Staphylococcus – Comprehensive Bullet-Point Notes

Taxonomy & Historical Background

• Discovery timeline
  • 1871: von Recklinghausen observed cocci in human pyogenic lesions.
  • 1880: Pasteur cultivated the cocci in pus → produced abscesses in rabbits.
  • 1880: Sir Alexander Ogston conclusively proved causation of abscesses & coined the term “Staphylococcus” (Greek: staphyle = bunch of grapes, kokkos = berry).
  • 1884–1885: Rosenbach differentiated pigmented species → Staph. aureus (golden colonies) vs Staph. albus (white colonies); Passet added Staph. citreus (lemon-yellow colonies).
• Early virulence indicators (haemolysis, gelatin liquefaction, lipase, urease, phosphatase) were unreliable; coagulase & mannitol fermentation correlated best with pathogenicity.
• Present genus structure: 32 species + 15 subspecies, classified by cell-wall composition & biochemical traits.
• Clinically relevant species
  • Coagulase-positive: Staph. aureus (main human pathogen).
  • Coagulase-negative that may cause disease: S. epidermidis, S. haemolyticus, S. saprophyticus.
  • Others (commensals or animal parasites): S. hominis, S. capitis, S. intermedius, S. hyicus.

Morphology & Staining

• Shape: spherical cocci ≈ $1\ \mu m$ diameter.
• Arrangement: characteristic grape-like clusters (division in three planes) but may appear singly, in pairs, or short chains (3–4 cells) in liquid culture; never long chains.
• Structures: non-motile, non-sporing; some strains have microscopic capsules or surface capsular material.
• Staining: uniformly Gram-positive; readily take aniline dyes.
• L-forms: develop under penicillin/chemical stress.

Cultural Characteristics

• Growth range: $10–42\ ^\circ C$ (optimum $37\ ^\circ C$); pH $7.4–7.6$; aerobe & facultative anaerobe.
• Nutrient agar (24 h)
  • Colonies: large 2–4 mm, circular, convex, smooth, shiny, opaque, easily emulsifiable.
  • Pigment: golden-yellow (carotene-like lipoprotein); enhanced at $22\ ^\circ C$ & in aerobic conditions (↑ with $1\%$ glycerol mono-acetate or milk).
  • Slope culture: “oil-paint” sheen.
• Blood agar: similar colonies; most strains haemolytic, especially with $20–25\%$ CO₂; haemolysis stronger on rabbit/sheep blood, weaker on horse blood.
• MacConkey agar: small pink colonies (lactose fermenter).
• Broth: uniform turbidity.
• Selective media for isolation
  • High-salt media (8–10 % NaCl: salt-milk agar, salt broth).
  • Lithium chloride + tellurite (Ludlam’s medium).
  • Polymyxin-containing media.
  • Primary isolation recommendation: sheep blood agar (avoid human blood—possible inhibitors).

Biochemical Profile of S. aureus

• Sugar fermentation: many sugars → acid only; mannitol positive (diagnostic).
• Catalase ++ (differentiates from streptococci).
• Urease ++ (usual).
• Nitrate reduction: $\text{NO}<em>3 \rightarrow \text{NO}</em>2$ (++).
• Gelatin liquefaction (++).
• MR & VP (++); indole –.
• Lipase production: opacity on egg-yolk media.
• Phosphatase: bright pink on phenolphthalein-diphosphate agar after NH₃ exposure—rapid for S. aureus.
• Tellurite reduction → black colonies.
• Thermostable nuclease (DNase) detectable after boiling culture.

Resistance & Environmental Survival

• Dry survival: viable $3–6$ months on dry threads; isolated from dried pus after $2–3$ months.
• Heat: survive $60\ ^\circ C$ for 30 min; thermal death point $62\ ^\circ C$/30 min; some need $80\ ^\circ C$/1 h; can grow at $45\ ^\circ C$.
• Salt: grow at $10\%$ NaCl (most) & some at $15\%$ → food preservation relevance.
• Chemicals: resist $1\%$ phenol (15 min); killed by $1\%$ HgCl₂ in 10 min.
• Dyes: crystal-violet lethal at 1:500 000; brilliant green lethal at 1:10 000 000.
• Fatty acids: unsaturated fatty acids inhibit growth (stronger effect on coagulase-positive strains).
• Lysozyme: staphylococci resistant (unlike some micrococci); lysostaphin generally active.

Antibiotic Resistance Mechanisms

1. $\beta$-lactamase (penicillinase) production (types A–D)
   • Plasmid-mediated; inducible; transmissible by transduction/conjugation.
2. Altered penicillin-binding proteins (PBPs)
   • Chromosomal; temperature-dependent (↑ expression at $30\ ^\circ C$); confers resistance to methicillin, cloxacillin, & other $\beta$-lactams → MRSA/EMRSA.
3. Tolerance (inhibition without killing).

• Additional plasmid-borne resistance to erythromycins, tetracyclines, aminoglycosides, etc.; vancomycin usually retained, though VISA/VRSA reported.

