Culture media: quick classification

Important correction

• Blood agar = enriched, non-selective medium
• RCM = anaerobic enrichment/reducing medium
• Enriched media and enrichment media are different terms.

One-line exam definitions

• Simple medium: basic medium for non-fastidious organisms.
• Enriched medium: simple medium plus extra nutrients for fastidious organisms.
• Enrichment medium: liquid medium that increases the number of a pathogen from a mixed sample.
• Selective medium: medium that inhibits unwanted microbes and permits selected microbes.
• RCM: reducing anaerobic culture medium, especially useful for cultivation of Clostridium.

Culture Media

Definition

Classification of culture media

1. Based on physical state

2. Based on purpose / function

A. Simple or basal media

• Nutrient broth
• Nutrient agar
• Peptone water

• Routine cultivation of bacteria
• Preparation of enriched media
• Maintenance of bacterial cultures

B. Enriched media

1. Blood agar
   • Basal agar plus 5-10% sheep blood.
   • Used for Streptococcus, Staphylococcus, pneumococci, etc.
   • Demonstrates hemolysis:
     • Alpha hemolysis: greenish partial hemolysis, for example Streptococcus pneumoniae
     • Beta hemolysis: complete clear-zone hemolysis, for example Streptococcus pyogenes
     • Gamma hemolysis: no hemolysis
2. Chocolate agar
   • Heated blood agar.
   • Releases X factor (hemin) and V factor (NAD).
   • Used for Haemophilus influenzae and Neisseria gonorrhoeae.
3. Loeffler serum slope
   • Used for Corynebacterium diphtheriae.

C. Enrichment media

D. Selective media

E. Differential or indicator media

• MacConkey agar
  • Lactose fermenters: pink colonies, for example E. coli
  • Non-lactose fermenters: pale/colorless colonies, for example Salmonella
• Blood agar: differentiates organisms by hemolysis
• CLED agar: used for urine culture
• TSI agar: differentiates bacteria by glucose/lactose/sucrose fermentation, gas, and H₂S production
• Urease medium: detects urease production

F. Anaerobic culture media

Robertson cooked meat medium (RCM)

• Also called Robertson's cooked meat broth.
• Contains cooked minced meat particles.
• The meat provides a reducing environment and helps maintain anaerobiosis.
• Used for cultivation and maintenance of anaerobes, particularly Clostridium species.
• Useful for demonstrating proteolytic activity and for maintaining anaerobic organisms.

• Thioglycollate broth
• Anaerobic blood agar
• Brucella blood agar
• Brain-heart infusion broth with reducing agents

G. Transport media

• Stuart medium
• Amies medium
• Cary-Blair medium, especially for enteric pathogens such as Vibrio, Salmonella, and Shigella

H. Storage or maintenance media

• Nutrient agar slope
• Dorset egg medium
• Robertson cooked meat medium for anaerobes

Short conclusion

• Blood agar: enriched medium
• Selenite F broth: enrichment medium for Salmonella
• MacConkey agar: selective and differential medium
• TCBS agar: Vibrio cholerae
• Lowenstein-Jensen medium: Mycobacterium tuberculosis
• RCM: anaerobic medium, especially for Clostridium

Type I Hypersensitivity Reaction

Definition

Mechanism

1. Sensitization phase: first exposure

1. Allergen enters the body, for example pollen, dust mite, food, drug, or insect venom.
2. Helper T cells produce cytokines, mainly IL-4 and IL-13.
3. B lymphocytes undergo class switching and produce allergen-specific IgE.
4. IgE binds to high-affinity FcεRI receptors on mast cells and basophils.

2. Effector phase: subsequent exposure

1. The same allergen enters again.
2. It cross-links mast-cell-bound IgE.
3. Mast cells and basophils degranulate.

Mediators released

• Histamine: vasodilation, increased vascular permeability, itching, bronchospasm
• Leukotrienes C4, D4, E4: prolonged bronchoconstriction, mucus secretion, edema
• Prostaglandin D2
• Cytokines, including TNF and IL-5

Phases of reaction

• Immediate phase: minutes after exposure. Caused mainly by histamine and other preformed mediators.
• Late phase: 2-24 hours later. Eosinophils, neutrophils, and Th2 cells cause persistent inflammation and tissue injury.

Clinical examples

• Anaphylaxis / anaphylactic shock
• Allergic rhinitis
• Allergic conjunctivitis
• Atopic bronchial asthma
• Urticaria
• Angioedema
• Food allergy
• Drug allergy, for example penicillin allergy
• Insect-sting allergy
• Atopic dermatitis, partly mediated by Type I mechanisms

Diagnosis

• Clinical history
• Skin-prick test
• Allergen-specific serum IgE
• Serum tryptase may support diagnosis after suspected anaphylaxis

Treatment

• Avoid the allergen where possible.
• Antihistamines for mild allergy.
• Corticosteroids or leukotriene modifiers in selected allergic conditions.
• Allergen immunotherapy for selected patients.
• Intramuscular adrenaline (epinephrine) is the first-line treatment for anaphylaxis.

Easy memory

Type I Hypersensitivity Reaction: Mechanism Chart

FIRST EXPOSURE TO ALLERGEN
(pollen, dust, food, drug, insect venom)
                ↓
Allergen taken up by antigen-presenting cell
                ↓
Activation of CD4+ T-helper 2 (Th2) cells
                ↓
Th2 cells release IL-4 and IL-13
                ↓
B lymphocyte class switching
                ↓
Plasma cells produce allergen-specific IgE
                ↓
IgE binds FcεRI receptors on
MAST CELLS and BASOPHILS
                ↓
SENSITIZED INDIVIDUAL

SECOND EXPOSURE TO SAME ALLERGEN
                ↓
Allergen binds and cross-links IgE
on surface of mast cell / basophil
                ↓
Mast cell and basophil activation
                ↓
DEGRANULATION
                ↓
Release of mediators

MEDIATOR EFFECTS
        ↓
Immediate phase: minutes
- Wheal and flare
- Urticaria and itching
- Rhinorrhea
- Bronchospasm
- Angioedema
- Hypotension

        ↓
Late phase: 2-24 hours
- Eosinophil-rich inflammation
- Persistent edema and bronchospasm
- Tissue damage

Examples

One-line recall