Gram-positive bacteria

Sorting Gram-positive cocci

• Catalase (bubbles with hydrogen peroxide): positive in staphylococci, negative in streptococci and enterococci.
• Staphylococci: coagulase positive means *S. aureus*; coagulase-negative species are *S. epidermidis* (novobiocin sensitive) and *S. saprophyticus* (novobiocin resistant).
• Streptococci are first sorted by haemolysis on blood agar, then beta-haemolytic ones by Lancefield grouping, which identifies the carbohydrate (C) antigen in the cell wall with specific antisera (groups A, B, C, D and so on).

• A sore throat swab showing complete (beta) haemolysis on sheep blood agar in a child is most likely group A streptococcus.

*Staphylococcus aureus*

• Virulence: protein A (binds the Fc of IgG, blocking opsonisation), coagulase, haemolysins, Panton–Valentine leucocidin (necrotising pneumonia, recurrent boils), enterotoxins, TSST-1, exfoliatins.
• Penicillin resistance: most strains make plasmid-coded beta-lactamase (penicillinase), so use flucloxacillin. MRSA carries *mecA*, coding the altered penicillin-binding protein PBP2a with low affinity for all beta-lactams; treat with vancomycin, teicoplanin, linezolid or daptomycin; decolonise with mupirocin and chlorhexidine.
• Diseases: the commonest cause of surgical wound infection, boils, carbuncles, bullous impetigo, cellulitis, osteomyelitis and septic arthritis (commonest cause), lactational breast abscess, acute endocarditis (IV drug users, tricuspid valve), post-influenza pneumonia with abscesses, abscesses anywhere (think *S. aureus* first for a lung or brain abscess with meningitis), parotitis.
• Toxin diseases: food poisoning (preformed heat-stable enterotoxin, vomiting at 1–6 hours; custards, dairy, cooked meat), toxic shock syndrome (tampons, nasal packs: fever, hypotension, rash then desquamation of palms), scalded skin syndrome (children; Nikolsky positive, mucosa spared).
• *S. epidermidis* forms biofilm on prosthetic joints, valves, lines and shunts, so infection persists despite sensitive antibiotics; it is also the commonest blood culture contaminant. *S. saprophyticus* causes UTI in young sexually active women (second to *E. coli*).

*Streptococcus pyogenes* (group A)

• Virulence: M protein (antiphagocytic; its antibodies cross-react with heart tissue), streptolysin O (gives the ASO titre), streptolysin S, hyaluronidase, streptokinase, DNase B, pyrogenic exotoxins (superantigens of scarlet fever and streptococcal toxic shock).
• Suppurative disease: pharyngitis, impetigo (honey crusts), erysipelas (sharply raised edge), cellulitis, necrotising fasciitis (surgical emergency), puerperal sepsis.
• Toxigenic: scarlet fever (sandpaper rash, strawberry tongue, circumoral pallor, peeling).
• Non-suppurative sequelae:

• Rheumatic carditis is a pancarditis; the typical new murmur is an apical pansystolic murmur of mitral regurgitation (sometimes with a soft mid-diastolic Carey Coombs murmur). Carditis with heart failure is the commonest cause of death in the acute attack; mitral stenosis comes years later. Recurrent streptococcal pharyngitis puts a child at risk of rheumatic fever.
• Penicillin remains uniformly active against *S. pyogenes*.

Other streptococci and enterococci

• Group B: early-onset neonatal sepsis, pneumonia and meningitis; intrapartum penicillin for colonised mothers. *E. coli* is the other leading cause of neonatal meningitis.
• *S. pneumoniae*: commonest cause of community-acquired pneumonia, adult meningitis (including over 60), otitis media, sinusitis; danger in asplenia and sickle cell disease. Meningitis: ceftriaxone (plus vancomycin if resistance is possible). Vaccines: PCV and PPV23.
• Viridans streptococci (*S. mutans*, *S. sanguinis*): dental caries; the commonest cause of subacute endocarditis on damaged valves (dextran helps them stick to fibrin–platelet vegetations). *S. anginosus* (milleri) group: brain and liver abscesses.
• *S. gallolyticus* (bovis) endocarditis means colonoscopy to look for colorectal cancer.
• Enterococci: UTI, biliary sepsis, endocarditis after urinary or bowel procedures; intrinsically resistant to cephalosporins; amoxicillin (with gentamicin for endocarditis); VRE needs linezolid or daptomycin.

