MGIT 960 Instrument: Principle for Culture

Mycobacteria Growth Indicator Tube (MGIT) contains:

• 7.0 ml of modified Middlebrook 7H9 broth base
• Growth supplement essential for the growth of Mycobacterium tuberculosis complex
• MGIT PANTA, antibiotic mix to prevent contamination
• Oxygen-quenched fluorochrome (tris 4, 7-diphenyl-1, 10-phenonthroline ruthenium chloride pentahydrate) embedded in silicone at bottom of the tube

Principle

• During bacterial growth within the tube, free oxygen is utilized and replaced with carbon dioxide.
• With the depletion of free oxygen, the fluorochrome is no longer inhibited, resulting in fluorescence within the MGIT tube.
• The intensity of fluorescence is directly proportional to the extent of oxygen depletion. 
• Inoculated MGIT tubes are either incubated at 37ºC, read manually under ultra violet light or incubated into MGIT 960 instrument and monitored for fluorescence
• Positive and negative results (Figure 1 and 2)
  • Growth of mycobacteria increases fluorescence; at the time of positivity, there are approximately 105-106 Colony-forming Units (CFU) per ml of medium
  • The instrument declares a tube negative if it remains negative for six weeks (42 days)
• The detection of growth in the medium is also visually observed for the:
  • Presence of non-homogeneous light turbidity, or
  • Small granular/ flaky appearance

Figure 1: Positive Growth indicated by Strong Fluorescence

Figure 2: Negative Growth indicated by Lack of Fluorescence

Resources

• BD BACTEC MGIT Instrument Manual.
• MGIT Procedure Manual.

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