Study of drug resistance pattern among isolated strains of Mycobacterium tuberculosis against individual antimycobacterial drugs

Background: In the laboratory in-vitro antimycobacterial sensitivity testing is done conventionally by exposing the test strain to individual drugs by one of the recommended methods. Thus the present work was planned to assess the effects of various antimycobacterial drugs individually. Methods: The study included 73 strains of Mycobacterium tuberculosis isolated from 216 patients suffering from pulmonary tuberculosis and who visited the hospital for the first time and who had not received any antimycobacterial chemotherapy. From all these patients early morning spot sample was collected in a wide-mouthed sterile container and transported immediately to the laboratory. Result: Out of 73 stains, 62 (84.93%) were sensitive to all primary antimycobacterial agents and 11 (15.07%) were resistant to one or more drugs. 6 (8.22%) were resistant to Isoniazid (INH), 9 (12.33%) and 2 (2.74%) strains were resistant to Rifampicin (R’cin) and Streptomycin respectively. Overall resistance to a single drug was higher (8.22%) than to two drugs (5.48%) followed by three drugs (1.37%). Conclusion: Treatment of tuberculosis with appropriate drugs for a defined period is an important factor in the complete remission of disease in the individual patient, as well as in the management of the disease in a community. drug resistance pattern among isolated strains of


Introduction
Every year, about 9 million people suffer from active tuberculosis, and about 2 million die because of this disease [1].
According to the World Health Organization and other international studies stated that the resistance against antituberculosis antibiotics exists all around the world, and the global prevalence of primary resistance is almost 10.7% [1,2].
The first clinically drug-resistant tuberculosis case Thus, it can be understood that tuberculosis treatment with appropriate drugs is the backbone of the Tuberculosis Control Programme.
In Tuberculosis Control Programme antituberculosis drugs are given in combination with two or more drugs to reduce the duration of therapy and to prevent toxicity and emergence of drug resistance.
In the laboratory in-vitro antimycobacterial sensitivity testing is done conventionally by exposing the test strain to individual drugs by one of the recommended methods. Thus the present work was planned to assess the effects of various antimycobacterial drugs individually.

Objectives Material and Methods
The present study was carried out in the department of microbiology in collaboration with the district tuberculosis center at Shri Chhatrapati For Streptomycin [12,13]. The present study carried out antimycobacterial susceptibility testing by using the individual drug.
To achieve these concentrations following dilutions were made.

MIC of H37 RV strain
Definition of resistance: Sensitive -RR of less than 2 Resistance -RR of greater than 8 Doubtful -RR of 4 The doubtful test was repeated from the control slope, if the same reading was obtained, it was called as resistant.    Overall resistance to a single drug was higher (8.22%) than to two drugs (5.48%) followed by three drugs (1.37%).   Thus, by this definition 6.85% of our strains are multidrug-resistant. This is within the Global Multidrug-resistance Rate of 0 to 10.8% [19].
8.22% of our strains were resistant to any one drug. M. tuberculosis acquires drug-resistance mainly through chromosomal mutations. Thus, it is a stepwise accumulation of individual mutations in several independent genes and not a-'block' acquisition of multidrug-resistance. On this basis, these 8.22% strains are important, as they may develop multidrug-resistance in the future.
In the present study, none of the strains were resistant to all 4 primary antimycobacterial agents.
Tuberculosis Research Control study comprising surveillance of drug-resistance tuberculosis has reported the overall resistance to anyone antituberculosis drugs to be from 12.7% to 34% and to any two drugs 1.8% to 15.8%, which is higher resistance pattern over our observations [13].
Resistance to one or more drugs varies in a geographical area and depends on the treatment regiment used [13]. A study conducted in New York City during April 1991 has also shown that 33% of the strains showed primary resistance to one or more antimycobacterial drugs [20]. Chavla and colleagues found an overall resistance of 30.9% in one or more drugs in Brooklyn [21]. have shown resistance to two drugs among which resistance to Isoniazid and Streptomycin was more (6.6%) as compared to Isoniazid and Rifampicin (0.7%) [24].
Jindal SK et al has also shown high resistance to Isoniazid and Streptomycin (6.5%) and resistance to Rifampicin and Isoniazid was 2-4% [25].
Similarly, a surveillance report of tuberculosis in Tamilnadu has reported a 1 to 6% resistance to Isoniazid and Streptomycin. Literature studied shows that acquired resistance to Rifampicin individually or with other drugs is very high (50-70%) which is an alarming situation as Rifampicin is extensively included in current regimens [13].

Conclusion
Treatment of tuberculosis with appropriate drugs for a defined period is an important factor in the complete remission of disease in the individual patient, as well as in the management of the disease in a community.
What does the study add to the existing knowledge?
There is a clear indication that resistance to Isoniazid alone does not readily affect the outcome of treatment but as soon as organisms develop resistance to Rifampicin also, there is a tremendous increase in treatment failure.