Staphylococcus
Introduction
Staphylococcus is a genus of Gram-positive cocci that form grape-like clusters (staphylo = bunch of grapes). They are facultative anaerobes, non-motile, non-spore forming, and catalase-positive.Found as normal flora on skin, anterior nares, throat, perineum, and sometimes GI tract.Some species are opportunistic pathogens causing pyogenic infections, food poisoning, and toxin-mediated diseases.
Staphylococcus Species
Staphylococcus aureus – most pathogenic; produces coagulase; causes pus-forming infections & toxin-mediated diseases.Staphylococcus epidermidis – coagulase-negative; causes device-related infections (catheters, prosthetics).Staphylococcus saprophyticus – coagulase-negative; causes UTIs in young sexually active females.
Others: S. haemolyticus, S. lugdunensis (emerging pathogens).
Morphology
Shape: Gram-positive cocci (0.5–1.5 μm), arranged in grape-like clusters.Staining: Gram-positive, purple on Gram stain.
Capsule: Some strains have polysaccharide capsule (antiphagocytic).
Cell wall: Thick peptidoglycan with teichoic acid.
Culture Media
Nutrient agar: Golden yellow colonies (S. aureus – due to carotenoid pigment staphyloxanthin).
Blood agar: β-hemolysis around colonies.
MacConkey agar: No lactose fermentation.
Selective media: Mannitol salt agar (7.5% NaCl) – S. aureus ferments mannitol (yellow colonies).
Liquid media: Uniform turbidity.
Biochemical Reactions
Catalase test: Positive (distinguishes from Streptococcus).
Coagulase test: Positive in S. aureus (tube or slide test).
Oxidase test: Negative.
Fermentation: Glucose, maltose, lactose (varies).
Novobiocin test:
Sensitive: S. epidermidis
Resistant: S. saprophyticus
Resistance
S. aureus shows resistance to many antibiotics due to:
β-lactamase production (penicillin resistance).
Altered PBP (PBP2a) encoded by mecA gene → Methicillin resistance (MRSA).
Resistance to erythromycin, aminoglycosides, tetracyclines also common.
Survival: Resistant to drying, high salt, heat (can survive on fomites).
Antigen Structure
Peptidoglycan – activates complement, induces cytokine release.
Teichoic acid – promotes adhesion to mucosal cells.
Capsular polysaccharide – antiphagocytic.
Protein A – binds Fc portion of IgG → prevents opsonization and phagocytosis.
Toxins & Enzymes
Cytotoxins: α-toxin (hemolysin, tissue necrosis), β, γ, δ toxins.
Panton-Valentine leukocidin (PVL): Kills neutrophils → severe skin infections & necrotizing pneumonia.
Exfoliative toxin: Causes staphylococcal scalded skin syndrome (SSSS).
Enterotoxins (A-E, G-I): Heat-stable; cause food poisoning (vomiting, diarrhea).
Toxic Shock Syndrome Toxin-1 (TSST-1): Superantigen → cytokine storm → toxic shock syndrome.
Enzymes
Coagulase – converts fibrinogen → fibrin (clotting).
Hyaluronidase – breaks down connective tissue.
Staphylokinase – fibrinolysis.
Lipase, DNase, Protease – tissue destruction.
β-lactamase – antibiotic resistance.
Pathogenesis
Entry: Breach in skin/mucosa.
Virulence factors: Adhesins, Protein A, toxins, enzymes.
Types of diseases:
Pyogenic infections: Boils, carbuncles, impetigo, abscesses, wound infections.
Deep-seated infections: Osteomyelitis, septic arthritis, pneumonia, endocarditis, bacteremia.
Toxin-mediated diseases:
Food poisoning (enterotoxin).
Scalded skin syndrome (exfoliative toxin).
Toxic shock syndrome (TSST-1)
Antibiotic Sensitivity
Methicillin-sensitive S. aureus (MSSA): Sensitive to cloxacillin, cephalosporins, vancomycin.
Methicillin-resistant S. aureus (MRSA): Sensitive to vancomycin, linezolid, daptomycin, tigecycline.
Vancomycin-resistant S. aureus (VRSA): Rare, treated with linezolid, daptomycin.
Epidemiology
Reservoir: Humans (nasal carriers, skin, throat).
Transmission: Direct contact, contaminated fomites, respiratory droplets, food (enterotoxin preformed).
Risk factors: Hospitalization, indwelling catheters, IV drug use, burns, immunosuppression.
Nosocomial infections: MRSA is a major hospital-acquired pathogen.
Laboratory Diagnosis
Specimen: Pus, wound swab, blood, sputum, urine, CSF, catheter tips.
Microscopy: Gram-positive cocci in clusters.
Culture: Blood agar, mannitol salt agar.
Biochemical tests: Catalase, coagulase.
Antibiotic sensitivity: Disc diffusion, MIC, detection of mecA gene by PCR for MRSA.
Toxin detection: ELISA, latex agglutination (for TSST, enterotoxins).
Treatment
Local infections: Drainage + antibiotics.
MSSA: Cloxacillin, cefazolin.
MRSA: Vancomycin, linezolid, daptomycin.
Supportive care: For toxin-mediated diseases (fluids, IVIG).
Prevention: Hand hygiene, asepsis in hospitals, screening carriers in ICUs.
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