2. Staphylococcus and Micrococcus

Overview of Genera Staphylococcus and Micrococcus

• Taxonomic Family: Both Staphylococcus and Micrococcus belong to the family Micrococcaceae.

• Characteristics of Micrococcus:

  • Common inhabitants of the environment and human skin.

  • Produce yellow-pigmented colonies when grown on culture media.

• Major Pathogenic Species of Staphylococcus:

  • Staphylococcus aureus: Recognized as major pathogen #1 within the genus.

  • Staphylococcus epidermidis: Common coagulase-negative pathogen.

  • Staphylococcus lugdunensis: Emerging pathogen capable of severe invasive infections.

  • Staphylococcus saprophyticus: Primary staphylococcal pathogen in urinary tract infections.

Characteristics and Clinical Significance of Staphylococcus aureus

• Microscopic & General Properties:

  • Gram-positive cocci arranged in pairs, short chains, and grape-like clusters.

  • Commonly designated as "golden staph" due to the production of yellow-pigmented colonies on agar.

  • Natural Reservoir: Resides as normal flora in the anterior nares and mucous membranes of human carriers; highly prevalent among hospitalized patients.

  • Pathogenicity: Serves as the primary pathogen of the genus Staphylococcus. As normal skin and mucous membrane flora, it requires a portal of entry (break in mucosal or cutaneous barriers) to establish infection.

  • Modes of Transmission: Traumatic introduction, direct person-to-person contact, or indirect contact via fomites.

• Clinical Diseases and Manifestations:

  • Produces pus-filled suppurative infections at any site of entry, with a high affinity for skin, breast tissue, surgical/wound sites, heart valves (endocarditis), and the bloodstream (bacteremia).

  • Serves as the most common overall cause of joint infections (septic arthritis) and bone infections (osteomyelitis).

  • Skin and Soft Tissue Infections (following trauma, cuts, burns, or surgery):

    • Impetigo and bullous impetigo.

    • Furuncles (boils) and carbuncles.

    • Surgical wound infections.

  • Toxin-Mediated Syndromes:

    • Staphylococcal Food Poisoning: Ingestion of preformed enterotoxins produced by enterotoxigenic strains; contamination originates typically from infected food handlers.

    • Staphylococcal Scalded Skin Syndrome (SSSS / Ritter's Disease): Exfoliative dermatitis characterized by cutaneous desquamation; most common in infants and previously healthy young children.

    • Toxic Shock Syndrome (TSS): Acute multisystem disease characterized by high fever, profound hypotension, rash, and shock.

  • Invasive & Systemic Infections:

    • Toxic Epidermal Necrolysis (TEN).

    • Staphylococcal Pneumonia.

    • Staphylococcal Bacteremia.

    • Staphylococcal Osteomyelitis.

    • Staphylococcal Endocarditis.

    • Septic Arthritis.

• Methicillin-Resistant S. aureus (MRSA):

  • Highly resistant strain that presents significant therapeutic challenges.

  • Demonstrates non-hemolytic colony morphology on blood agar.

  • Possesses a polysaccharide capsule that covers cell surface components (Protein A and clumping factor), which can interfere with standard latex agglutination test clumping unless specific anti-capsular antibodies are present.

Coagulase-Negative Staphylococci

• General Features:

  • Defined by the absence of the coagulase enzyme.

  • Normal commensals of human skin and mucous membranes.

  • Common human isolates include Staphylococcus epidermidis, Staphylococcus saprophyticus, and Staphylococcus haemolyticus.

• Staphylococcus epidermidis:

  • Gram-positive, coagulase-negative coccus found in normal skin flora.

  • Clinical Presentation: Causes skin lesions characterized by erythema and oozy discharge. Frequently infects implanted medical hardware (prosthetic joints, metal pins, plates, screws), producing pain, swelling, tissue erosion, and hardware loosening.

  • Central Nervous System: Infects cerebrospinal fluid shunts, leading to staphylococcal meningitis.

  • Nosocomial Infections: Represents one of the most frequent causes of hospital-acquired urinary tract infections secondary to urinary catheterization.

  • Major Virulence Factor: Production of an extracellular slime layer (biofilm) that firmly anchors bacteria to biomaterials, making eradication with antimicrobial agents extremely difficult and necessitating physical hardware removal.

  • Mode of Transmission: Inoculation via surgical placement or catheterization of medical devices.

  • Colonial Morphology: Smooth, creamy, white, small-to-medium-sized colonies; typically non-hemolytic.

• Staphylococcus saprophyticus:

  • Gram-positive, coagulase-negative coccus.

  • Habitat: Normal flora of human skin and genitourinary mucous membranes.

  • Clinical Significance: Leading Gram-positive cause of community-acquired urinary tract infections in young, sexually active females. Even low colony counts in urine culture specimens are clinically significant.

  • Colonial Morphology: Smooth, creamy colonies; approximately $50\%$ of strains produce a yellow pigment.

  • Diagnostic Differentiation: Resistant to novobiocin.

• Staphylococcus lugdunensis:

  • Skin commensal that acts as an aggressive opportunistic pathogen, producing invasive clinical manifestations similar in severity to S. aureus.

