Staphylococcus Overview

Staphylococcus Overview

Taxonomy

  • Family: MICROCOCCACEAE

  • Genus: Micrococcus

  • Genus: Staphylococcus

  • Family: STREPTOCOCCACEAE

  • Genus: Streptococcus

  • Genus: Enterococcus

  • Classification: GRAM POSITIVE COCCI

Basic Characteristics of Gram Positive Cocci

  • Staphylococcus and Streptococcus

  • Divide into:

    • Catalase Positive: Staphylococcus

    • Catalase Negative: Streptococcus

Staphylococcus aureus

General Properties

  • Morphology: Gram-positive cocci

    • Seen individually, in pairs, or in irregular, grapelike clusters

  • Characteristics:

    • Nonmotile

    • Non-spore-forming

    • Catalase-positive

    • Coagulase-positive

    • Bound Coagulase (Clumping Factor): Reacts with fibrinogen causing aggregation of organisms.

    • Extracellular Coagulase: Converts prothrombin to thrombin, thus converting fibrinogen to fibrin.

  • Resilience:

    • Resistant to temperatures around ~50°C

    • Able to withstand high salt concentrations and drying

  • Colony Characteristics:

    • Golden/yellow colonies

    • Strongly β-hemolytic on blood agar

Colonization and Carriage

  • Human Flora Locations:

    • Found in the axillae, inguinal, perineal areas, and anterior nares

  • Patterns of Carriage:

    • Persistent carriers (higher risk of infection) typically seen in children

    • Intermittent or noncarriers (lower risk of infection) seen more in adults

Molecular Typing

  • Technique Used: Pulsed-field gel electrophoresis (PFGE)

  • Findings: In S. aureus bacteremia, isolates from patients' blood are often identical to those found in anterior nares

Decolonization Strategies

  • In hospitalized patients with methicillin-resistant S. aureus (MRSA) strains:

    • Topical Mupirocin

    • Chlorhexidine Gluconate Washes

    • Oral Rifampin plus Doxycycline for 7 days

Virulence Factors of Staphylococcus aureus

Key Adhesins

  • Protein A (SpA)

  • Fibronectin-binding Proteins A and B

  • Collagen-binding Protein

  • Clumping Factor A and B Proteins

Enzymes Produced

  • Coagulase

  • Lipase

  • Hyaluronidase

  • Staphylokinase

  • Nuclease

Toxins

  • α-Toxin

  • β-Toxin

  • γ-Toxin

  • Panton-Valentine Leukocidin (PVL)

  • Enterotoxins

  • Exfoliative Toxin

  • Toxic Shock Syndrome Toxin (TSST)

Superantigens

  • The toxins can lead to severe immune responses via nonspecific T-cell stimulation.

Tissue Invasion

Mechanism of the Major Toxin

  • α-Toxin Function:

    1. Causes pore formation in cell membranes

    2. Interrupts epithelial and endothelial integrity by breaching adherens junctions and compromising the cytoskeleton

Panton-Valentine Leukocidin (PVL)

  • Pore Formation: Induces damage in leukocyte membranes

  • Epidemiology: Found in less than 5% of S. aureus strains, prevalent in those causing necrotic skin lesions and severe pneumonia

  • Associations: Frequently linked with community-associated MRSA (CA-MRSA) infections, particularly affecting skin and soft tissue

Effects on Host Immunity

  • Immunosuppression Mechanisms:

    • Decrease in neutrophil-mediated killing

    • Reduced neutrophil activation and migration to the infection site

    • Impaired bacterial opsonization and phagocytosis

Small-Colony Variants (SCVs)

  • Description: Quasi-dormant, slow-growing bacteria linked to persistent infections

  • Characteristics:

    • Smaller colonies on agar plates

    • Quiescent metabolism

    • Reduced hemolytic and coagulase activities

    • Decreased carbohydrate utilization

    • Low virulence potential and increased antibiotic resistance

  • Emergence Triggers: Reactive oxygen species, low pH, cationic peptides, and nutrient limitation can induce SCVs

  • Clinical Relevance: Associated with cystic fibrosis patients

Summary of Virulence Factors

  • All Strains Include:

    • Leukocidin

    • Enzymes: Lipases, hyaluronidase, cytotoxins (α, β, γ, δ), enterotoxins (various types), exfoliative toxins, nucleases, proteases, collagenase

  • Mechanism: Convert local host tissues into nutrients for bacteria

  • Pyrogenic Toxin Superantigens: Bind major histocompatibility complex II, inducing severe clinical diseases via nonspecific T-cell activation

