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.
Related Posts
Peptone Water Composition Preparation Uses
Introduction Peptone water is one of the simplest basal liquid…
Sugar Media Composition Preparation Uses
Introduction Sugar media are specialized differential culture media used in…
Biochemical Tests in Microbiology – Types & Examples
Biochemical Tests in Microbiology –Types & Examples Introduction Biochemical tests…