Löffler's medium (also written as Loeffler's medium) is a nutrient agar used in microbiology for the cultivation and identification of diphtheroids, particularly Corynebacterium diphtheriae. It is a non‑selective, non‑differential medium that supports the rapid growth of these organisms and promotes the formation of characteristic metachromatic granules (Babes Ernst granules) that aid microscopic identification.
Composition
Typical formulations contain the following components per liter of distilled water:
- Peptones (e.g., pancreatic digest of casein) – 10 g
- Sodium chloride – 5 g
- Potassium phosphate – 0.5 g
- Calcium chloride – 0.07 g
- Magnesium sulfate – 0.5 g
- Sodium sulfite – 10 g (provides a reducing environment)
- Ferrous sulfate – 1 g (facilitates the formation of metachromatic granules)
- Agar – 15 g
The medium is sterilized by autoclaving and then cooled to about 45 °C before the addition of heat‑labile components such as ferrous sulfate, which may be filter‑sterilized separately.
Usage
- Primary isolation – Specimens suspected of containing C. diphtheriae (e.g., throat swabs) are streaked onto Löffler's medium and incubated at 35–37 °C for 24–48 hours. Colonies typically appear as small, gray‑white, raised, and may display a buttery consistency.
- Microscopic identification – After growth, a smear is prepared and stained with Wayson or methylene blue. The presence of metachromatic granules within the bacilli confirms a diphtheroid identification.
- Differential purposes – While not selective for C. diphtheriae alone, the medium can be used in conjunction with selective media (e.g., tellurite agar) for differential diagnosis.
Historical background
The medium is named after Friedrich August Johannes Loeffler (1852–1915), a German bacteriologist who, together with Emil von Behring, discovered the diphtheria toxin and contributed to the development of the diphtheria antitoxin. Loeffler introduced this agar formulation in the early 20th century to facilitate the culture of the diphtheria bacillus.
Limitations
- Löffler's medium lacks specific inhibitory agents; therefore, contaminating flora may grow alongside the target organism.
- It is generally used as a preliminary medium; definitive identification often requires additional biochemical or molecular tests.
References
- Forbes, B. A., Sahm, D. F., & Weissfeld, A. S. (2007). Bailey & Scott’s Diagnostic Microbiology (12th ed.). Mosby.
- Atlas, R. M. (2010). Handbook of Microbiological Media (4th ed.). CRC Press.
All information reflects current scientific understanding as documented in standard microbiology references.
