHSA; BNF