2. Isolation and Identification of Staphylococcus and Streptococcus

Introduction

This study guide covers laboratory techniques for isolating and identifying Staphylococcus and Streptococcus species from various sources. It includes the properties of these bacteria, methods for their isolation, and key differences between the two genera. These topics are fundamental to understanding microbial growth, laboratory diagnostics, and host-microbe interactions in microbiology.

Staphylococcus: Isolation and Properties

Overview of Staphylococcus aureus

• Gram-positive cocci: Spherical bacteria that stain purple with Gram stain due to a thick peptidoglycan cell wall.

• Cell arrangement: Divide in multiple planes, forming irregular clusters (resembling grape clusters).

• Colony morphology: On trypticase soy agar or blood agar, colonies are 1–3 mm in diameter and may appear yellow, orange, or white.

• Coagulase positive: Can clot serum, distinguishing S. aureus from other staphylococci.

• Hemolysis: Produces alpha toxin, resulting in a clear zone of beta hemolysis on blood agar.

• Mannitol fermentation: Ferments mannitol sugar to produce acid, which can be detected on mannitol salt agar.

• Salt tolerance: Can grow in high-salt environments (e.g., mannitol salt agar).

Laboratory Isolation Procedure

• Inoculate three tubes of Staphylococcus broth:

  • One from a known culture of Staph

  • One from a fomite (inanimate object)

  • One from the nose using a sterile cotton swab

Comparison of Staphylococcus Species

The following table summarizes key properties of three clinically relevant Staphylococcus species:

Key: + = positive/present; - = negative/absent

Streptococcus: Isolation and Characterization

Overview of Streptococcus Species

• Gram-positive cocci: Spherical bacteria that form chains or pairs.

• Lancefield grouping: Classified into groups (A, B, C, D, or none) based on cell wall carbohydrate antigens.

• Hemolysis patterns: Can be alpha (partial), beta (complete), or gamma (none) hemolytic on blood agar.

Laboratory Isolation Procedure

• Isolate Streptococcus from the throat using a sterile cotton swab and tongue depressor.

• Swab the back of the throat and inoculate a blood agar plate to observe hemolysis and aid identification.

Key Differences Between Staphylococcus and Streptococcus

Cell Arrangement

• Staphylococcus: Grows in clusters (grape-like appearance).

• Streptococcus: Grows in chains or pairs.

Catalase Test

• Staphylococcus: Catalase positive (produces bubbles when hydrogen peroxide is added).

• Streptococcus: Catalase negative (no bubbles with hydrogen peroxide).

Summary Table: Staph vs. Strep

Applications and Importance

• Identification of Staphylococcus and Streptococcus is crucial in clinical microbiology for diagnosing infections such as pharyngitis, skin infections, and sepsis.

• Understanding their properties aids in selecting appropriate antibiotics and infection control measures.

Additional info:

• The catalase test distinguishes between these genera by detecting the presence of the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen:

• Lancefield grouping is especially important for identifying pathogenic Streptococcus species, such as Group A (S. pyogenes) and Group B (S. agalactiae).