Revisiting the susceptibility testing of Mycobacterium tuberculosis to ethionamide in solid culture medium.

Background & objectives: Increase in the isolation of drug resistant phenotypes of Mycobacterium tuberculosis necessitates accuracy in the testing methodology. Critical concentration defining resistance for ethionamide (ETO), needs re-evaluation in accordance with the current scenario. Thus, re-evaluation of conventional minimum inhibitory concentration (MIC) and proportion sensitivity testing (PST) methods for ETO was done to identify the ideal breakpoint concentration defining resistance. Methods: Isolates of M. tuberculosis (n=235) from new and treated patients were subjected to conventional MIC and PST methods for ETO following standard operating procedures. Results: With breakpoint concentration set at 114 and 156 μg/ml, an increase in specificity was observed whereas sensitivity was high with 80 μg/ml as breakpoint concentration. Errors due to false resistant and susceptible isolates were least at 80 μg/ml concentration. Interpretation & conclusions: Performance parameters at 80 μg/ml breakpoint concentration indicated significant association between PST and MIC methods.

Exposure to cetyl pyridinium chloride and loss of integrity of cell wall of mycobacteria.

Background: Cetyl pyridinium chloride (CPC) liquefied sputum was shown to reduce AFB smear positivity presumably damaging cell wall of M. tuberculosis. Settings: National Institute for Research in Tuberculosis, Chennai, (Tamil Nadu). Objective: To assess the cell wall damage of mycobacteria in CPC liquefied sputum, by Transmission Electron Microscopy (TEM) and mycobacteriophage adsorption studies. Methods: Pooled sputum sample from smear positive pulmonary TB patients was homogenized and liquefied with CPC. It was examined in TEM daily for four days, to assess cell wall damage of M. tuberculosis, and photomicrographs were taken. M. smegmatis mc2155, treated with CPC, was infected with mycobacteriophage (phAE129) to study phage adsorption on cell wall and plaque formation. CPC untreated sputum and M. smegmatis formed controls. Results: Photomicrographs showed that cell wall of M. tuberculosis was intact in controls and damaged in CPC preserved sputum for 96 hours. Plaque formation was seen and absent respectively in CPC untreated and treated M. smegmatis cells. Conclusion: Exposure to CPC damaged the cell wall of M. tuberculosis within 96 hours. Mycobacteriophage failed to form plaques after M. smegmatis mc2155 was treated with CPC implying inhibition of phage adsorption on damaged cell wall and thus providing a clue for poor staining and smear positivity in microscopy.

Trends in the prevalence of Pulmonary Tuberculosis over a period of seven and half years in a rural community in south India with DOTS.

Setting: Tiruvallur district In Tamil Nadu where DOTS was implemented by the State Government as the tuberculosis control measure in 1999, and monitored by the National Institute for Research in Tuberculosis for over five years. Objective: To estimate trends in TB prevalence in a rural community with DOTS. Design: Surveys of pulmonary tuberculosis were undertaken in representative samples of subjects aged >15 years (N = 83,000 – 92,000), initially and after two and half, five and seven and half years of implementation of DOTS. Sputa were collected from those with abnormal radiograph and/or presence of chest symptoms, and examined by direct smear and culture. Results: The prevalence of culture-positive tuberculosis was 607, 454, 309 and 388 per 100,000 in the four surveys, and that of smear-positive tuberculosis was 326, 259, 168 and 180. In the first five years; annual decrease was 12.4% (95% CI 10.4 - 14.4%) for culture-positive tuberculosis, and 12.2% (95% CI 8.0–16.2) for smear-positive tuberculosis. This was, however, followed by a significant increase in the next two and half years. The average new smear-positive case-notification rate was 75 per 100,000 during first four years but declined to 49 in subsequent years. There were no methodological differences during this period and information on changes in socio-economic indicators and nutritional standards was unavailable. Conclusion: Despite the average annual success rate (78%) in this tuberculosis unit being lower than the expected rate of 85%, the implementation of DOTS was followed by a substantial decrease in the prevalence of pulmonary tuberculosis over the seven and half year period. Our findings suggest that sustaining the high effectiveness of DOTS programme needs vigilant supervision.

Efficacy of the 6-month thrice-weekly regimen in the treatment of new sputum smear-positive pulmonary tuberculosis under clinical trial conditions.

