Microbiology Study Guide - Staph & Strep

Enriched Media

• Definition: Contains extra nutrients for fastidious organisms.
• Examples: Blood agar.

Differential Media

• Definition: Shows differences in bacterial properties (e.g., metabolism).
• Examples:
  • Blood agar (hemolysis)
  • MacConkey agar (lactose fermentation).

Selective Media

• Definition: Promotes growth of some organisms while inhibiting others.
• Examples:
  • Mannitol Salt Agar (MSA) - selects Staphylococcus species.
  • MacConkey agar.

Bacteria That Grow on Blood Agar

• Fastidious organisms like Streptococcus.

Staphylococci Characteristics

• Type: Gram-positive cocci in clusters.
• Catalase: Positive.
• Motility: Non-motile, facultative anaerobes.

Where Staphylococci Are Found on Body

• Sites: Skin, nose, mucous membranes.

Important Staphylococcus Species

1. S. epidermidis

   • Found: Skin
   • Diseases: Infections from catheters, implants.

2. S. saprophyticus

   • Found: Urinary/genital tract
   • Diseases: Urinary Tract Infections (UTIs), especially in young women.

3. S. aureus

   • Found: Nose, skin
   • Diseases: MRSA, boils, food poisoning, pneumonia, Toxic Shock Syndrome (TSS).

Toxins and Enzymes Produced by S. aureus

• Hemolysins: Lyse red blood cells (RBCs).
• Coagulase: Clots plasma.
• Leukocidin: Destroys white blood cells (WBCs).
• Hyaluronidase: Spreads infection.
• Staphylokinase: Dissolves clots.
• Enterotoxin: Causes food poisoning.

MSA Media Characteristics

• Selective:
  • Selects for: Staphylococcus species
  • Ingredient: High salt concentration (7.5% NaCl).
• Differential:
  • Differentiates: S. aureus (mannitol fermentation).
  • Ingredients: Mannitol + phenol red (color indicator).

Hospital Strains

• Issue: Antibiotic-resistant strains (e.g., MRSA).
• Risk: Difficult to treat and spreads in clinical settings.

Normal Flora of the Respiratory System

• Includes Staphylococcus, Streptococcus, Neisseria, Corynebacterium.
• Locations: Upper respiratory tract (nose, throat).
• Defenses: Mucus, cilia, immune cells, coughing/sneezing.

Streptococcus Genus Characteristics

• Type: Gram-positive cocci in chains.
• Catalase: Negative.
• Fastidious: Requires extra nutrients to grow.
• Media: Grows on blood agar.

RBC-Destroying Substances in Streptococcus

• Hemolysins: Enzymes that lyse RBCs.

Types of Hemolysis

1. Alpha: Green, partial RBC breakdown.
2. Beta: Clear, complete RBC breakdown.
3. Gamma: No hemolysis.

Normal Flora Hemolysis Types

• Groups: Alpha and gamma-hemolytic strep are often found in normal flora.

Pathogenic Strep Group

• Group A (S. pyogenes):
  • Diseases: Strep throat, scarlet fever, impetigo, necrotizing fasciitis.

Antigen and Classification

• Antigen: A molecule that triggers an immune response.
• Naming Strep: Based on Lancefield groups (A, B, etc.).

Human Pathogen Groups

• Group A: S. pyogenes
• Group B: S. agalactiae.

Staph Comparison Chart