CDST_LT: Growth detection

BACTEC MGIT 960 System Growth Detection

Principle:

The MGIT (Mycobacteria Growth Indicator Tube) consists of a liquid broth medium in the form of Middlebrook 7H9 liquid media that is known to yield better recovery and faster growth of mycobacteria. The MGIT tube also contains an oxygen-quenched fluorochrome, tris 4, 7-diphenyl-1, 10-phenanthroline ruthenium chloride pentahydrate embedded in silicone at the bottom of the tube.

 During bacterial growth within the tube, the free oxygen is utilized and is replaced with carbon dioxide. With the depletion of free oxygen, the fluorochrome is no longer inhibited, resulting in fluorescence within the MGIT tube when visualized under UV light. The intensity of fluorescence is directly proportional to the extent of oxygen depletion. MGIT tubes may be incubated at 37ºC and read manually under a UV light or entered into an MGIT 960 instrument, where they are incubated and monitored for increasing fluorescence every 60 minutes.

 The growth of bacteria, as well as mycobacteria, increases the fluorescence. In the case of M. tuberculosis, 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 can also be visually observed by the presence of non-homogeneous light turbidity or a small granular/flaky appearance in the medium.

Notifications:

Positive Cultures:

 The system will indicate the presence of presumptive positive vials in several ways: 

1.  The positive indicator lamp at the front of the drawer illuminates.

2.  The tube count for each drawer, next to the filled circle with a plus sign icon, increments in the Summary window display.

3.  When the drawer is opened, the "remove positive tubes" soft key appears on the screen.

4. The audible alert sounds until the condition is acknowledged.

Negative Cultures:

 Negative cultures exist as ongoing negatives (in the protocol) or out-of-protocol negatives. Notification of these conditions includes:

1.  Ongoing Negatives - In the summary region of the display, the ongoing tube count for each drawer appears next to the filled circle icon,

2.  Out-of-protocol Negatives - In the summary region of the display, the tube count for each drawer appears next to the filled circle with a minus (- )sign icon.

3.  The negative indicator light for the drawer(s) illuminates.

   Resources

   1. MGIT Procedure manual, FIND Diagnostics

   2. BACTEC™ MGIT™ 960 System

   Assessment

The different steps of tube removal are as follows:

1. Visual Observation: ​
 Characteristic flake-like growth (Presumptive Positive) ​
 Uniform Turbidity (Presumptive Contamination) ​

2. Smear: ​
 Corded AFB smear-positive (Culture positive) ​
 Both Acid Fast or Non-Acid Fast Bacilli (Culture with contamination) ​
 Only Non-Acid Fast Bacilli (Contamination)​

3. Inoculate LJ slant/BHI for contamination check by the rate of growth, morphology, smear and immunochromatographic assays

 Growth in 24 – 48 hours (Contamination)

​ No Growth (Contamination free)

​ Cultures should be checked for purity and identification of M. tuberculosis using a rapid immunochromatographic species identification test, such as MPT 64 Ag Assay​ 

Resource

Mycobacteriology Laboratory Manual

Assessment

MGIT results are interpreted as described in Flowcharts 1-3:

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  Resource

Mycobacteriology Laboratory Manual 

Assessment