Background. Under the Revised National Tuberculosis Control Programme of India, patients with new smear-positive pulmonary tuberculosis are treated with a thrice-weekly regimen of antitubercular drugs (2H3R3Z3E3/4H3R3 [H isoniazid, R rifampicin, Z pyrazinamide and E ethambutol]) for 6 months. We conducted a retrospective analysis of the efficacy and tolerability of this regimen under clinical trial conditions in HIV-negative patients with newly diagnosed smear-positive pulmonary tuberculosis. Methods. We retrospectively analysed the data on patients assigned to the control regimen (2H3R3Z3E3/4H3R3) in two clinical trials during 2001–06 at the National Institute for Research in Tuberculosis, Chennai, India. Results. Of the 268 patients treated with this regimen, data for efficacy analysis were available for 249. At the end of treatment, of 249 patients, 238 (96%) had a favourable status. Treatment failure occurred in the remaining 11: 7 in whom the organisms were initially drug-susceptible and 4 with initial drug resistance. Of the 238 patients who had a favourable status at the end of treatment, 14 (6%) had recurrence of tuberculosis during the following 24 months. In the intention-to-treat analysis, 245 (94%) of 262 patients had a favourable status at the end of treatment. Of the 28 patients with initial drug resistance, 24 (86%) had a favourable outcome. Only 4 of these 24 patients were found to have recurrence of tuberculosis in 2 years of follow-up. Among the 221 patients initially infected with drug-susceptible organisms, drug resistance did not develop in any of the 7 patients in whom the treatment failed or the 10 who had recurrence of tuberculosis. Further, 5 of the 7 patients in whom the treatment failed continued to excrete drug-susceptible bacilli at 6 months. Adverse drug reactions were observed in 38 (14%) of the 262 patients. Only 3 (1.1%) needed a modification in the treatment. Conclusion. This thrice-weekly 6-month regimen of antitubercular drugs, when administered under full supervision, is associated with a high rate of favourable treatment outcomes in HIV-negative patients with newly diagnosed sputum smearpositive pulmonary tuberculosis. There are few adverse drug reactions in these patients.

Elucidating pyrazinamide resistance in mycobacterium tuberculosis by molecular docking.

Pyrazinamide (PZA) - an important drug in the anti-tuberculosis therapy, activated by an enzyme Pyrazinamidase (PZase). The basis of PZA resistance in Mycobacterium tuberculosis (Mtb) is owing to mutation in pncA gene coding for PZase. The identification of the structural or functional defects in the mutant enzymes leading to resistance still remains an area to be explored. The Wild-type (WT) and five mutant models Asp8Gly, Lys96Thr, Ser104Arg, Cys138Ser and Cys138Tyr were docked with PZA and its derivatives. In the present study, docking results has aided in predicting the best form of PZA to bind with mutants of PZase to be compounds-10 and 4. These models represent the first in-silico evidence for the binding interaction of PZase with PZA derivatives/analogues. The models may provide useful chemical insights for designing new anti-TB agents in order to overcome the resistance developed with PZA.

Prevalence of pulmonary tuberculosis amongst thebaigas A primitive tribe of Madhya Pradesh, Central India.

Background: A community-based cross-sectional tuberculosis (TB) disease prevalence survey was undertaken amongst the Baiga primitive tribal community of Baiga Chak in central India. Material and Methods: A population of 2,359 was covered under the study. Sputum samples were collected from chest symptomatics and examined for smear microscopy and culture. Results: Overall prevalence of PTB was 146 (95% C.I: 0 - 318) per 100,000 population. Conclusion: The findings suggest that TB is not a major public health problem amongst this tribal group. However, there is still the need to maintain and further strengthen TB control measures on a sustained and long term basis in the area.

Pot method for storage & detection of acid-fast bacilli from sputum samples.

