Thursday, August 25, 2011

Automated Blood Culture Systems

These systems employ equipment that automatically detects an early sign of bacterial growth in a special blood culture bottle. The system most used is Bactec (Becton Dickinson). It depends on the release of carbon dioxide (C02) into the atmosphere in the culture bottle by the bacterial degradation nutrients in a special culture medium and PH of the bottle becomes acidic leading to release of fluorescent signal detected by specific sensor in Bactec system. Other system like Bact/alert use chromogenic substrate that change in color due to PH change by bacterial growth.
Separate bottles of special medium are supplied for aerobic and anaerobic culture, and bottles of medium containing resins to inactivate any antibiotics that may be present in the specimen.
Normally 10 ml blood would be collected in a syringe and up to 5 ml injected through their rubber caps into aerobic and anaerobic culture. Bottles containing 30 ml medium. Up to 60 bottles of the one kind seeded from different patients are loaded into an instrument in which they are incubated, agitated and periodically flushed with an appropriate gas (air + CO2, or N2 + H2 + C02) through two heat-sterilized needles inserted automatically through the rubber cap). It can indicate early bacterial growth, which is often detectable on the same day as the specimen is received in the laboratory.
When growth is thus first detected, the bottle is removed and examined in the usual way by filming and subculture. Negatively reacting bottles do not need to be filmed or subcultured at any stage.
Automated Culture for Mycobacterium tuberculosis
- Radiometrie BACTEC 460 TB system: BACTEC 460 TB system and BACTEC 12B vials (Becton Dickinson Microbiology system, Cockeysville, Md.) are used for radiometrie culture of MTB. BACTEC 12B vials are prepared by addition of antimicrobial agents "PANTA" which contains polymyxin, amphotericin B, nalidixic acid, trimethoprim, and azlocillin. One half ml of concentrated sputum sediments are inoculated into each BACTEC 12B vial and kept at 37°C and then tested 3 times a week. Positive cultures as indicated by BACTEC460 are subjected to further identification by NAP test (P-Nitro-acetyl amino hydroxyl-Proionophenone) for the presence of typical Myucobacterium tuberculosis.
The BACTEC MGIT 960 System is an in vitro diagnostic instrument for rapid detection of Mycobacteria in clinical specimens other than blood.
The system is designed to meet the needs of medium and high volume labs,capable of processing about 8,000 cultures per year.
This system is simple, efficient , safe to use and occupies small laboratory space. The MGIT 7ml tube contains modified middle brook 7H9 broth. Culture tubes contain a fluorescent sensor at the bottom which responds to the concentration of oxygen.
Initial concentration of dissolved oxygen quenches the emission from the compound, and little fluorescence can be detected.
Actively respiring micro organisms consume the oxygen which allows the compound to fluorescence.

MGIT can be used in laboratory without the system and read by UV lamps of the electrophoresis

-Antimicrobial Drug susceptibility testing (AST): Isolated TB colonies from BACTEC 12B media and from LJ media will be subjected to AST using both Agar proportion method and BACTEC 460 TB system.
Recently BACTEC system is replaced by non radiometric system MGIT

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