Mycobacterium tuberculosis is an obligate aerobic bacterium with high nutritional requirements. Its optimal pH range is 6.5–6.8, and it grows slowly, requiring nutrient-rich media for initial isolation. The commonly used Löwenstein-Jensen (LJ) solid medium contains egg yolk, glycerol, potato, inorganic salts, and malachite green. Malachite green inhibits the growth of contaminating bacteria, facilitating isolation and prolonged culture. The egg yolk provides lipid growth factors that stimulate bacterial growth. Depending on the inoculum size, colonies typically appear in 2–4 weeks. On solid media, colonies appear as dry, granular, raised, rough, and cauliflower-like with a grayish-yellow-white color. In liquid media, a thick, wrinkled pellicle forms at the surface while the medium remains clear. Adding Tween 80 to the medium promotes uniform dispersed growth, usually within 1–2 weeks.
Liquid culture yields a several-fold higher positivity rate than solid culture for clinical specimens. The bacteria stain red with acid-fast staining and exhibit strong resistance to drying and acids. They are prone to developing drug resistance and L-form variants. M. tuberculosis appears as slender, slightly curved bacilli, occurring singly or in branching arrangements, with no spores or flagella but possessing fimbriae. Traditional detection methods are time-consuming (up to 8 weeks) and have low positivity rates (≤20%), causing significant delays in diagnosis and treatment, highlighting the need for improvement.
Liquid culture, due to its optimized detection principle, offers unparalleled advantages over solid culture. As the gold standard in methodology, it provides unmatched reliability. M. tuberculosis is resistant to acids and alkalis, highly resistant to drying, but sensitive to ethanol, moist heat, and ultraviolet radiation. It can infect various tissues and organs, particularly the lungs, causing tuberculosis and severely impacting human health. Diagnosing tuberculosis typically requires culturing M. tuberculosis to confirm clinical cases and guide treatment.
Liquid culture is generally regarded as a rapid method for mycobacterial growth, significantly shortening the time required for clinical diagnosis. It is more sensitive than solid culture, with a higher detection rate. The liquid medium provides more abundant nutrients, promoting faster growth—positive results can be obtained in 1–2 weeks, much sooner than with solid culture. If no growth is observed after 6 weeks, a negative report can be issued, greatly reducing waiting time.
The BACTEC™ MGIT™ (Mycobacteria Growth Indicator Tube) system, an automated mycobacterial culture monitoring instrument, is compatible with liquid culture tubes, offering high detection rates and significantly improving clinical testing efficiency.
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