BACKGROUND & OBJECTIVES: Sputum acid-fast bacilli (AFB) microscopy services are not available in all health facilities. Alternative procedures are needed to transport sputum samples to the diagnostic centres for detection of AFB. The objective of the present study was to evaluate sputum smears made by pot-method with the direct smears made immediately after sputum collection by Ziehl-Neelsen (ZN) method. METHODS: Ninety three sputum samples from 49 pulmonary tuberculosis suspects were studied. Their direct smears (ZN smears) were stained by hot ZN method. The samples were then mixed with phenol ammonium sulphate basic fuchsin solution and stored at ambient conditions. The smears (pot smears), made on day 7, were then, decolourized and counter-stained for detection of AFB (pot method). The ZN and pot smears were read blind. After excluding 18 samples for various reasons, the results of pot and ZN smears of 63 samples from smear positive (2 of 3 direct smears were positive) and 12 from smear negative (3 of 3 direct smears were negative) patients were analysed. ZN method was the gold standard. RESULTS: Pot and ZN smears were positive in 61 of 63 samples from smear-positive patients and negative in 11 of 12 smear-negative patients (kappa = 0.87). The sensitivity and specificity of pot method were 96.8 and 91.7 per cent respectively. INTERPRETATION & CONCLUSION: Sputum samples can be stored for up to seven days in the sputum container with phenol ammonium sulphate basic fuchsin solution. However, a comprehensive study needs to be done confirm the accuracy of the pot method for storage and transportation of sputum to microscopy centres for detection of AFB.

Phenol ammonium sulphate basic fuchsin staining of sputum in pot for the detection of acid-fast bacilli.

BACKGROUND & OBJECTIVES: Improper practices of making direct smears of sputum for detection of acid-fast bacilli (AFB) and of disposing sputum cups are hazardous. The present study was undertaken with the objective to stain sputum samples in their containers by 'phenol (10%) ammonium sulphate (4%) basic fuchsin (2%) solution' and to decolourize and counterstain their smears for detection of AFB- (henceforth called pot method) and to compare the smear results of pot method with the standard Ziehl-Neelsen (ZN) method. METHODS: A total of 575 selected sputum samples from pulmonary tuberculosis patients were stained by the standard ZN and pot methods and the proportions of AFB positive smears were compared. RESULTS: Of the 575 samples, 126 were AFB positive for both the staining methods and the difference was not statistically significant. Pot method missed 9 ZN positive smears (8 scanty and one 1+) and ZN method missed 9 pot positive smears (9 scanty) and the difference was not significant. High grade smears (3+) were seen more in pot method (42) than in ZN method (25) and the difference was significant. INTERPRETATION & CONCLUSION: Our findings showed that pot method was comparable to standard ZN method and had many advantages. Pot method can be explored further for the detection of AFB in sputum samples obtained from pulmonary tuberculosis suspects.

Sputum conversion at the end of intensive phase of Category-1 regimen in the treatment of pulmonary tuberculosis patients with diabetes mellitus or HIV infection: An analysis of risk factors.

BACKGROUND & OBJECTIVE: New smear-positive pulmonary tuberculosis (PTB) patients in the Revised National Tuberculosis Control Programme (RNTCP) are treated with a 6-month short-course chemotherapy (SCC) regimen irrespective of co-morbid conditions. We undertook this retrospective analysis to compare sputum conversion rates (smear, culture) at the end of intensive phase (IP) of Category-1 regimen among patients admitted to concurrent controlled clinical trials: pulmonary tuberculosis alone (PTB) or with type 2 diabetes mellitus (DM-TB) or HIV infection (HIV-TB), and to identify the risk factors influencing sputum conversion. METHODS: In this retrospective analysis sputum conversion rates at the end of intensive phase (IP) in three concurrent studies undertaken among PTB, DM-TB and HIV-TB patients, during 1998 - 2002 at the Tuberculosis Research Centre (TRC), Chennai, were compared. Sputum smears were examined by fluorescent microscopy. HIV infected patients did not receive anti-retroviral treatment (ART). Patients with DM were treated with oral hypoglycaemic drugs or insulin (sc). RESULTS: The study population included 98, 92 and 88 patients in the PTB, DM-TB and HIV-TB studies. At the end of IP the smear conversion (58, 61, and 62%) and culture conversion (86, 88 and 92%) rates were similar in the three groups respectively. The variables associated with lack of sputum smear or culture conversion were age >45 yr, higher pre-treatment smear and culture grading, and extent of the radiographic involvement. INTERPRETATION & CONCLUSION: Our findings confirm that the current policy of the control programme to treat all pulmonary TB patients with or with out co-morbid conditions with Category-I regimen appears to be appropriate.

Management of multi drug resistance tuberculosis in the field: Tuberculosis Research Centre experience.

