Gram-Positive Cocci Overview

Gram-Positive Cocci

• Gram-positive cocci are medically important bacteria
• Two medically important genera are Staphylococcus and Streptococcus
• Staphylococcus and Streptococcus are nonmotile and do not form spores
• These bacteria are distinguished by microscopic and biochemical criteria

Staphylococcus

• Microscopic Appearance: Staphylococci appear in grape-like clusters
• Biochemical Properties: All staphylococci produce catalase
• Virulence Factor: Catalase degrades hydrogen peroxide, important for virulence
• Important Human Pathogens: S. aureus, S. epidermidis, S. saprophyticus

Staphylococcus Aureus

• Diseases caused by S. aureus include abscesses, pyogenic infections (e.g., endocarditis, septic arthritis, osteomyelitis), food poisoning, scalded skin syndrome, and toxic shock syndrome.
• Common Causes: Hospital-acquired pneumonia, septicemia, surgical-wound infections, skin and soft tissue infections (folliculitis, cellulitis, impetigo), and conjunctivitis
• Transmission: Human's bodies are the reservoir. The nose is the main site of colonization for S. aureus and approximately 30% of people are colonized. The skin, particularly of hospital personnel and patients, is also a common site of S. aureus colonization. Hand contact is an important mode of transmission.
• Pathogenesis: Toxigenic (superantigen) and pyogenic (abscess) infections. Local and disseminated infections. (refer to subsequent slides for specific toxins)
• Clinical findings: Pyogenic and toxin-mediated. Pyogenic infections include abscess, impetigo, furuncles, cellulitis, folliculitis, conjunctivitis, eyelid infections, and septicemia (originates from any localized lesions). Toxin-mediated diseases include food poisoning, toxic shock syndrome, and scalded skin syndrome.
• Treatment: Mostly resistant to penicillin G. Treated with β-lactamase-resistant penicillins (e.g., nafcillin or cloxacillin), some cephalosporins, or vancomycin. Combination therapy using a β-lactamase-sensitive penicillin (e.g., amoxicillin) and a β-lactamase inhibitor (e.g., clavulanic acid) is also useful. Methicillin-resistant S. aureus (MRSA) or nafcillin-resistant S. aureus (NRSA) strains require vancomycin.
• Prevention: Reduce time of tampon use and refrigerate food. Maintain cleanliness and handwashing to reduce nasal carriage. Reduce IV drug use. Handle IV catheters promptly.

Staphylococcus Epidermidis

• Diseases caused by S. epidermidis include prosthetic valve endocarditis and prosthetic joint infections, central nervous system shunt infections, and cause of sepsis in newborns
• Treatment: Penicillins. Prolonged treatment with penicillin.

Staphylococcus Saprophyticus

• Diseases caused by S. saprophyticus: urinary tract infections, especially cystitis
• Treatment: Penicillins.

Streptococcus

• Important Properties: Family of G+ve, nonmotile, non-spore forming, catalase-negative cocci
• Common Facultative Anaerobic Cocci: The most common facultative anaerobic cocci
• Hemolysis: Divided into three groups based on their ability to induce hemolysis on blood agar: α-hemolytic, β-hemolytic, γ-hemolytic

Streptococcus Pyogenes

• Diseases caused by S. pyogenes: Leading bacterial cause of pharyngitis, cellulitis, impetigo, necrotizing fasciitis, streptococcal toxic shock syndrome, rheumatic fever, and acute glomerulonephritis (AGN)
• Transmission: Part of the normal flora of the throat, skin, and intestines, but produce disease when accessing tissues or blood. Found in the pharynx in small numbers, but also skin.
• Pathogenesis: Pyogenic, toxigenic, immune-mediated (post-streptococcal)
• Clinical findings: Pharyngitis (sore throat, inflamed throat, and tonsils), Otitis media, mastoiditis, sinusitis, and meningitis
• Treatment: Penicillin G or Amoxicillin. Penicillin V for mild cases, Erythromycin or Azithromycin for penicillin-allergic patients.

Streptococcus Agalactiae

• Diseases caused by S. agalactiae: Leading cause of neonatal sepsis and meningitis
• Transmission: Found in the vagina and colon

Streptococcus Pneumoniae

• Important Properties: Gram-positive lancet-shaped cocci, arranged in pairs (diplococci) or short chains
• Diseases caused by S. pneumoniae: Pneumonia, bacteremia, meningitis, otitis media, mastoiditis, sinusitis
• Transmission: Part of the normal flora in the oropharynx
• Pathogenesis: Capsular polysaccharide, lipoteichoic acid, pneumolysin
• Clinical findings: Sudden chill, fever, cough, pleuritic pain, red or brown sputum (“rusty” sputum). Bacteremia occurs in 15-25% of cases. Spontaneous recovery in 5-10 days. Leading cause of sepsis in patients without a functional spleen.
• Treatment: Penicillins and Erythromycin. In penicillin-allergic patients, Erythromycin or Azithromycin (long-acting derivatives) are used. In severe cases, penicillin G is preferred; in mild cases, oral penicillin V might suffice. Vancomycin is used for penicillin-resistant pneumococci. Ceftriaxone or levofloxacin for less severely ill patients.
• Prevention: Vaccination with 13-valent pneumococcal conjugate vaccine (important for immunocompromised and young children). Unconjugated 23-valent pneumococcal vaccine for healthy adults age 50+