Chapter 14: Staphylococci

General Characteristics of Staphylococci:

  • Gram-positive cocci producing catalase.

  • Spherical cells (0.5 to 1.5 µm) arranged singly, in pairs, or clusters, resembling "bunches of grapes."

  • Differentiated by the coagulase test: Coagulase-positive species include Staphylococcus aureus, Staphylococcus intermedius, Staphylococcus delphini, and others.

  • Staphylococcus aureus: Most clinically significant, causing skin infections, abscesses, bacteremia, and toxin-related diseases like food poisoning, scalded skin syndrome, and toxic shock syndrome.

  • Coagulase-negative staphylococci: Includes Staphylococcus epidermidis (hospital-acquired infections), Staphylococcus saprophyticus (urinary tract infections), Staphylococcus haemolyticus, and Staphylococcus lugdunensis.

Virulence Factors of Staphylococcus aureus:

  1. Enterotoxins: Heat-stable, causing food poisoning and toxic shock syndrome.

  2. Exfoliative toxin: Causes scalded skin syndrome and bullous impetigo.

  3. Cytolytic toxins: Include hemolysins (alpha, beta, gamma, delta) that damage red blood cells, platelets, and tissues.

  4. Protein A: Binds immunoglobulin G, blocking phagocytosis.

Epidemiology:

  • Main reservoir: Human nares, with colonization on skin and mucous membranes.

  • Hospital outbreaks are common in high-risk areas like burn units and post-surgery wards.

Infections:

  • Skin and wound infections: Staphylococcus aureus causes abscesses and deeper infections.

  • Scalded skin syndrome: Primarily affects newborns, caused by exfoliative toxin.

  • Toxic shock syndrome: Rare, potentially fatal multisystem disease linked to toxin production.

  • Food poisoning: Enterotoxins (A, D, B) cause gastrointestinal symptoms.

Coagulase-Negative Staphylococci Infections:

  • Staphylococcus epidermidis: Hospital-acquired infections and prosthetic valve endocarditis.

  • Staphylococcus saprophyticus: Urinary tract infections in young women.

  • Staphylococcus lugdunensis: Mimics Staphylococcus aureus, causing endocarditis and resistance to oxacillin.

Isolation and Identification:

  • Staphylococci grow well on sheep blood agar and selective media like mannitol salt agar.

  • Colonies are round, smooth, and white, with Staphylococcus aureus often showing hemolysis.

Cultural Characteristics of Staphylococci:

  • Staphylococci: Produce round, smooth, white, creamy colonies on sheep blood agar (SBA) after 18 to 24 hours of incubation at 35°C to 37°C.

  • Staphylococcus aureus: Often forms hemolytic zones around colonies and can occasionally exhibit yellow pigment with extended incubation.

  • Staphylococcus epidermidis: Colonies are typically small to medium-sized, non-hemolytic, gray to white; some may exhibit weak hemolysis.

  • Staphylococcus saprophyticus: Forms slightly larger colonies, with around 50% of strains producing yellow pigment.

  • Staphylococcus haemolyticus: Produces medium-sized colonies with moderate or weak hemolysis and variable pigment.

  • Staphylococcus lugdunensis: Colonies are often hemolytic and medium-sized, though small colony variants may occur.

Identification Methods:

  1. Oxidation-Fermentation (O/F) Test: Staphylococci ferment glucose, while micrococci do not. However, this test may not distinguish certain weak acid producers.

  2. Coagulase Test: Staphylococcus aureus is identified using coagulase tests. Clumping factor converts fibrinogen to fibrin, causing agglutination. Negative results must be confirmed with the tube coagulase method.

  3. Novobiocin Susceptibility: Used for the presumptive identification of Staphylococcus saprophyticus, which is resistant to novobiocin.

Methicillin-Resistant Staphylococci (MRSA):

  • Methicillin-resistant Staphylococci: Refers to staphylococci resistant to nafcillin or oxacillin, commonly termed as MRSA (methicillin-resistant Staphylococcus aureus) or MRSE (methicillin-resistant Staphylococcus epidermidis).

  • Types of MRSA:

    • Hospital-associated MRSA (HA-MRSA): Typically occurs in healthcare settings.

    • Community-associated MRSA (CA-MRSA): Increasingly common in patients without traditional risk factors.

    • Healthcare-associated community-onset MRSA (HACO-MRSA): A hybrid form appearing outside hospitals but in individuals with healthcare-associated risk factors.