SETTING: Multi-drug TB resistant (resistant to isoniazid and rifampicin) patients identified from a rural and urban area. OBJECTIVE: To study the feasibility of managing MDR TB patients under field conditions where DOTS programme has been implemented. METHODS: MDR TB Patients identified among patients treated under DOTS in the rural area and from cases referred by the NGO when MDR TB was suspected form the study population. Culture and drug susceptibility testing were done at Tuberculosis Research Centre (TRC). Treatment regimen was decided on individual basis. After a period of initial hospitalization, treatment was continued in the respective peripheral health facility or with the NGO after identifying a DOT provider in the field. Patients attended TRC at monthly intervals for clinical, sociological and bacteriological evaluations. Drugs for the month were pre-packed and handed over to the respective center. RESULTS: A total of 66 MDR TB patients (46 from the rural and 20 from the NGO) started on treatment form the study population and among them 20 (30%) were resistant to one or more second line drugs (Eto, Ofx, Km) including a case of "XDR TB". Less than half the patients stayed in the hospital for more than 10 days. The treatment was provided partially under supervision. Providing injection was identified to be a major problem. Response to treatment could be correctly predicted based on the 6-month smear results in 40 of 42 regular patients. Successful treatment outcome was observed only in 37% of cases with a high default of 24%. Adverse reactions necessitating modification of treatment was required only for three patients. IMPLICATIONS: Despite having reliable DST and drug logistics, the main challenge was to maintain patients on such prolonged treatment by identifying a provider closer to the patient who can also give injection, have social skills and manage of minor adverse reactions.

Sensitivity of Ziehl-Neelsen method for centrifuged deposit smears of sputum samples transported in cetyl-pyridinium chloride.

BACKGROUND & OBJECTIVES: Sensitivity of Ziehl-Neelsen (ZN) method is known to be low for liquefied sputum smears. Information on the ZN sensitivity for centrifuged deposit smears is not known. This study was carried out to determine the sensitivity of ZN method for acid fast bacilli (AFB) in centrifuged deposit smears and liquefied sputum smears made from sputum samples transported in cetyl-pyridinium chloride (CPC) solution. METHODS: Liquefied sputum smears and the corresponding centrifuged deposit smears from each of the 607 consecutive sputum samples collected from tuberculosis patients admitted to receive treatment transported in CPC were read by the same readers and their results compared with culture results. RESULTS: A significantly (P<0.001) higher proportion of samples were positive in centrifuged deposit smears (40%) compared to liquefied sputum smears (30%). The results of 341 culture-positive specimens revealed that the sensitivity of ZN method was 47 per cent using liquefied sputum smears and 63 per cent using centrifuged deposit smears (P<0.001). INTERPRETATION & CONCLUSION: Our study demonstrated that the sensitivity of ZN method for AFB in centrifuged deposit smears and liquefied sputum smears was reduced if sputum samples are transported in CPC solution.

Influence of drug susceptibility on treatment outcome and susceptibility profile of failures to category II regimen.

Objective: To assess the influence of drug resistance on treatment outcome among patients treated with Category-II regimen and document drug susceptibility pattern of “Failures” to this regimen. Design: A retrospective analysis of patients registered from May 1999 through December 2004. Results: Treatment success was 42% among 572 patients and was similar among patients with fully susceptible or resistant but non-MDR organisms (41% of 254 and 40% of 128 patients, respectively). Among 49 MDR-TB patients, 27% had successful treatment outcome. The failure rates among patients with fully susceptible, resistant but non-MDR and MDR bacilli, were 6%, 12% and 27% respectively. Default was significantly higher among males (53% vs. 34%: p<0.01) smokers (57% vs. 36%: p <0.001), alcoholics (58% vs. 39%: p <0.001) and patients with higher initial smear grading (2+ or 3+, 56% vs. scanty or 1+, 44%: p <0.01). DST results were available for 60% (31 of 52) of failures and 10 had MDR-TB. Conclusion: The low success rate to the re-treatment regimen was mainly due to non-compliance. Failure was observed among 9% of patients and MDR-TB was 32% among Category II failures. The currently recommended Category II regimen appears to be adequate for majority of re-treatment cases.

Association of conversion & cure with initial smear grading among new smear positive pulmonary tuberculosis patients treated with Category I regimen.

