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

Staphylococci

General Characteristics

Staphylococci are a group of Gram-positive cocci commonly found on the skin and mucous membranes. They are arranged in irregular clusters and are important opportunistic pathogens.

• Gram-positive cocci

• Arranged in irregular clusters

• Common inhabitant of skin and mucous membranes

Staphylococcus aureus

S. aureus is the most clinically significant species, known for its virulence and resistance to antibiotics.

• Beta-hemolysis on blood agar due to α-toxin

• Transmission: direct contact

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

• Risk factors: poor hygiene, tissue injury, diabetes, immunodeficiency

• Increasing prevalence of MRSA (methicillin-resistant S. aureus)

Cutaneous Lesions of S. aureus

Cutaneous infections range from superficial to deep tissue involvement.

• Folliculitis: Superficial inflammation of hair follicle

• Furuncle: Boil; deeper inflammation of hair follicle or sebaceous gland

• Carbuncle: Aggregation of furuncles forming a larger, deeper lesion

• Impetigo: Bubble-like swellings, common in newborns

Staphylococcal Disease: Toxigenic Forms

Toxigenic diseases are caused by toxins produced by S. aureus.

• Food intoxication: Ingestion of heat-stable enterotoxins causing 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 infections, especially in immunocompromised patients.

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

• S. hominis: Apocrine sweat glands

• S. capitis: Scalp, face, external ear

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

Identification of Staphylococcus

Laboratory identification relies on biochemical tests.

• Catalase test: Add H2O2; positive result indicates staphylococci

• Coagulase test: Plasma inoculation; positive result (clumping) for S. aureus

• Agglutination test: Antibodies added to bacteria; positive result indicates specific staphylococcal antigens

Clinical Concerns in Staphylococcal Infections

• 95% produce penicillinase; resistant to penicillin and ampicillin

• MRSA: Multiple drug resistance; some strains only sensitive to vancomycin

• Abscesses often require surgical drainage

• Systemic infections need intensive therapy

Treatment of Staphylococcal Infections

• S. aureus: Cephalexin, sulfa drugs, tetracyclines, clindamycin

• MRSA: Vancomycin, ceftaroline, linezolid, daptomycin (often in combination)

• VISA/VRSA: Quinupristin/dalfopristin

Prevention of Staphylococcal Infections

• Universal precautions in healthcare settings

• Exclusion of carriers from high-risk hospital areas

• Carrier treatment: bactroban, dicloxacillin

• Hygiene and cleansing

• Vaccine in clinical trials

Streptococci and Related Genera

General Characteristics

Streptococci are Gram-positive cocci arranged in long chains, non-spore-forming, nonmotile, and facultative anaerobes. They can form capsules and slime layers.

• Gram-positive, long chains

• Non-spore-forming, nonmotile

• Capsules and slime layers possible

• Facultative anaerobes

Separation of Streptococcal Subgroups

Streptococci are classified by hemolysis type and Lancefield grouping.

• Beta-hemolytic: Complete hemolysis (e.g., S. pyogenes)

• Alpha-hemolytic: Partial hemolysis (e.g., S. pneumoniae)

Table: Major Species of Streptococcus and Related Genera

Species	Lancefield Group	Hemolysis Type	Habitat	Pathogenicity to Humans
S. pyogenes	A	Beta (β)	Human throat	Skin, throat infections, scarlet fever
S. agalactiae	B	β	Human vagina, cow udder	Neonatal, wound infections
Enterococcus faecalis	D	α, β, N	Human, animal intestine	Endocarditis, UTI
E. faecium, E. durans	D	Alpha (α)	Human, animal intestine	Similar to E. faecalis
S. bovis	D	N	Cattle	Subacute endocarditis, bacteremia

Beta-Hemolytic Streptococci: Streptococcus pyogenes

S. pyogenes is the most serious streptococcal pathogen, a strict parasite inhabiting the throat, nasopharynx, and occasionally skin.

