Gram-Positive Cocci of Medical Importance: Staphylococcus, Streptococcus, and Enterococcus

Gram-Positive Cocci of Medical Importance

Overview

Gram-positive cocci are a significant group of bacteria in clinical microbiology, including common normal flora and important human pathogens. They are characterized by their spherical shape, Gram-positive cell wall, and lack of endospore formation. Many infections caused by these organisms are pyogenic, meaning they stimulate pus formation.

• Common genera: Staphylococcus, Streptococcus, Enterococcus

• Clinical importance: Range from mild skin infections to life-threatening systemic diseases

• Notable feature: Many are part of the normal human flora but can become opportunistic pathogens

Staphylococcus

General Characteristics

Staphylococcus species are Gram-positive cocci that typically form clusters. They are catalase positive, non-motile, and may possess capsules. These bacteria are common inhabitants of the skin and mucous membranes.

• Arrangement: Clusters (staphyle)

• Catalase test: Positive (distinguishes from Streptococcus)

• Flagella: Absent

• Capsules: May be present

• Species: Over 45 identified

Staphylococcus aureus

Staphylococcus aureus is the most clinically significant species, known for its golden-yellow colonies and numerous virulence factors. It is a facultative anaerobe and can withstand harsh environmental conditions.

• Colony morphology: Large, round, opaque, golden yellow on certain media

• Growth temperature: Optimum at 37°C (mesophile)

• Resistance: Withstands high salt, extremes in pH, and high temperatures

• Virulence factors: Capsule/slime layer, Protein A, lipases, coagulase, cytolysins, exfoliative toxins, enterotoxins, toxic shock syndrome toxin

Virulence Factors

• Glycocalyx: Capsule or slime layer for evasion of host defenses

• Protein A: Binds Fc region of IgG, interfering with opsonization

• Coagulase: Promotes clot formation, protecting bacteria from immune cells

• Cytolysins: Damage cell membranes (e.g., alpha, beta, gamma, delta toxins, PV leukocidin)

• Exfoliative toxins: Proteases that damage skin proteins

• Enterotoxins & Toxic Shock Syndrome Toxin: Superantigens causing systemic effects

Epidemiology and Pathogenesis

• Reservoir: Anterior nares (nostrils) of healthy adults (carriage rate ~60%)

• Transmission: Fomites, direct contact

• Predisposing factors: Poor hygiene, injury, diabetes, immunodeficiency

• Antibiotic resistance: >95% produce beta-lactamases; MRSA (mecA gene), VISA/VRSA emerging

Staphylococcal Diseases

Localized Cutaneous Infections

• Folliculitis: Inflammation of hair follicle (stye if on eyelid)

• Furuncle: Boil; abscess formation

• Carbuncle: Larger, deeper lesion formed by cluster of furuncles

• Impetigo: Pus-filled lesions on reddened base

• Mastitis: Infection of breast during lactation

Systemic Infections

• Osteomyelitis: Bone infection, especially in children and adults (vertebrae)

• Endocarditis: Infection of heart valves

• Pneumonia, septic arthritis

• Treatment: Long-term antibiotics (e.g., clindamycin, TMP-SMX)

Toxigenic Diseases

• Food intoxication: Ingestion of heat-stable enterotoxins; rapid onset of GI symptoms

• Staphylococcal scalded skin syndrome: Exfoliative toxin causes skin desquamation

• Toxic shock syndrome: Superantigen-mediated toxemia, shock, organ failure

Other Staphylococci

• S. epidermidis: Normal flora; can cause endocarditis, bacteremia, UTI

• S. saprophyticus: Second most common cause of UTIs in young women

• S. lugdunensis, S. hominis, S. haemolyticus, S. capitis: Opportunistic infections

Identification of Staphylococcus

• Sample types: Pus, tissue exudates, sputum, urine, blood

• PCR: For specific DNA or resistance genes

• Cultivation: Blood agar, mannitol salt agar

• Catalase test: Positive for Staphylococcus

• Coagulase test: Positive for S. aureus

Streptococcus

General Characteristics

Streptococcus species are Gram-positive cocci that form chains. They are catalase negative, non-motile, and often require enriched media for growth. Many are sensitive to drying and disinfectants.

