Production of the enzyme dihydrofolate reductase by methotrexate‐resistant bacteria isolated from soil

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Abstract
Six bacterial cultures isolated from soil were capable of growing in the presence of methotrexate (MTX). Two strains, PFR-1 and 3, developed resistance to 500 μg cm−3 MTX in the medium and produced elevated levels of the enzyme dihydrofolate reductase (EC 1.5.1.3): 2580 and 2702 U dm−3 compared to the sensitive parent strains (28 and 35 U dm−3). Isolate PFR-3 showed maximum enzyme production (4950 U dm−3, specific activity 12.56 U mg−1 in flasks and 5737 U dm−3, specific activity 14.80 U mg−1 in 5-dm3 fermenter) during exponential phase of growth (6 h) at 37°C and pH 7.0.