BACKGROUND & OBJECTIVES: Early diagnosis of tuberculosis (TB) is important for initiating treatment to gain cure. The present investigation was undertaken to study the association of conversion and cure with initial smear grading among pulmonary tuberculosis (TB) patients registered in a directly observed treatment - short course (DOTS) programme in Tiruvallur district, south India. METHODS: All new smear positive cases registered from May, 1999 to December, 2002 were analysed for conversion and cure related to initial smear grading. RESULTS: Of the 1463 patients, 1206(82.4%) were converted at the end of the intensive phase and 1109 (75.8%) were declared 'cured' after the completion of treatment. The cure rate decreased as the initial smear grading increased and the decrease in trend was statistically significant (P=0.01). Similarly, a significant decrease in conversion rate was also observed with increase in initial smear grading (P<0.001). In multivariate analysis, lower cure rate was significantly associated with patient's age (AOR=1.5, 95% CI=1.1-2.1), alcoholism (AOR=1.7, 95% CI 1.2- 2.4) and conversion at the end of intensive phase (AOR=3.5, 95% CI= 2.6-4.8). INTERPRETATION & CONCLUSION: Cure and conversion rates were linearly associated with initial smear grading. High default and death rates were responsible for low cure and conversion. The proportion of patients who required extension of treatment and those who had an unfavourable treatment outcome were significantly higher among patients with a 3+ initial smear grading. This reiterates the need to pay more attention in motivating these patients to return to regular treatment and sustained commitment in the control of tuberculosis. There is a need to extend the treatment for one more month in the intensive phase of treatment.

A novel method of staining acid-fast bacilli in sputum containers.

BACKGROUND & OBJECTIVES: Making centrifuged deposit smears from sputum to detect acid-fast bacilli (AFB) is considered hazardous. We carried out this study to stain the centrifuged deposits with carbol-fuchsin in sputum containers and to decolourize and counterstain their smears made on glass slides. METHODS: The centrifuged deposits of 180 sputum samples from pulmonary tuberculosis patients were used for making smears (initial deposit smears) and staining by Ziehl-Neelsen (ZN) method for the detection of AFB. Each of the sputum deposit was then treated with one ml of 1 per cent carbol-fuchsin and a smear made between 2 to 3 h was then decolourized and counterstained by the same procedures followed in ZN method (2 h stained deposit smear). The coded initial deposit smears and the corresponding 2 h stained deposit smears were read by the same readers and the results compared. RESULTS: One hundred and fifty (70 positive and 80 negative) 2 h stained deposit smears were compared with initial deposit smears and the difference was not statistically significant. INTERPRETATION & CONCLUSION: Centrifuged deposits of sputum in sputum containers can be stained by carbol-fuchsin within 2-3 h and their smears made subsequently on glass slides can then be decolourized and counterstained by the procedures followed in ZN method for detection of AFB by light microscopy.

Is it worth treating Category I failure patients with Category II regimen.

Background: Very little information is available on the drug susceptibility profile among patients who are treated with standardized short-course chemotherapy regimens under programme conditions. Methods: Sputum samples were collected from new sputum smear-positive patients declared ‘failure’ after treatment with Category I regimen under tuberculosis control programme using DOTS strategy from a rural area of Tamil Nadu. Results: Of 1463 patients started on Category I regimen between May1999 and December 2002, 74 cases were declared as ‘failures’ (smear positive at 5/6 months of treatment). We collected sputum samples from 60 (81%) of 74 ‘failures’ and 27% (16 of 60) of them were culture-negative for M tuberculosis and 17% (10 of 60) had organisms resistant to Isoniazid and Rifampicin (MDR TB). Conclusion: Based on the drug susceptibility profile at the time of ‘failure’, treating Category I ‘failures’ with Category II regimen with close monitoring appears to be justified.

Estimation of burden of tuberculosis in India for the year 2000.

BACKGROUND AND OBJECTIVE: Data on the burden of tuberculosis (TB) in India are vital for programme planners to plan the resource requirements and for monitoring the nation-wide TB control programme. There was a need to revise the earlier estimate on the burden of TB in India based on the increase in population and current epidemiological data. This study estimates the burden of disease for the year 2000 based on recent prevalence of TB and annual risk of tuberculosis infection (ARTI) estimates. METHODS: Data on prevalence generated among adults by the Tuberculosis Research Centre (TRC), Chennai, among children by National Tuberculosis Institute (NTI), Bangalore, and the ARTI estimates from the nation-wide sample survey by NTI and TRC were used for the estimation. The prevalence of disease corresponding to 1 per cent ARTI was extrapolated to different parts of the country using the estimates of ARTI and the population in those areas and added together to get the total cases. Abacillary cases that required treatment were estimated from X-ray abnormals. The estimates of bacillary, abacillary and extrapulmonary cases were then combined to get the national burden. RESULTS: The estimated number of bacillary cases was 3.8 million (95% CI: 2.8-4.7). The number of abacillary cases was estimated to be 3.9 million and that for extrapulmonary cases was 0.8 million giving a total burden of 8.5 million (95% CI: 6.3-10.4) for 2000. INTERPRETATION AND CONCLUSION: The present estimate differs from the earlier estimates because we have included the disease burden of X-ray cases that are likely to breakdown to bacillary cases in a one year period, and extrapulmonary TB cases. The current estimates provided baseline information for advocacy and planning resource allocation for TB control activities. Also, these estimates can be compared with that in future years to measure the long term impact of TB control activities in India.

