Study Notes on Staphylococcus aureus

Overview of Staphylococcus aureus

  • Classification: Staphylococcus aureus, often referred to as Staph aureus, is a species of bacteria characterized by its positive response to Gram staining, which classifies it as a gram-positive bacterium.
  • Shape and Arrangement:
    • Cocci: Staph aureus is spherical (coccus-shaped) in structure.
    • Cluster Formation: Under microscopic examination, Staph aureus appears in clusters, akin to grapes. This clustering helps in differentiating Staphylococcus from Streptococcus species, which form chains (referred to as streptococci).
  • Color: The name "aureus" derives from the Latin word for gold, referring to the golden-yellow color of its colonies when grown on agar plates, attributed to a pigment called staphloxanthin.

Microbiological Characteristics

Gram Staining

  • Staph aureus reacts positively in a Gram stain, appearing as purple spherical clusters.

Laboratory Characteristics

  • Catalase Test:

    • Catalase Positive: Staph aureus produces the enzyme catalase, which breaks down hydrogen peroxide into water and carbon dioxide.
    • Laboratory Procedure: When hydrogen peroxide is introduced to a culture, the presence of catalase is indicated by the release of bubbles (carbon dioxide).
    • Differentiation: This test helps distinguish Staph (catalase positive) from Streptococcus (catalase negative).

  • Coagulase Test:

    • Coagulase Positive: Staph aureus produces coagulase, an enzyme causing clotting of blood plasma, an essential factor for identification.
    • Mechanism: Coagulase converts fibrinogen to fibrin, resulting in blood clot formation.
    • Laboratory Procedure: In a tube test, adding bacterial culture to plasma will result in the formation of clumps if coagulase is present.
    • Distinction from Other Staphylococci: This test differentiates Staph aureus from coagulase-negative species like Staphylococcus epidermidis.

Hemolytic Activity

  • Beta Hemolytic: Staph aureus is classified as beta hemolytic, suggesting that it completely lyses red blood cells in blood agar.

Mannitol Fermentation

  • Fermentation Process: Staph aureus can ferment mannitol, causing a pH change that can be measured as a color change in mannitol broth, making it another specific test for Staph aureus.

Virulence Factors

Protein A

  • Function: Staph aureus expresses protein A, which binds to the Fc region of Immunoglobulin G (IgG).
  • Immunological Impact: This binding disrupts the opsonization process, inhibiting phagocytosis and thereby aiding in the bacterium's survival and virulence.

Penicillinase

  • Definition: Staph aureus produces an enzyme known as penicillinase, which cleaves the beta-lactam ring in penicillin, rendering it ineffective.
  • Mechanism: By breaking down penicillin, Staph aureus poses a challenge in treatment, necessitating the use of alternative antibiotics.

Resistance to Penicillins

  • Penicillinase Resistant Penicillins: Although resistant to many forms of penicillin due to penicillinase, Staph aureus remains susceptible to specific penicillinase-resistant penicillins such as nafcillin, oxacillin, and methicillin.
  • Mnemonic: The phrase "use Naf for Staph" helps remember the use of nafcillin for treatment.

Summary of Key Points

  • Classification: Staphylococcus aureus (Staph aureus) is a gram-positive cocci, appearing in clusters.
  • Characteristics:
    • Catalase positive
    • Coagulase positive
    • Beta hemolytic on blood agar
    • Mannitol fermentation
  • Virulence Factors: Include Protein A for immune evasion and Penicillinase for antibiotic resistance.
  • Treatment Considerations: Penicillinase-resistant penicillins are the treatment of choice for infections caused by Staph aureus.