Gram-Positive Cocci Overview

Study Notes

Gram-Positive Cocci

Gram-positive Cocci: A group of bacteria characterized by their spherical shape and the ability to retain crystal violet dye during the Gram stain.
Common Types: Staphylococcus, Streptococcus, Enterococcus

Staphylococcus

Staphylococcus: Gram-positive cocci that grow in grape-like clusters; they are commonly found on skin and mucosal membranes.
Staphylococcus aureus: The most pathogenic staphylococcal species; responsible for various infections, including toxic shock syndrome, abscesses, endocarditis, food poisoning, pneumonia, surgical wound infections, and sepsis.
- Habitat: Human nose and skin.
- Transmission: Via hands.
- Diagnosis: Gram-positive cocci in clusters, coagulase-positive, catalase-positive, often produces β-lactamase.
- Predisposing factors to infection: Breaks in the skin, foreign bodies, low neutrophil levels, intravenous drug use, tampon use.
Staphylococcus epidermidis: A commensal organism found on human skin and mucous membranes; can cause infections in individuals with compromised immune systems or after implantation of prosthetic devices.
- Diseases: Endocarditis (especially on prosthetic heart valves), prosthetic hip infections, intravascular catheter infections, cerebrospinal fluid shunt infections, neonatal sepsis.
- Habitat: Normal flora of human skin and mucous membranes.
- Transmission: Patient's own strains or from person to person via hands.
- Diagnosis: Gram-positive cocci in clusters, coagulase-negative, catalase-positive.
Staphylococcus saprophyticus: A species that commonly causes urinary tract infections in young women.
- Diagnosis: Gram-positive cocci in clusters, coagulase-negative, resistant to novobiocin.

Streptococcus

Streptococcus: Gram-positive cocci that typically grow in chains; classified into different groups (A, B, D, Viridans) based on their cell wall carbohydrate antigenicity.
Streptococcus pyogenes (Group A): Responsible for various infections, including pharyngitis, cellulitis, rheumatic fever, and acute glomerulonephritis.
- Habitat: Human throat and skin.
- Transmission: Via respiratory droplets.
- Diagnosis: Gram-positive cocci in chains, β-hemolytic, catalase-negative, bacitracin-sensitive.
Streptococcus agalactiae (Group B): A significant cause of neonatal meningitis and sepsis.
- Habitat: Human vagina.
- Transmission: During birth.
- Diagnosis: Gram-positive cocci in chains, β-hemolytic, catalase-negative, bacitracin-resistant.
Streptococcus pneumoniae: A common cause of pneumonia, meningitis, otitis media, and sinusitis.
- Habitat: Human upper respiratory tract.
- Transmission: Via respiratory droplets.
- Diagnosis: Gram-positive "lancet-shaped" cocci in pairs (diplococci), β-hemolytic under anaerobic conditions, catalase-negative, sensitive to bile and optochin, notable polysaccharide capsule; one of three classical encapsulated pyogenic bacteria (along with Neisseria meningitidis and Haemophilus influenzae).
Viridans Group Streptococci (e.g., S. sanguis, S. mutans): Often found in the human mouth; associated with endocarditis and brain abscesses (typically in mixed infections). S. mutans contributes to dental caries.
- Habitat: Human oropharynx.
- Transmission: Organism enters blood during dental procedures.
- Diagnosis: Gram-positive cocci in chains, α-hemolytic, catalase-negative, resistant to bile and optochin.

Enterococcus

Enterococcus faecalis: A significant cause of urinary tract and biliary tract infections, including endocarditis, which can be life-threatening. It's found in the human colon, urethra, and female genital tract.
- Habitat: Human colon, urethra, and female genital tract.
- Transmission: May enter the bloodstream during procedures in the gastrointestinal or genitourinary tracts, leading to infections in various locations like endocarditis.
- Diagnosis: Gram-positive cocci in chains, catalase-negative.