Pathogenicity Overview

• Disease types:
  1. Infections: organism invades damaged skin/mucosa → adherence, immune evasion, multiplication, tissue damage.
  2. Intoxications: disease due to pre-formed or in-vivo-produced toxins (e.g., food poisoning, TSS, SSSS).
• Virulence determinants
  • Cell-associated polymers: peptidoglycan (complement activation, cytokine release), teichoic acid (adhesion, anti-opsonic), capsular polysaccharide (anti-phagocytic).
  • Surface proteins:
    • Protein A (binds Fc of IgG except IgG3 → antiphagocytic, basis of coagglutination; B-cell mitogen).
    • Clumping factor (bound coagulase) → slide test; may be masked by capsule.
  • Extracellular enzymes:
    • Free coagulase (requires CRF; tube test) vs bound coagulase.
    • Lipases, hyaluronidase, staphylokinase (fibrinolysin), proteases, fatty-acid–modifying enzymes, thermostable nuclease.
    • Protein receptors for fibronectin, fibrinogen, IgG, C1q.
  • Toxins (see separate heading).

Major Exotoxins

• Cytolytic (membrane-active) toxins
  • $\alpha$-toxin: lyses rabbit RBCs, leucocidal, dermonecrotic, neurotoxic; heat inactivated at $70\ ^\circ C$ but reactivated at $100\ ^\circ C$ (heat-labile inhibitor destroyed).
  • $\beta$-hemolysin (sphingomyelinase): ‘hot-cold’ lysis of sheep RBCs.
  • $\gamma$-hemolysin: bicomponent.
  • $\delta$-hemolysin: detergent-like broad membrane damage.
  • Panton–Valentine leucocidin (PVL): bicomponent S & F → potent leucocidal; classed with synergohymenotropic toxins.
• Enterotoxins (food poisoning)
  • Types A, B, C₁, C₂, C₃, D, E, H (often multiple produced).
  • Heat-stable: endure $100\ ^\circ C$ for 10–40 min.
  • Clinical: nausea, vomiting, diarrhoea within $2–6$ h; self-limited (~24 h).
  • Mechanism: direct autonomic nervous system stimulation; potent (µg doses).
  • Detection: latex agglutination, ELISA.
• Toxic Shock Syndrome Toxin-1 (TSST-1)
  • Formerly enterotoxin F/pyrogenic exotoxin C.
  • Multisystem disease: fever, hypotension, rash → desquamation; often tampon-associated; blood cultures negative; antibody absence predisposes.
• Exfoliative (epidermolytic) toxin (ET/exfoliatin)
  • Causes Staphylococcal Scalded Skin Syndrome (SSSS): Ritter’s disease (newborn), toxic epidermal necrolysis, bullous impetigo.
• Superantigen property: Enterotoxins & TSST-1 are Vβ-restricted T-cell mitogens → massive cytokine release (IL-1, IL-2, TNF, IFN-γ) → systemic effects.

Clinical Manifestations

• Skin/Soft tissue: folliculitis, furuncle, carbuncle, abscess (e.g., breast), wound infection, impetigo, paronychia, occasional cellulitis.
• Musculoskeletal: osteomyelitis, septic arthritis, bursitis, pyomyositis.
• Respiratory: tonsillitis, sinusitis, otitis, bronchopneumonia, lung abscess, empyema; rarely frank pneumonia.
• CNS: brain abscess, meningitis, intracranial thrombophlebitis.
• Endovascular: bacteremia, septicemia/pyemia, endocarditis.
• Urinary: usually linked to instruments/implants/DM; low counts still significant.
• Toxin-mediated: staphylococcal food poisoning, TSS, SSSS.

Epidemiology & Carriage

• Reservoir: humans (patients + carriers) > animals > fomites.
• Carriage rates: nasal 10–30 %, perineal 10 %, vaginal 5–10 % (↑ during menses) → relevance to TSS; early life colonisation (umbilical stump); ‘shedders’ disseminate heavily.
• Modes: direct contact, fomites, dust, airborne droplets.
• Hospital (“nosocomial”) infections
  • Post-operative wounds, cross-infection; dominated by antibiotic-resistant ‘hospital strains’ (limited phage types, e.g., former 80/81 → now MRSA & G-ve bacilli).

Prevention & Control in Hospitals

• Isolation of patients with open staph lesions.
• Screen & exclude infected colonised staff until healed.
• Strict theatre asepsis.
• Hand-washing: cheapest & most effective yet often neglected.
• Outbreak management: carrier search → treat with neomycin + chlorhexidine; sometimes deliberate colonisation with benign strains; topical antiseptics (hexachlorophene).

