<prompt> Resistance to isoniazid, the likely mechanism for resistance isb-LactamaseMutations in the katG geneAlterations in the rpoB geneMutations in the DNA gyrase gene  <response>: Strains of M. tuberculosis resistant to individual drugs arise by spontaneous point mutations in the mycobacterial genome that occur at low but predictable rates 10-7-10-10for the key drugs. Resistance to rifampin is associated with mutations in the rpoB gene in 95 of cases; That to isoniazid with mutations mainly in the katG 50-95 of cases and inhA up to 45 genes; That to pyrazinamide in the pncAgene up to 98; That to ethambutol in the embB gene 50-65. Because there is no cross-resistance among the commonly used drugs, the probability that a strain will be resistant to two drugs is the product of the probabilities of resistance to each drug and thus is low. 