• Arrangement: Chains

• Catalase test: Negative (distinguishes from Staphylococcus)

• Capsules/slime layers: May be present

• Colony morphology: Small, non-pigmented

Classification of Streptococcus

• Hemolysis on blood agar:

  • Alpha (α): Partial (greenish, "bruising")

  • Beta (β): Complete (clear zone)

  • Gamma (γ): None

• Lancefield grouping: Based on cell wall antigens (Groups A, B, C, etc.)

Streptococcus pyogenes (Group A Streptococcus, GAS)

• Hemolysis: Beta-hemolytic

• Habitat: Throat, nasopharynx, occasionally skin

• Strict parasite: Humans only reservoir

Virulence Factors

• C-carbohydrates: Protect from lysozyme

• Fimbriae: Adherence to surfaces

• M-protein: Resists phagocytosis, blocks complement

• Hyaluronic acid capsule: Mimics host tissue

• C5a protease (ScpA): Inactivates complement components

• Streptolysins (O, S): Damage cell membranes

• Streptococcal pyrogenic exotoxins (SPE): Induce fever, rash

• Streptokinase: Digests clots

• Hyaluronidase: Breaks down connective tissue

• DNAse: Degrades DNA in neutrophil extracellular traps (NETs)

Epidemiology and Pathogenesis

• Transmission: Contact, droplets, food, fomites

• Portal of entry: Skin or pharynx

• Population affected: Mostly children

• Carriers: Up to 15% inapparent carriers

Clinical Diseases Caused by Streptococcus pyogenes

Skin Infections

• Impetigo: Contagious, crusted lesions

• Erysipelas: More invasive, well-demarcated borders

• Cellulitis: Deeper, less defined borders

• Necrotizing fasciitis: "Flesh-eating bacteria"; rapid tissue destruction, high mortality

Throat Infections

• Streptococcal pharyngitis (strep throat): Sore throat, fever, red pharynx

Systemic Diseases

• Scarlet fever: SPE toxin causes fever, sandpapery rash

• Septicemia, pneumonia, toxic shock syndrome

Sequelae (Post-infection Complications)

• Rheumatic fever: Autoimmune reaction after pharyngitis; affects heart, joints, CNS

• Acute glomerulonephritis: Immune complex deposition in kidneys; can lead to chronic renal failure

Group B Streptococcus: Streptococcus agalactiae

• Hemolysis: Beta-hemolytic

• Habitat: Vagina, throat, large intestine (25% carriage)

• Clinical importance: Severe neonatal infections (pneumonia, sepsis, meningitis)

• Prevention: Screening pregnant women (weeks 35-37), intrapartum antibiotics

Identification of Beta-Hemolytic Streptococcus spp.

• Rapid diagnostic tests: Monoclonal antibodies for Lancefield groups

• Cultural tests: Bacitracin sensitivity (GAS +), CAMP test (GBS +)

Treatment and Prevention

• Groups A and B: Penicillin (some reduced sensitivity emerging)

• Alternatives: Used for penicillin-allergic patients

• Long-term prophylaxis: For history of rheumatic fever or recurrent strep throat

Alpha-Hemolytic Streptococci: Viridans Group

• Species: S. mutans, S. oralis, S. salivarius, others

• Habitat: Oral cavity, nasopharynx, genital tract, skin

• Diseases: Dental caries, tooth abscesses, subacute endocarditis (especially in heart disease)

Streptococcus pneumoniae (Pneumococcus)

• Shape: Lancet-shaped diplococci

• Hemolysis: Alpha-hemolytic

• Virulence factor: Capsule (90 types)

• Diseases: Pneumonia, meningitis, otitis media

• Diagnosis: Gram stain, Quellung reaction, optochin sensitivity, bile solubility

• Prevention: Vaccines (23-valent polysaccharide, 13-valent conjugate)

Enterococci

General Characteristics

• Lancefield Group D: Enterococcus faecalis, E. faecium, E. durans

• Habitat: Normal flora of large intestine

• Diseases: Opportunistic UTIs, wound, and skin infections (especially in debilitated patients)

• Antibiotic resistance: Increasing, second most common nosocomial infection after Staphylococci

• Hemolysis: Can be alpha, beta, or gamma

Identification of Enterococci

• Colony morphology: Usually large colonies

• Bile esculin test: Positive for Enterococcus

• Optochin resistance: Enterococcus resistant

• PYR test: Production of L-pyrrolidonyl arylamidase (cherry red positive)