Clostridia: anaerobic spore-formers

• Tetanus prophylaxis for a dirty wound: full primary course with the last dose over 10 years ago (or over 5 years for a heavily contaminated wound): toxoid booster plus wound care; unknown or incomplete course: toxoid plus tetanus immunoglobulin. Neonatal tetanus is prevented by vaccinating mothers.

*Corynebacterium diphtheriae*

• Club-shaped rods in Chinese-letter arrangement with metachromatic (Babes–Ernst) granules; grows on Loeffler serum and tellurite (black colonies). The toxin gene comes from a beta-prophage (lysogenic conversion); the Elek test shows whether a strain makes toxin.
• Toxin ADP-ribosylates EF-2: grey adherent pseudomembrane that bleeds when removed, bull neck, airway obstruction, myocarditis (main cause of death), palatal and cranial nerve palsies.
• Give antitoxin at once on clinical suspicion (it cannot reach toxin already inside cells), plus penicillin or erythromycin; isolate; immunise contacts.

*Listeria monocytogenes*

• Small Gram-positive rod; tumbling motility at 25 °C; grows at fridge temperature; intracellular, spreading cell to cell by actin rockets.
• Sources: soft cheese, unpasteurised milk, deli meat. Disease: sepsis in pregnancy (miscarriage, stillbirth), neonatal sepsis and meningitis, meningitis in the elderly and immunosuppressed.
• Amoxicillin or ampicillin (with gentamicin); cephalosporins do not cover it.

*Bacillus*

• *B. anthracis*: large boxcar rods, spore-forming, non-motile, "medusa head" colonies. Its capsule is poly-D-glutamic acid (protein), not polysaccharide; with polychrome methylene blue the blue rods sit in purple-pink capsular material (McFadyean reaction). Cutaneous anthrax (wool sorters, hide and livestock handlers): painless black eschar with oedema. Inhalational anthrax: haemorrhagic mediastinitis, widened mediastinum. Treat with ciprofloxacin or doxycycline.
• *B. cereus*: reheated fried rice; emetic type (preformed toxin, 1–6 hours) or diarrhoeal type (8–16 hours).

Branching Gram-positives and others

• *Actinomyces israelii*: anaerobic branching filaments, sulphur granules; cervicofacial "lumpy jaw" with sinuses after dental work, abdominal, pelvic with a long-standing IUCD; long-course penicillin plus drainage.
• *Nocardia*: aerobic, branching, weakly acid-fast; lung and brain abscess in the immunosuppressed; co-trimoxazole.
• *Cutibacterium acnes*: acne, shunt and prosthetic joint infections.
• Rat-bite fever (fever, migratory polyarthritis, rash after a rat bite): *Streptobacillus moniliformis* in the West, *Spirillum minus* in Asia; both are Gram-negative, treated with penicillin.

Remember Catalase splits staphylococci from streptococci; coagulase marks *S. aureus*; haemolysis and Lancefield groups sort the streptococci.

Exam trap Listeria and enterococci resist cephalosporins; add amoxicillin.

Exam trap Tetanus gives spastic paralysis (loss of inhibition); botulism gives flaccid paralysis (no acetylcholine release).

How FCPS asks about this

• Grouping streptococci: Lancefield, by cell-wall carbohydrate antigen; group A is bacitracin sensitive, pneumococcus optochin sensitive.
• Staphylococcal penicillin resistance: beta-lactamase; MRSA through mecA and PBP2a; biofilm explains persistent prosthetic infection.
• Commonest causes: *S. aureus* for osteomyelitis, wound infection and lactational mastitis; viridans streptococci for subacute endocarditis.
• Rheumatic fever: pharyngitis only; apical pansystolic murmur; death from carditis; PSGN shows low C3 and follows throat or skin infection.
• Clostridia: tetanus blocks glycine and GABA release; infant botulism from honey; *C. perfringens* lecithinase; *C. difficile* treated with oral vancomycin.
• Anthrax: painless black eschar in wool sorters; McFadyean reaction shows a protein capsule; ciprofloxacin.
• Diphtheria: EF-2 toxin, pseudomembrane, myocarditis; Elek test; antitoxin first.