Mycobacterium Tuberculosis Morphology

Morphology and Identification

• Mycobacteria are slender, straight rods approximately 0.4 x 3 µm in size.
• Exhibit variable morphology, including coccoid and filamentous forms, depending on the species.
• Do not efficiently stain with standard aniline dyes; appear as "Gram-invisible" or clear zones (often referred to as "ghosts").
• Certain mycobacteria may be identified as beaded Gram-positive rods, though branching is usually absent.
• Other bacteria with acid-fast staining characteristics include Nocardia, Rhodococcus, Gordonia, and Tsukamurella.

Ziehl-Neelsen Technique

• A staining method specifically for detecting acid-fast bacteria, crucial in identifying Mycobacterium species.
• Mycobacteria exhibit yellow-orange fluorescence when treated with fluorochrome stains, enhancing visibility.
• Fluorochrome stains are preferred for clinical specimens due to their efficacious detection of acid-fast bacteria.
• Advanced fluorescence microscopy, utilizing ultrabright LED microscopes, has improved the provision of diagnostics in resource-limited settings.

Culture

• Primary culture of mycobacteria requires both nonselective and selective media to ensure appropriate growth conditions.
• Selective media incorporate antibiotics to inhibit the proliferation of contaminating bacteria and fungi.
• Various formulations can be deployed for both nonselective and selective media applications.

Semisynthetic Agar Media

• Middlebrook 7H10 and 7H11 are examples of semisynthetic agar media enriched with essential nutrients.
• These media contain defined salts, vitamins, cofactors, oleic acid, albumin, catalase, and glycerol.
• Casein hydrolysate is included in the 7H11 medium, providing additional nutrients.
• Albumin acts to neutralize the toxic effects of fatty acids present in the media, promoting mycobacterial growth.