An Agar Medium Designed for Pyrazinamide Susceptibility Testing Of the Mycobacterium Tuberculosis Strains

Abstract

Background: Pyrazinamide (PZA) is an important front-line antituberculosis agent. This drug plays a unique role in shortening the therapy, besides of metabolically active and replicating bacilli; it kills a population of semi-dormant organisms that are not killed by other first-line antituberculosis drugs. The activity of PZA correlates with the acidity of the medium, being most active at PH 5.5 less active at PH 6 and inactive at neutral PH. The problem is that such an acidic environment is quite unfavorable for Mycobacterium tuberculosis growth. Therefore the PZA susceptibility testing is difficult and often unreliable because of the acid PH requirement for drug activity. For this reason, many clinical microbiology laboratories do not perform PZA susceptibility testing and most drug-resistance surveys do not have PZA resistance data. For this reason a special condition which could be to support the optimal growth of organisms & allow to performing PZA susceptibility testing at favorable PH has been developed. Methods: The continuously buffered Middlebrook 7H10 agar base with an acidic PH of 6.0 used, to provide optimal conditions for PZA acidity, it also differs from conventional 7H10 medium in that supplemented with animal serum instead of oleic acid to support optimal growth of organism at low PH of 6.0. Individual critical concentrations of PZA were used according to the Hassle- Bausch´s enzyme- substrate activity correlation in this medium made it possible to differentiate between PZA-susceptible and PZA-resistant clinical isolates. Results: During two years survey following results was obtained; approximately 2.6٪ of isolated were identified as PZA-resistance together other drugs resistance, with about 1٪ only PZA-resistance. PZA positive & PZA negative standard strains as control shown the method was used in this study obtains reliable results. Conclusion: Compared to a liquid medium this agar medium also has the following advantages: it allows determination of the actual proportion of PZA-resistant bacteria in the isolate and it is simple and inexpensive. In addition, it has the potential of being used for a direct susceptibility test with PZA