Chapter 18: The Gram-Positive and Gram-Negative Cocci of Medical Importance

Staphylococci: General Characteristics

Staphylococci are a diverse group of Gram-positive cocci commonly found on the skin and mucous membranes. They are important in both normal flora and as opportunistic pathogens.

• Habitat: Common inhabitant of skin and mucous membranes.

• Cell Morphology: Spherical cells arranged in irregular clusters.

• Gram Reaction: Gram-positive.

• Spores/Flagella: Lack spores and flagella.

• Capsules: May have capsules.

• Diversity: More than 40 species identified.

Staphylococcus aureus: Characteristics and Growth

Staphylococcus aureus is the most clinically significant species, known for its robust growth and resistance to environmental stress.

• Colony Morphology: Large, round, opaque colonies.

• Optimal Temperature: 37°C.

• Oxygen Requirement: Facultative anaerobe.

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

• Virulence: Produces many virulence factors.

Virulence Factors of S. aureus

S. aureus possesses a wide array of enzymes and toxins that contribute to its pathogenicity.

Epidemiology and Pathogenesis of S. aureus

S. aureus is widespread in human environments and can be carried asymptomatically.

• Reservoir: Present in most environments frequented by humans.

• Transmission: Readily isolated from fomites.

• Carriage Rate: 20-60% of healthy adults, mainly in anterior nares, skin, nasopharynx, intestine.

• Risk Factors: Poor hygiene, tissue injury, preexisting infection, diabetes, immunodeficiency.

• Antibiotic Resistance: Increase in community-acquired methicillin resistance (MRSA).

Staphylococcal Disease: Clinical Manifestations

Diseases caused by S. aureus range from localized skin infections to severe systemic illnesses.

Localized Cutaneous Infections

• Folliculitis and Hidradenitis: Superficial inflammation of hair follicles and glands; usually resolves but can progress.

• Furuncle: Boil; inflammation of hair follicle or sebaceous gland, progresses into abscess or pustule.

• Carbuncle: Larger, deeper lesion from aggregation of furuncles.

• Impetigo: Bubble-like swellings that break and peel away; common in newborns.

Systemic Infections

• Osteomyelitis: Infection in bone metaphysis; abscess formation.

• Bacteremia: Bacteria from other infected sites or medical devices; risk of endocarditis.

Toxigenic Diseases

• Food Intoxication: Ingestion of heat-stable enterotoxins; causes gastrointestinal distress.

• Staphylococcal Scalded Skin Syndrome: Toxin induces red flush, blisters, desquamation of epidermis.

• Toxic Shock Syndrome: Toxemia leading to shock and organ failure.

Other Important Staphylococci

Coagulase-negative staphylococci are significant in healthcare-associated and opportunistic infections, especially in immunocompromised patients.

• S. epidermidis: Skin and mucous membranes; endocarditis, bacteremia, UTI.

• S. hominis: Around apocrine sweat glands.

• S. capitis: Scalp, face, external ear.

• S. saprophyticus: Skin, intestine, vagina; UTI.

Identification of Staphylococcus

Laboratory identification is essential for diagnosis and treatment.

• Sample Sources: Pus, tissue exudates, sputum, urine, blood.

• Tests: Cultivation, catalase test, biochemical testing, coagulase test.

Key Laboratory Tests

• Catalase Test: Differentiates staphylococci (catalase-positive) from streptococci (catalase-negative).

• Coagulase Test: Differentiates S. aureus (coagulase-positive) from other staphylococci.

Identification Flowchart for Clinically Important Species of Staphylococcus

Clinical Concerns in Staphylococcal Infections

• Antibiotic Resistance: 95% produce penicillinase; resistant to penicillin and ampicillin.

• MRSA: Methicillin-resistant S. aureus; multiple drug resistance, some strains only sensitive to vancomycin.

• Treatment: Abscesses require surgical drainage; systemic infections need intensive therapy.

Treatment of Staphylococcal Infections

• S. aureus: Cephalexin, sulfa drugs, tetracyclines, clindamycin for less resistant forms.

• MRSA: Vancomycin, ceftaroline, linezolid, daptomycin; combination therapy to reduce resistance.

• VISA/VRSA: Quinupristin/dalfopristin as last resort.

Prevention of Staphylococcal Infections

• Universal Precautions: Healthcare providers must prevent nosocomial infections.

• Carrier Management: Nasal carriers may be restricted from sensitive hospital areas; treated with antimicrobials.

• Hygiene: Regular cleansing reduces risk.

• Vaccines: Vaccine against S. aureus in clinical trials.

Concepts Check

• Which test differentiates S. aureus from other Staphylococci species? Answer: D. Coagulase

General Characteristics of Streptococci and Related Genera

Streptococci are Gram-positive cocci, often found in chains or pairs, and are important pathogens in humans.

• Cell Morphology: Spherical/ovoid cocci in chains or pairs.

• Spores/Flagella: Non-spore-forming, nonmotile.

• Capsules/Slime Layers: Can form capsules and slime layers.

• Oxygen Requirement: Facultative anaerobes.

• Catalase: Do not form catalase; have peroxidase system.

• Colony Morphology: Small, nonpigmented colonies.

• Sensitivity: Sensitive to drying, heat, disinfectants.

Separation of Streptococcal Subgroups

Streptococci are classified by Lancefield groups (cell wall carbohydrates) and hemolysis patterns.

• Lancefield Groups: 20 groups (A, B, C, ...).

• Hemolysis: β-hemolytic: Groups A, B, C, G, some D strains α-hemolytic: S. pneumoniae, viridans group

Species of Streptococcus and Related Genera

Beta-Hemolytic Streptococci: Streptococcus pyogenes

S. pyogenes is the most serious streptococcal pathogen, strictly parasitic, inhabiting the throat, nasopharynx, and occasionally skin.

Virulence Factors of S. pyogenes

• Streptolysins: Hemolysins (SLO, SLS) cause cell and tissue injury.

• Erythrogenic toxin: Induces fever and red rash.

• Superantigens: Stimulate monocytes/lymphocytes, release necrotic factor.

• Streptokinase: Digests fibrin clots.

• Hyaluronidase: Breaks down connective tissue.

• DNase: Hydrolyzes DNA.

Epidemiology and Pathogenesis of S. pyogenes

• Reservoir: Humans only.

• Transmission: Contact, droplets, food, fomites.

• Portal of Entry: Skin or pharynx.

• Risk Group: Children are most affected.

• Complications: Systemic infections and sequelae if untreated.

Scope of Clinical Disease: S. pyogenes

Skin Infections

• Impetigo (pyoderma): Superficial, contagious crusted lesions; common in children.

• Erysipelas: Spreads to dermis/subcutaneous tissue; can be systemic.

Necrotizing Fasciitis (Flesh-Eating Disease)

• Rapid spread through subcutaneous tissue; severe disfigurement or death.

• Enzymes digest tissue; toxins poison skin layers; some act as superantigens.

Concepts Check

• Which of the following is NOT a possible complication of strep throat? Answer: D. Otitis media

Additional info:

• Tables and flowcharts have been expanded for clarity and completeness.

• Images referenced in the slides are described in text for study purposes.