Faculty of Dental Medical: Gram Positive Cocci Lecture 9

Gram-Positive Cocci: Staphylococcus

• There are over 40 species of Staphylococcus, but only three are medically important: S. aureus, S. epidermidis, and S. saprophyticus.
• Staphylococcus are Gram-positive cocci that arrange in pairs or clusters (grape-like).
• They are facultative anaerobes and can grow on simple media.
• They are catalase positive, which means they can break down hydrogen peroxide (H2O2) into water and oxygen.

Staphylococcus aureus

• Morphology: Gram-positive cocci arranged in clusters, with some strains being capsulated.
• Culture: Can grow in nutrient agar and blood agar at 37°C for 24 hours, forming large, yellow-golden colonies.
• They are β-hemolytic on blood agar.
• They can ferment mannitol sugar in mannitol salt agar, producing acid.

Virulence Factors of S. aureus

• Surface adhesions: allow the bacteria to adhere to host tissues and form biofilms, providing protection from host defenses and antimicrobial agents.
• Coagulase: an enzyme that converts fibrinogen into fibrin, creating a protective fibrin clot around the bacteria, shielding it from phagocytosis.
• Leukocidin: destroys white blood cells.
• Catalase enzyme: breaks down hydrogen peroxide (H2O2) produced by macrophages, allowing the bacteria to survive.
• Nucleases: DNase and RNase.
• Protein A: binds to antibodies, preventing the killing of bacteria and allowing S. aureus to evade the immune response.
• β-lactamase: breaks down β-lactam antibiotics.
• Exotoxins:
  • Enterotoxin: causes food poisoning.
  • Exfoliative toxin: causes scalded skin syndrome (SSSS).
  • Toxic shock syndrome toxin (TSST): can cause toxic shock syndrome.

Diseases Caused by S. aureus

• Skin and soft tissue infections, such as impetigo, pus, and red, swollen, and painful areas on the skin.
• Staphylococcal scalded skin syndrome (SSSS): a severe skin infection caused by S. aureus that produces exfoliative toxins.
• Toxic shock syndrome (TSS): caused by TSST, leading to fever, multi-organ dysfunction, and potentially life-threatening conditions.
• Bacteremia and endocarditis: can cause bloodstream infections.
• Pneumonia.
• Food poisoning: characterized by nausea, vomiting, diarrhea, and a short incubation period of 1-6 hours.

Lab Diagnosis of S. aureus

• Specimen: swap from lesions, pus, sputum, CSF, blood culture, or urine culture.
• Direct smear: examination of the specimen under a microscope.
• Cultivation: on nutrient agar or blood agar at 37°C for 24 hours.
• Identification: on nutrient agar, the colonies are golden yellow or blood agar, with β-hemolysin colonies.
• Identification is also done through Gram staining, catalase test, and mannitol salt agar.

Food Poisoning Diagnosis

• Cultivation of the vomiting or diarrhea on mannitol salt agar.
• ELISA test.

Toxic Shock Syndrome Diagnosis

• Cultivation of the specimen from tampon or vaginal swab on MSA for 24 hours at 37°C.
• ELISA test through serum.

Treatment of S. aureus Infections

• Most S. aureus are resistant to penicillin, so semi-synthetic penicillin (methicillin, oxacillin, nafcillin) or other antibiotics like linezolide or daptomycin are used.
• No vaccine available.
• Prevention involves hand washing and avoiding sharing personal items.

S. epidermidis and S. saprophyticus

• Both grow on nutrient agar at 37°C for 24 hours, forming white colonies.
• Both grow on MSA, forming pink colonies.
• Both are coagulase-negative and catalase-positive.
• S. epidermidis:
  • Commonly found on human skin as normal flora.
  • Can act as an opportunistic pathogen, causing wound infections, UTI, and subacute endocarditis.
  • Adheres to devices, such as teeth, by glycocalyx.
  • Produces β-lactamase.
• S. saprophyticus:
  • Found in soil and water.
  • Attaches to the genitourinary tract and causes honeymoon cystitis.
• Treatment:
  • S. epidermidis: vancomycin, gentamicin.
  • S. saprophyticus: ciprofloxacin.