Drug susceptibility of Mycobacterium tuberculosis to primary antitubercular drugs by nitrate reductase assay.

BACKGROUND & OBJECTIVES: Rapid susceptibility testing of Mycobacterium tuberculosis strains is imperative for therapy selection but traditional drug susceptibility tests take weeks or are expensive. In this study we evaluated nitrate reductase assay which utilizes the detection of nitrate reduction as an indication of growth and therefore results can be obtained faster than by visual detection of colonies. METHODS: One hundred clinical isolates of M. tuberculosis were tested for four first line antitubercular drugs by nitrate reductase assay (NRA) and were compared with standard proportion method. The bacteria were inoculated on Lowenstein-Jensen (LJ) medium with primary antitubercular drugs and potassium nitrate was incorporated. After incubation for 7- 14 days, nitrate reduction indicating growth could be detected by colour change when reagents were added. RESULTS: Resistance of isolates as determined by both methods for isoniazid, rifampicin, streptomycin and ethambutol was 32, 35, 62 and 15 per cent respectively. Agreement between NRA and proportion method was 99 per cent for isoniazid and ethambutol. Complete agreement (100%) was found for rifampicin and streptomycin. Results were available in 7-14 days by NRA as compared to proportion method which takes 4-6 wk. INTERPRETATION & CONCLUSION: Nitrate reductase assay is a rapid and inexpensive method for susceptibility testing of M. tuberculosis for primary antitubercular drugs and could be an appropriate alternative to existing methods, particularly in resource-poor settings.

Significance and characterisation of pseudomonads from urinary tract specimens.

Pseudomonads are commonly encountered in clinical samples. Usually ignored as contaminants, these organisms are known to cause nosocomial opportunistic infections like urinary tract infections (UTI). One hundred and two pseudomonads obtained in pure culture and significant numbers from 8400 consecutive urinary tract (UT) specimens were biochemically characterised upto species level by a battery of biochemical tests. Modified Stoke's disk diffusion method was followed for testing antibiotic susceptibility. Beta-lactamase production was checked by nitrocefin disk method. Minimum Inhibitory Concentration for some of these strains against imipenem was done by agar dilution method of NCCLS. Etiological significance of isolating these organisms from UT specimens was also assessed. P. aeruginosa was the commonest (76) followed by B. pickettii (10), P. putida (6), P.fluorescence (2), P. stutzeri (20) P. vesicularis (2), S. putrefaciens (2) and Stenotrophomonas maltophilia (2). Seventy six per cent of P. aeruginosa produced beta-lactamases as compared to 45% of other pseudomonads. The frequency of antibiotic resistance was gentamicin and ciprofloxacin (68.6%) followed by netilmicin (60.7%), ceftazidime (58.8%), amikacin (43.1%) and piperacillin (39.2%). In 42 patients (51.2%) the etiological significance of isolating a pseudomonad could be confirmed. Risk factors for development of UTI were present in 62(75%). Obstructive uropathy (20) followed by post operative period and surgery on urinary tract were the commonest risk factors involved. A high level of resistance was observed for imipenem (P. aeruginosa 43.7% and other pseudomonads 25%).