In-depth Notes on Gram-Positive and Gram-Negative Cocci

Gram-Positive and Gram-Negative Cocci of Medical Importance

Staphylococci General Characteristics

• Common inhabitant of the skin and mucous membranes.
• Spherical cells arranged in irregular clusters.
• Gram-positive, meaning they retain violet stain after Gram staining.
• Lack spores and flagella.
• May have capsules.
• More than 40 species exist.

Staphylococcus aureus

Characteristics

• Grows in large, round, opaque colonies.
• Optimum temperature: 37^ ext{°C}.
• Facultative anaerobe: can grow with or without oxygen.
• Well adapted to high salt conditions, extremes in pH, and high temperatures.
• Produces numerous virulence factors important in pathogenesis.

Virulence Factors

Epidemiology and Pathogenesis

• Presence in most environments frequented by humans.
• Readily isolated from fomites (objects likely to carry infection).
• Healthy adult carrier rate is 20 ext{-}60 ext{%}.
• Predominantly colonizes the anterior nares, skin, nasopharynx, and intestine.
• Higher infection risk due to poor hygiene, nutrition, tissue injury, preexisting infections, diabetes, and immunodeficiency.
• Community-acquired methicillin resistance (MRSA) has increased.

Staphylococcal Diseases

• Localized infections:

  • Invade skin through wounds, follicles, or glands.
  • Folliculitis, hidradenitis lead to inflammation or can progress to suppuration.
  • Furuncle (boil) involves hair follicle or sebaceous gland.
  • Carbuncle is deeper, connected cluster of furuncles.
  • Impetigo appears as bubble-like swellings, mostly in newborns.

• Miscellaneous Systemic Infections:

  • Osteomyelitis: infection in metaphysis leading to abscess.
  • Bacteremia: bacteria from infected sites or medical devices; can lead to endocarditis.

• Toxigenic Diseases:

  • Food intoxication from heat-stable enterotoxins leads to gastrointestinal distress.
  • Staphylococcal scalded skin syndrome results in red flush and skin desquamation.
  • Toxic shock syndrome from toxemia can cause shock and organ failure.

Other Important Staphylococci

• Coagulase-negative staphylococci are significant in healthcare-associated infections in immunocompromised patients:
  • S. epidermidis: endocarditis, bacteremia, UTI.
  • S. hominis: associated with apocrine sweat glands.
  • S. capitis: found on scalp and external ear.
  • S. saprophyticus: infrequent in skin; causes UTI.

Identification of Staphylococcus

• Isolated from pus, tissue exudates, sputum, urine, and blood.
• Techniques used:
  • Cultivation (culture media), catalase, biochemical testing, coagulase testing.

Clinical Concerns in Staphylococcal Infections

• 95% produce penicillinase, indicating resistance to penicillin and ampicillin.
• MRSA exhibits resistance to many major drugs; treatment often requires combinations of remaining effective drugs such as vancomycin.
• Abscesses need surgical intervention; systemic infections provoke lengthy therapies.

Treatment of Staphylococcal Infections

• Treatment selection based on culture and sensitivity testing:
  • Non-resistant S. aureus: cephalexin, sulfa drugs, tetracyclines, clindamycin.
  • MRSA: vancomycin, ceftaroline, linezolid, daptomycin (often in combination).
  • VISA and VRSA: quinupristin/dalfopristin last resort.

Prevention of Staphylococcal Infections

• Universal precautions in healthcare settings.
• Carriers may be treated with a combination of antimicrobials.
• Hygiene and cleansing practices are crucial.
• Vaccine development against S. aureus is in clinical trials.

General Characteristics of Streptococci

• Gram-positive spherical/ovoid cocci in long chains or pairs.
• Non-spore forming and nonmotile.
• Capable of forming capsules and slime layers.
• Exhibit a peroxidase system but do not produce catalase.
• Sensitive to drying, heat, and disinfectants.

Separation of Streptococcal Subgroups

• Lancefield classification based on cell wall carbohydrates: 20 groups (A, B, C, …).
• Hemolytic classification:
  • β-hemolytic: groups A, B, C, G, and some D strains.
  • α-hemolytic: S. pneumoniae and viridans streptococci.

Species of Streptococcus and Related Genera

Virulence Factors of S. pyogenes

• C-carbohydrate: polysaccharides protect from lysozyme.
• M-protein: resist phagocytosis and aid adherence (over 80 subtypes).
• Capsule: protects from immune response.
• Streptolysins: break down red blood cells and tissues.

Epidemology and Pathogenesis of S. pyogenes

• Humans are the only significant reservoir.
• Transmitted via contact, droplets, food, and fomites.
• Generally impacts children for skin and throat infections; untreated infections can lead to serious complications.
• High risk for necrotizing fasciitis.

Group B Streptococcus S. agalactiae

• Regularly resides in the human vagina and can cause severe neonatal infections during delivery. Screening in pregnant women is essential.

Gram-Negative Cocci: Neisseria

• Includes Neisseria gonorrhoeae (gonorrhea) and Neisseria meningitidis (meningitis).
• Gram-negative, bean-shaped, diplococci with no flagella/spores, but can recruit capsules and pili for adherence.

Neisseria gonorrhoeae - Gonococcus

• Causes gonorrhea: prevalent sexually transmitted infection.
• Factors contributing to pathogenicity: fimbriae and IgA protease.
• Transmission is human-to-human, with high risks in certain populations, notably sexually active adults.

Neisseria meningitidis - Meningococcus

• Virulence factors: capsule, fimbriae, IgA protease, endotoxins.
• Causes widespread meningitis; high risk in crowded living conditions and certain age groups.
• Rapid disease onset with serious complications possible.