Laboratory Diagnosis

1. Specimen selection
   • Pus, sputum, blood, CSF, urine, nasal/perineal swabs, feces & suspected food (food poisoning), hair/umbilical stump (neonates).
2. Direct microscopy: Gram-positive cocci in clusters helpful in pure pus; limited value in mixed flora.
3. Culture
   • Plate on blood agar (colonies >! overnight). For scant numbers → selective media (Ludlam, salt-milk agar, 10 % NaCl RCM).
4. Identification
   • Gram stain from culture.
   • Coagulase tests
     • Tube (free coagulase + CRF; 0.1 ml culture + 0.5 ml plasma; incubate $37\ ^\circ C$ 3–6 h; clot = positive).
     • Slide (bound coagulase/clumping factor; rapid).
   • Phosphatase, DNase, mannitol, tellurite, pigment tests as adjuncts.
5. Antibiotic susceptibility: disc diffusion/automated; essential due to resistance variability.
6. Typing for epidemiology: bacteriophage typing (international set: groups I–V, e.g., 29, 52/52A/79/80 etc.), antibiogram, plasmid profile, DNA fingerprint, ribotyping, PCR.
7. Serology: rising antistaphylolysin (anti-$\alpha$-toxin) >2 units/ml may suggest deep infection.

Treatment Principles

• Base choice on sensitivity.
• If sensitive: benzyl penicillin is best.
• Penicillinase producers: cloxacillin (oxacillin class) but MRSA resistant.
• MRSA / severe infections: vancomycin (± teicoplanin, linezolid newer); VISA/VRSA emerging ⇒ stewardship critical.
• Mild superficial lesions: topical non-systemic agents (bacitracin, chlorhexidine, mupirocin).
• Carriers: topical bacitracin/chlorhexidine; refractory → rifampicin + 2nd oral drug for suppression.
• Beware “tolerance”: MIC ≠ MBC; bacteriostatic effect only → treatment failure.

Coagulase-Positive Non-aureus species

• S. intermedius, S. hyicus: animal parasites; not human pathogens.

Coagulase-Negative Staphylococci (CoNS)

• Major human flora component; generally less virulent but pathogenic with breached defences or prosthetics.
• Staph. epidermidis
  • Universal skin commensal; stitch abscess; prosthetic device infections (heart valves, shunts, IV catheters) → bacteremia/endocarditis; hospital strains multidrug-resistant.
• Staph. saprophyticus
  • Skin/peri-urethral coloniser; symptomatic UTIs in sexually active young women; may ascend; usually antibiotic-sensitive except intrinsic nalidixic-acid resistance; novobiocin-resistant (diagnostic).
• Staph. haemolyticus: opportunistic device-related infections, high multi-drug resistance.
• Differentiation table (key tests)
  • S. aureus: coagulase +, mannitol +, phosphatase +, novobiocin sensitive.
  • S. epidermidis: coagulase –, mannitol –, phosphatase ± (weak), novobiocin sensitive.
  • S. saprophyticus: coagulase –, mannitol variable, phosphatase –, novobiocin resistant.

Micrococci (Genus Micrococcus)

• Gram-positive cocci in pairs/tetrads/clusters; catalase & oxidase +; strict aerobes (respiratory metabolism).
• Larger, more Gram-variable cells; smaller colonies vs staph.
• Hugh & Leifson O/F test: micrococci show oxidative pattern; staph fermentative.
• Usually harmless skin commensals.

Connections & Clinical Relevance

• Increasing MRSA & VRSA underline importance of antimicrobial stewardship, hospital hygiene, & rapid diagnostics.
• Protein A coagglutination applied in streptococcal grouping & gonococcal typing—example of cross-application of staph biology.
• Superantigen concept (enterotoxins, TSST-1) pivotal for immunology & vaccine research.
• Food safety: heat-stable enterotoxins survive common cooking; emphasises cold-chain & hygiene for meat/dairy.
• Epidemiology relies on phage typing/DNA methods → informs infection-control measures during outbreaks.

Ethical & Practical Implications

• Hospital outbreak management must balance patient privacy with staff screening and exclusion.
• Over-the-counter misuse of topical antibiotics fosters resistance in community strains.
• Tampon design changes & public education reduced TSS incidence—shows interplay between microbiology & product safety.

Numerical & Statistical Highlights

• Carriage prevalence: nasal $10–30\%$; perineum $\sim10\%$; vaginal $5–10\%$ (↑ during menses).
• Food-poisoning incubation: $2–6$ h; toxin active at $\mu g$ levels.
• Heat resistance: survive $60\ ^\circ C$ / 30 min; enterotoxin resists $100\ ^\circ C$ / 10–40 min.
• Phage typing scheme: 23 standard phages grouped I–V.

Quick Revision Checklist

• Gram-positive clusters, catalase +, coagulase differentiates pathogenic S. aureus.
• Virulence = coagulase, Protein A, toxins ($\alpha$, PVL, enterotoxin, TSST-1, ET), enzymes (lipase, nuclease).
• MRSA: $\beta$-lactamase vs PBP alteration.
• Diseases: local pyogenic → systemic; toxins → food poisoning, TSS, SSSS.
• Diagnosis: culture + coagulase; selective media for carriers; phage typing for epidemiology.
• Treatment: penicillin if sensitive; cloxacillin for penicillinase producers; vancomycin/linezolid for MRSA.
• CoNS: device-related infections (S. epidermidis), UTI in young women (S. saprophyticus).
• Hospital control: isolation, staff screening, hand hygiene.