Virulence Factors of S. pyogenes

• Surface antigens: C-carbohydrates (protect against lysozyme), fimbriae (adherence), M-protein (resistance to phagocytosis), hyaluronic acid capsule (no immune response), C5a protease (hinders complement and neutrophil responses)

• Major extracellular toxins:

  • Streptolysins: hemolysins causing cell and tissue injury

  • Erythrogenic toxin (pyrogenic): causes scarlet fever, red rash

  • Superantigens: release tissue necrotic factor, cytokines

• Extracellular enzymes:

  • Streptokinase: digests fibrin clots

  • Hyaluronidase: breaks down connective tissue

  • DNase: hydrolyzes DNA

Epidemiology and Pathogenesis of S. pyogenes

• Humans are the only reservoir

• Unapparent carriers

• Transmission: mainly airborne (droplets), contact, food, fomites

• Portal of entry: skin or pharynx

• Children mainly affected for cutaneous and throat infections

• Systemic infections and sequelae possible if untreated

Scope of Clinical Disease

Skin Infections

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

• Erysipelas: Pathogen enters broken skin, spreads to dermis and subcutaneous tissues; can become systemic

• Necrotizing fasciitis: Enzymes digest connective tissue, toxins poison epidermal and dermal tissue

Throat Infections

• Streptococcal pharyngitis: Strep throat

Systemic Infections

• Scarlet fever: Strain carrying prophage for erythrogenic toxin

• Septicemia

• Pneumonia

• Streptococcal toxic shock syndrome

Long-Term Complications of Group A Infections

• Rheumatic fever: Carditis, valve damage, arthritis, chorea, fever

• Acute glomerulonephritis: Nephritis, increased blood pressure, heart failure, possible chronic kidney failure

Diagnosis of Group A Infections

• Rapid diagnostic tests: monoclonal antibodies reacting with C-carbohydrates

Treatment and Prevention of Streptococcal Infections

• Group A: treated with penicillin

• Long-term penicillin prophylaxis for history of rheumatic fever or recurrent strep throat

Streptococcus pneumoniae: The Pneumococcus

General Characteristics

• Major cause of bacterial pneumonias, meningitis, and otitis media

• Cells arranged in pairs and short chains

• Culture requires blood or chocolate agar, growth improved by 5-10% CO2

• Lack catalase and peroxidases; cultures die in O2

Epidemiology and Pathology

• Transmission: airborne

• 5-50% carry as normal flora in nasopharynx; infections usually endogenous

• Induces overwhelming inflammatory response

• Can access middle ear

Laboratory Cultivation and Diagnosis

• Specimens: blood, sputum, pleural fluid, spinal fluid

• Gram stain: presumptive identification

• Quellung test: Serological test for capsular swelling (antibodies on capsule surface)

• α-hemolytic, optochin sensitivity, bile solubility, inulin fermentation

Treatment and Prevention

• Treated with penicillin G or V

• Increased drug resistance noted

• Two vaccines for high-risk individuals:

  • Capsular antigen vaccine (older adults, effective 5 years)

  • Conjugate vaccine (children 2-23 months)

The Family Neisseriaceae: Gram-Negative Cocci

General Characteristics

• Residents of mucous membranes of warm-blooded animals

• Two primary human pathogens: Neisseria gonorrhoeae, Neisseria meningitidis

Genus Neisseria

• Gram-negative, bean-shaped, diplococci

• No flagella or spores

• Capsules on pathogens, fimbriae for adherence

• Strict parasites; do not survive long outside host

• Aerobic or microaerophilic

• Produce catalase and cytochrome oxidase

• Pathogenic species require enriched complex media

Neisseria meningitidis: The Meningococcus

• Transmission: airborne

• Human reservoir: nasopharynx (10% of adults)

• High risk: closed populations (military, college), children 6mo-3yr, young adults 10-20yr

• Attaches to nasopharynx cells, enters bloodstream

• Meningococcemia: intravascular coagulation, tissue destruction, shock

• Meningitis symptoms: fever, sore throat, headache, vomiting, stiff neck, rapid progression to coma/death

Epidemiology and Pathogenesis

• Endotoxins cause hemorrhage, coagulation, vascular damage, necrosis (especially extremities)

• Petechiae: crops of lesions on trunk and appendages (in ~50% cases)

Clinical Diagnosis

• Gram stain of CSF, blood, nasopharyngeal sample

• Cultures for differentiation

• Rapid tests for capsular polysaccharide

Immunity, Treatment, and Prevention

• Natural antibodies effective in most adults

• Preliminary diagnosis based on symptoms; treatment started before culture results

• Treated with penicillin G, cephalosporin

• Prophylactic treatment for close contacts

• Prevention: vaccine

Additional info: These notes cover the major Gram-positive and Gram-negative cocci of medical importance, focusing on their characteristics, pathogenic mechanisms, clinical manifestations, laboratory identification, treatment, and prevention. The included table summarizes key species of Streptococcus and related genera for quick reference.