  • Control Measures: Requires strict infection control, including barrier protection, contact isolation, and hand hygiene.

  • Resistance Detection:

    • Resistance is often due to the mecA gene, which codes for an altered penicillin-binding protein (PBP2a), rendering β-lactam antibiotics ineffective.

    • The gold standard for detecting MRSA is PCR amplification of the mecA gene.

  • Treatment: Vancomycin is the treatment of choice for MRSA, though increasing resistance to glycopeptides is a concern.

Vancomycin-Resistant Staphylococci (VRSA):

  • Vancomycin: Often the drug of choice, and sometimes the only effective treatment for serious staphylococcal infections.

  • Vancomycin-Intermediate Staphylococcus aureus (VISA): First discovered in Japan in 1996. Automated antimicrobial susceptibility tests may not always detect these strains reliably.

  • Vancomycin-Resistant Staphylococcus aureus (VRSA): First reported in the United States in 2002 from patients undergoing long-term vancomycin treatment. Most isolates recovered in the U.S. were from patients with underlying conditions.

  • Screening: A vancomycin agar plate, according to CLSI performance guidelines, enhances detection of VISA and VRSA.

Macrolide Resistance:

  • Clindamycin: A macrolide frequently used in staphylococcal skin infections. Testing with a D-zone test may be necessary when there are discrepancies in macrolide susceptibility test results, especially between erythromycin and clindamycin.

  • Inducible Clindamycin Resistance: Detected when staphylococcal resistance is inducible by erythromycin. A disk diffusion test, where an erythromycin disk is placed near a clindamycin disk, is used to identify this resistance.

    • If inducible resistance exists, a D-shaped zone of inhibition will appear around the clindamycin disk where the drugs overlap.


Key Terms:

  1. Staphylococci: Gram-positive cocci, often found in clusters.

  2. S. aureus: Pathogenic species causing a wide range of infections, including skin infections, pneumonia, and sepsis.

  3. S. epidermidis: Coagulase-negative staphylococcus, commonly found on skin; significant in device-related infections.

  4. S. saprophyticus: Causes urinary tract infections, especially in young women; novobiocin resistant.

  5. Hemolysis: Destruction of red blood cells, observed as clear zones around colonies on blood agar.

  6. Coagulase: Enzyme that clots plasma; used to differentiate S. aureus (coagulase-positive) from other staphylococci.

  7. Clumping Factor: Surface protein in S. aureus that causes agglutination of plasma; detected in coagulase slide test.

  8. Protein A: Binds to IgG, inhibiting opsonization and phagocytosis; increases virulence of S. aureus.

  9. MRSA (Methicillin-resistant Staphylococcus aureus): Resistant to β-lactam antibiotics due to mecA gene encoding PBP2a.

  10. CA-MRSA: Community-associated MRSA infections, often in healthy individuals.

  11. HA-MRSA: Hospital-associated MRSA, seen in healthcare settings.

  12. VISA/VRSA: Vancomycin-intermediate or vancomycin-resistant S. aureus; concerns in long-term vancomycin use.

  13. Exfoliative Toxins: Cause scalded skin syndrome by cleaving skin proteins.

  14. TSST-1: Toxic shock syndrome toxin-1; causes toxic shock syndrome.

  15. Enterotoxins: Cause staphylococcal food poisoning; heat-stable toxins affecting the gastrointestinal tract.

  16. D-zone test: Used to detect inducible clindamycin resistance in staphylococci.

  17. Oxacillin-Salt Agar: Medium used to screen for MRSA.

  18. PBP2a: Altered penicillin-binding protein, responsible for methicillin resistance in MRSA.

  19. MecA gene: Gene responsible for oxacillin resistance in MRSA.

  20. Novobiocin Test: Differentiates S. saprophyticus (resistant) from other coagulase-negative staphylococci.

  21. Chromogenic Media: Used to rapidly detect MRSA directly from clinical samples.

  22. PCR: Molecular method for detecting mecA gene or other staphylococcal virulence genes rapidly.

  23. Vancomycin Agar Plate: Screening tool for VISA/VRSA isolates.

  24. Bacteremia: Presence of bacteria in the bloodstream, potentially leading to severe infections like sepsis.

  25. Scalded Skin Syndrome: Caused by exfoliative toxins leading to skin detachment.

  26. Toxic Shock Syndrome: Caused by TSST-1, leads to systemic effects including shock and multi-organ failure.

  27. BORSA: Borderline oxacillin-resistant S. aureus, resistant due to hyperproduction of β-lactamase.