Types and Presentations of Staphylococcus aureus Infection

  • Common Infections:

    • Skin and Soft Tissue: Impetigo, folliculitis, furuncle, carbuncle, paronychia

    • Invasive: Pneumonia, osteomyelitis, septic arthritis, bacteremia, endocarditis, thrombophlebitis, deep tissue abscess

    • Toxin-Related: Classic food poisoning, toxic shock syndrome, scalded skin syndrome (Ritter disease)

Specific Presentations

  • Impetigo: Small erythematous area progressing to bullae that heal with honey-colored crusts

  • Folliculitis: Tender pustule at hair follicle

  • Furuncle: Abscesses exuding purulent material from a single opening, affecting skin and subcutaneous tissues

  • Carbuncle: Aggregate of furuncles with multiple pustular openings

  • Paronychia: Infection around fingernail starting as cellulitis, can progress to abscess

    • Acute: Frequently caused by S. aureus

    • Chronic: Usually of fungal origin

    • Treatment: Warm water soaks, surgery, antibiotics

  • Osteomyelitis: Sudden fever and painful limp in children, severe presentation may be subtle in neonates

    • Diagnosis: Blood cultures (positive in only 30-50% of pediatric cases) and culture of bone aspirate

    • Minimum treatment duration: 4-6 weeks

  • Septic Arthritis: Symptoms include decreased motion, warmth, tenderness of join, often absent in infants

    • Diagnosis: Joint fluid examination and culture

  • Endocarditis: Presents with fever, malaise, and potential peripheral emboli; diagnosis by blood culture

Imaging Examples

  • Findings in Endocarditis:

    • Large vegetation on mitral valve visible via echocardiography

Specific Infections by S. aureus

  • Pneumonia: Blood cultures more positive in secondary than primary cases; specimens should be adequate (e.g., lung tap, pleural fluid)

  • Thrombophlebitis: Affects IV catheter sites, diagnosed through blood cultures

  • Abscess Formation: Painful puss collections; S. aureus is the common cause; management includes drainage

  • Scalded Skin Syndrome (Ritter Disease): Fragile blisters with tender bases, may accompany fever; treatment with high-volume antibiotics

  • Toxic Shock Syndrome: Progresses rapidly with severe symptoms; involves T-cell stimulation and cytokine release leading to shock

Treatment Strategies

MSSA Treatment

  • Regimens:

    • Penicillinase-resistant penicillin (nafcillin, oxacillin)

    • First or second generation cephalosporins

    • Combination therapy with clindamycin

    • Other agents: fluoroquinolones, tetracycline, tigecycline, aminoglycosides

MRSA Treatment

  • Options:

    • Vancomycin, teicoplanin for bacteremia

    • Daptomycin

    • Linezolid and tedizolid (pneumonia)

    • Quinupristin/dalfopristin

    • Telavancin

    • Ceftaroline

    • Local antibiotics (mupirocin)

    • MecA Detection: Via PCR

Prevention and Control

  • Source: Skin and nasal colonization

  • Transmission: Hands, objects, surfaces

  • Prevention Strategies:

    • Hand hygiene

    • Use of mupirocin to reduce nasal colonization

  • Important Sites for Infection Control: Newborn nurseries, ICUs, operating theaters

Coagulase-Negative Staphylococci (CNS)

  • Species Examples:

    • S. epidermidis

    • S. haemolyticus

    • S. hominis

    • S. capitis

    • S. saprophyticus (novobiocin resistant)

    • S. warneri

    • S. xylosus

    • S. lugdunensis

Clinical Role

  • Characteristics: Opportunistic pathogens; biofilm producers on plastic surfaces (e.g., prosthetic appliances)

  • Associated Conditions: Catheter-associated sepsis, infections following immunosuppression

Staphylococcus lugdunensis

  • Key Features:

    • Coagulase-negative but produces bound coagulase

    • Causes SSTI, infective endocarditis (high mortality associated)

    • Treatment: Generally sensitive to oxacillin

Treatment Options for CNS Infections

  • Resistance: 50-60% oxacillin resistant; treat with:

    • Vancomycin, teicoplanin (glycopeptides)

    • Tigecycline, linezolid, tedizolid

    • Quinupristin/dalfopristin

Laboratory Diagnosis for Staphylococci

  • Methods:

    • Microscopy

    • Culture

    • Biochemical reactions (catalase, coagulase tests)

    • Automatic identification systems (e.g., ID32Staph, bioMerieux)

    • MALDI-TOF (mass spectrometry)