Learning Objective
By the end of this section, students should be able to differentiate culture requirements for obligate intracellular, facultative, and extracellular microorganisms, identify selective and differential media, and recall special growth requirements of clinically important pathogens.
Culture of Microorganisms
Growth Requirements
| Type of Organism | Culture Method | Examples / Notes |
|---|---|---|
| Obligate intracellular pathogens | Cannot grow on artificial media; requires tissue/cell culture, embryonated eggs, or animals | Viruses, Rickettsia, Chlamydia; some cannot be cultured at all (e.g., Treponema pallidum) |
| Facultative intracellular or extracellular organisms | Can grow on inert lab media such as broth or agar | Most bacteria |
| Specialized media | Selective media: favors the growth of specific organisms by adding nutrients or antibiotics Differential media: distinguishes organisms by colony morphology, color, or biochemical reactions |
Example: MacConkey agar selects for Gram-negative bacteria and differentiates lactose fermenters |
Special Media for Selected Organisms
| Organism | Medium | Type |
|---|---|---|
| Anaerobes | Thioglycolate broth | Growth support |
| Corynebacterium | Löffler coagulated serum medium | Supportive (S) |
| Tellurite agar | Differential (D) | |
| Enteric bacteria | Eosin methylene blue (EMB) | Differential (D) |
| MacConkey agar | Differential & Selective (D & S) | |
| Enteric pathogens | Hektoen enteric agar | Differential (D) |
| Xylose-lysine-deoxycholate agar | Differential (D) | |
| Vibrio cholerae | TCBS (Thiosulfate Citrate Bile Salts Sucrose) agar | Selective (S) |
| Legionella | Charcoal-yeast extract agar (CYE) | Selective (S) |
| Mycobacterium | Löwenstein-Jensen medium | Supportive (S) |
| Neisseria (sterile sites) | Chocolate agar | Growth support |
| Neisseria (sites with normal flora) | Thayer-Martin agar* | Selective (S) |
Note: Thayer-Martin agar = chocolate agar + vancomycin, nystatin, colistin to inhibit normal flora, including nonpathogenic Neisseria.
Miscellaneous Growth Requirements
| Requirement | Organism |
|---|---|
| Cholesterol, purines, pyrimidines | Mycoplasma |
| Cysteine* | Francisella, Brucella, Legionella, Pasteurella |
| X factor (protoporphyrin) and V factor (NAD) | Haemophilus influenzae & H. aegyptius |
Mnemonic: The 4 sisters “Ella” worship in the Cysteine Chapel → Francisella, Brucella, Legionella, Pasteurella
Activity
Teaching Tips / Memory Aids
- Obligate intracellular = “needs a host” → cannot grow on plates, needs cells/eggs/animals.
- Facultative/extracellular = “happy on plates” → can grow on broth or agar.
- Selective vs Differential media:
- Selective: includes inhibitors to favor certain bacteria.
- Differential: uses color or morphology to distinguish bacteria.
- Special growth factor mnemonic:
- Cholesterol → Mycoplasma
- Cysteine → 4 sisters Ella
- X & V → Haemophilus influenzae / aegyptius
