Effect of metronidazole on hydrogen production by Clostridium acetobutylicum

Summary

• 1. When metronidazole (1-β-hydroxyethyl-2-methyl-5-nitroimidazole) was added at 50 μg/ml to a liquid (35°C) culture of Clostridium acetobutylicum (0.5 mg dry wt of organisms/ml), it inhibited growth for only 2 h; thereafter, growth resumed at a slower and arithmetic rate. Consumption of glucose and excretion of acetate plus butyrate continued, though at diminished rates, during the period of bacteriostasis, and the ATP content of the organisms did not decline.

• 2. Metronidazole inhibited production of H₂ by a culture of Cl. acetobutylicum, yet it only temporarily halted formation of H₂ from pyruvate by cell extracts, and had no deleterious effect on the simultaneous production of acetyl phosphate and CO₂. After a relatively short time no residual metronidazole was detectable in this cell-free system and H₂ production resumed at its former rate.

• 3. The drug did not inhibit the hydrogenases of either Cl. acetobutylicum or Cl. pasteurianum, but having an E′ ₀ at pH 7 of approx. -0.23 V (on the hydrogen scale), it acted as a preferential acceptor of electrons from reduced ferredoxin (or reduced viologen dyes). Reduction of metronidazole was a 6 electron process compatible with complete reduction of its 5-nitro group. Yet, although the expected decrease in absorbance at 320 nm was observed, the 5-amino derivative did not accumulate (probably due to its intrinsic lability).

• 4. Since many aryl nitro compounds are similarly able to accept electrons from reduced ferredoxin, it seems that additional reasons might have to be sought to explain the unique effectiveness of metronidazole as an antimicrobial agent.