  • Spectrum of Clinical Infections: Abscesses, meningitis, skin and soft-tissue cellulitis, shunt infections, spondylodiscitis, prosthetic joint infections, catheter-related bacteremia, septic arthritis, osteomyelitis, endocarditis, peritonitis (in patients undergoing peritoneal dialysis), and toxic shock syndrome.

  • Microscopic Morphology: Gram-positive cocci in grape-like clusters.

  • Colonial Morphology: $2\text{--}4\,\text{mm}$ in diameter after $48\,\text{hrs}$ of incubation; white to opaque, smooth, glossy, beta-hemolytic; pigmented yellow, yellow-orange, or cream.

  • Diagnostic Profile & Misidentification Warnings:

    • Frequently misidentified as S. aureus because it produces bound coagulase (clumping factor), yielding a positive slide coagulase test.

    • Universally negative for free coagulase (tube coagulase test negative).

    • Manual Identification Profile: Catalase positive, tube coagulase negative, pyrrolidonyl arylamidase (PYR) test positive, and ornithine decarboxylase test positive.

Laboratory Diagnosis and Specimen Processing

• Specimen Handling & Collection Protocols:

  • Lesions, Wounds, and Abscesses:

    • Decontaminate surface area prior to collection using sterile saline or alcohol.

    • Superficial lesions: Swab along the active outer edge and place immediately into a swab transport system.

    • Deep abscesses: Aspirate purulent material using a sterile needle and syringe, then transfer into an anaerobic transport system.

    • Storage: Maintain at room temperature.

    • Processing: Direct Gram stain examination and primary culture inoculation onto blood agar, chocolate agar, MacConkey agar or eosin methylene blue (EMB) agar, anaerobic blood agar, and thioglycollate broth.

    • Mixed flora protocol: Incorporate colistin-nalidixic acid (CNA) agar or phenylethyl alcohol (PEA) agar if the direct Gram stain indicates a mixture of Gram-positive and Gram-negative organisms.

  • Urine (Clean Catch Midstream):

    • Patient cleans external genitalia, begins voiding, discards the initial portion, and collects the midstream sample without stopping urine flow into a sterile, screw-cap container or urine transport tube.

    • Storage: Store at room temperature if preserved; refrigerate if unpreserved.

    • Processing: Inoculate directly onto blood agar and MacConkey agar or EMB agar.

• Direct Microscopic Examination:

  • Microscopic morphology reveals Gram-positive cocci occurring singly, in pairs, short chains, and clusters.

  • Smear Interpretation Notes: Direct identification from smears can be challenging because overdecolorized Gram-positive cells stain red. Mixed Gram-positive cocci in pairs, chains, and clusters (e.g., co-infection of Streptococcus pyogenes and Staphylococcus aureus) are common in impetigo, ecthyma, and cutaneous abscesses.

  • Observation of large, spherical Gram-positive cocci in grape-like clusters strongly indicates staphylococci. Red mucinous background strands indicate high-quality sputum specimens.

Biochemical and Phenotypic Differentiation Methods

• Differentiation of Staphylococcus from Micrococcus:

  • Oxidation-Fermentation (O/F) Glucose Test: Staphylococcus species ferment glucose anaerobically with acid production; Micrococcus species are oxidative non-fermenters (with rare exceptions).

  • Microdase (Modified Oxidase) Test: Micrococcus species test positive; Staphylococcus species test negative.

  • Antimicrobial & Chemical Susceptibility Profiling:

    • Bacitracin ($0.04\,\text{U}$ disk): Staphylococcus species are resistant (zone of inhibition $< 10\,\text{mm}$); Micrococcus species are susceptible (zone of inhibition $\ge 10\,\text{mm}$).

    • Furazolidone ($100\,\mu\text{g}$ disk): Staphylococcus species are susceptible (zone of inhibition $\ge 15\,\text{mm}$); Micrococcus species are resistant (zone of inhibition $< 15\,\text{mm}$).

    • Lysostaphin ($200\,\mu\text{g/mL}$): Staphylococcus species are susceptible; Micrococcus species are resistant.

• Catalase Test:

  • Differentiates catalase-positive Staphylococcus species from catalase-negative Streptococcus species.

• Coagulase Test:

  • Differentiates S. aureus (coagulase positive) from coagulase-negative staphylococci.

  • Slide Coagulase Test: Detects cell-bound coagulase ("clumping factor").

  • Tube Coagulase Test: Detects extracellular "free coagulase".

• Novobiocin Susceptibility Test:

  • Differentiates coagulase-negative staphylococci isolated from urine specimens.

  • Staphylococcus saprophyticus is resistant (zone of inhibition $< 16\,\text{mm}$).

  • Other CoNS species (e.g., S. epidermidis) are susceptible (susceptibility breakpoint $\ge 16\,\text{mm}$).

• Deoxyribonuclease (DNase) Test:

  • Used to differentiate Staphylococcus aureus (DNase positive) from other staphylococcal species (DNase negative).

• Commercial Rapid Identification Kits (e.g., Pastorex Staph Plus):

  • Latex agglutination tests used for rapid identification of S. aureus.

  • Reagents detect cell-bound clumping factor, Protein A, and capsular polysaccharides.

  • Note on Capsular Strains: Strains possessing a surface capsule (including MRSA) may mask underlying Protein A and clumping factor, yielding false-negative results on standard clumping assays unless anti-capsular antibodies are incorporated into the reagent.