Gram-Positive Cocci Overview

Study Notes

Gram-Positive Cocci

Gram-positive cocci have a high peptidoglycan content and low lipid in their cell walls.
They are common inhabitants of skin and mucous membranes.
Infections often result from direct contact.
Pyogenic infections (pus formation) can be caused by staphylococcal infections.
Clinically significant genera include Staphylococcus (catalase-positive) and Streptococcus (catalase-negative).

Stomatococcus mucilaginosus

The only member of the genus Stomatococcus.
Recently reclassified as Rothia mucilaginosus.
Normal flora of the respiratory tract.
Cannot grow in the presence of 5% NaCl.
Weakly catalase-positive, coagulase-negative, and has a capsule.
Can be an emerging pathogen in immunocompromised individuals.

Staphylococcus

Common isolates include S. aureus, S. epidermidis, and S. saprophyticus.
Gram-positive cocci, catalase-positive.
Nonmotile and non-spore-forming bacteria.
Facultative anaerobes.
Modified oxidase-negative.
Reduce nitrates to nitrites.
Grow in 7.5% to 10% NaCl (mannitol salt agar).
Susceptible to lysostaphin, resistant to bacitracin.

Staphylococcus aureus (SAU)

Virulence factors:
- Surface structures: Protein A, capsular polysaccharide, peptidoglycan, teichoic acid
- Enzymes: Catalase, bound coagulase (clotting factor), free coagulase (tube coagulase), staphylokinase, lipase, hyaluronidase, DNase, β-lactamase
- Toxins: Exfoliatin, leukocidin, hemolysins, enterotoxins, superantigens (TSST-1).
Diseases:
- Local abscesses (folliculitis, furuncles, carbuncles, sty)
- Focal suppuration (osteomyelitis, pneumonia, meningitis, empyema, endocarditis, sepsis, impetigo)
- Exotoxin elaboration (food poisoning, scalded skin syndrome, toxic shock syndrome)

Staphylococcus epidermidis

Virulence factor: Biofilm production, adherence to surfaces of prosthetic devices.
Gram-positive cocci, catalase-positive, coagulase-negative, and DNAse-negative.
White and creamy, nonhemolytic growth on blood agar plates (BAP).
Grows on Columbia Nalidixic Agar (CNA).
Does not ferment mannitol on mannitol salt agar.
Susceptible to novobiocin.
Commonly associated with hospital-acquired infections (nosocomial).

Staphylococcus saprophyticus

Primarily causes urinary tract infections (UTIs) in sexually active women.
Associated with catheter-associated UTIs.
Gram-positive cocci, catalase-positive, coagulase-negative, and DNAse-negative.
Nonhemolytic growth on blood agar plates (BAP).
Mannitol salt agar fermentation varies.
Resistant to novobiocin (zone of inhibition ≥16mm is considered susceptible).

Methicillin-Resistant Staphylococcus aureus (MRSA)

Methicillin-resistant S. aureus (MRSA) strains are resistant to beta-lactam antibiotics.
Oxacillin, cloxacillin, and methicillin are common drugs used to treat MRSA infections.

Description

This quiz explores the characteristics and clinical significance of gram-positive cocci, including the genera Staphylococcus and Streptococcus. It covers their role in infections, normal flora, and the specific features of Stomatococcus mucilaginosus. Test your knowledge on these important pathogens!