Chapter 7.7 : Susceptibility Testing by Liquid Media Methods

Abstract:

The BACTEC MGIT 960 SIRE kit is a nonradiometric antimicrobial susceptibility (AST) system for testing Mycobacterium tuberculosis from isolated culture. It provides susceptibility results for streptomycin (STR), isoniazid (INH), rifampin (RIF), and ethambutol (EMB) in approximately the same time frame as the BACTEC 460TB system. The BBL MGIT 7-ml mycobacteria growth indicator tube (MGIT tube) contains a modified Middlebrook 7H9 broth, which supports the growth and detection of mycobacteria. The MGIT tube contains a fluorescent compound embedded in silicone on the bottom of a 16- by 100-mm round-bottom tube. The fluorescent compound is sensitive to the presence of oxygen dissolved in the broth. The initial concentration of dissolved oxygen quenches the emission of fluorescence from the compound, and little fluorescence can be detected. Later, actively growing and respiring microorganisms consume the oxygen, which allows the compound to fluoresce.