Proficiency to read sputum AFB smears by senior tuberculosis laboratory supervisors under training at a reference laboratory in India.

Background: A national reference laboratory imparting training on sputum AFB smear microscopy to fresh Senior Tuberculosis Laboratory Supervisors (STLS). Aim: To assess the proficiency of STLSs under training to read sputum AFB smears. Methods: Each of 342 trainees read the same set of 15 to 20 Ziehl Neelsen stained smears in a blinded fashion on day– 1 and on day-15 of the training programme. The smear results were matched with the original results. Observations: The sensitivity, specificity, positive predictive value and negative predictive value of smear reading were 75%, 88%, 93%, 63% and 94%, 99%, 99%, 89% respectively on day-1 and day –15. Conclusion: The sensitivity to read sputum AFB smears by fresh STLSs with little or no experience increased from 75% to 94% during the carefully planned training programme; the specificity increased from 88% to 99%. The study highlights the importance of training in improving the microscopy results.

Role of laboratory in RNTCP.

Tuberculosis (TB) remains a major public health problem in many parts of the world. Sputum smear microscopy is the mainstay of diagnosis of TB. RNTCP follows the international guidelines which recommend the establishment of microscopy centre for every 100,000 population. All patients with a cough of three weeks or more should undergo 3 sputum diagnostic examinations for acid-fast bacilli (AFB). A separate TB laboratory register is maintained in each microscopy centre. Disposal of laboratory waste should be destroyed. One slide from each patient is sent to the District TB Centre (DTC) for external quality assurance (EQA). The activities of the microscopy centres in a TB unit are supervised by a Senior TB Laboratory Supervisor (STLS). The STLS visits every microscopy centre at least once a month. Sputum samples from patients who fail RNTCP treatment are sent to the nearest reference laboratory for culture of M. tuberculosis and drug susceptibility testing; however there is no need to send patient samples routinely for culture.

Bioluminescence assay of adenosine triphosphate in drug susceptibility testing of Mycobacterium tuberculosis.

Twenty three clinical isolates M. tuberculosis and the reference strain, M. tuberculosis H37Rv were tested for their susceptibility to trifluoperazine (TFP) by the standard broth dilution method and the bioluminescence assay. The results showed that in 15 of the 23 isolates, the minimal inhibitory concentration (MIC) was identical in both the methods and in the remaining 8 isolates the difference in the MIC values between the methods, was less than two fold and was not significant. The findings suggest that the measurement of adenosine triphosphate (ATP) by bioluminescence assay can be employed as an alternative method for the rapid screening of clinical isolates for their susceptibility to anti-mycobacterial agents.

In vitro susceptibility of clinical isolates of Mycobacterium tuberculosis to cefadroxil--a cephalosporin antibiotic.

The bactericidal activity (BA) of cefadroxil, a semisynthetic cephalosporin antibiotic, against M. tuberculosis H37Rv was studied in Middlebrook 7H9 medium. Cefadroxil showed good BA (average fall of viable counts = log10 0.32 colony forming units/ml/day) against the log phase culture of M. tuberculosis H37Rv. Its minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) were found to be 15 micrograms/ml or less. The MIC of cefadroxil for 29 clinical isolates of M. tuberculosis and a laboratory strain, M. tuberculosis H37Rv was also determined by agar dilution method using Middlebrook 7H11 agar as a screening procedure. The MIC of cefadroxil was found to be 10 micrograms/ml or less for M. tuberculosis H37Rv and 16 (55.1%) of 29 clinical isolates tested. The MIC for 3 of 10 drug sensitive and 9 of 19 drug resistant isolates was 40 or more, a concentration much higher than the peak plasma concentration (28 micrograms/ml) attained in human beings. The higher MIC observed in 12 of 29 clinical isolates irrespective of their susceptibility pattern requires further studies to assess the usefulness of cefadroxil in the treatment of tuberculosis.