The bacteriocidal effects of transition metal complexes containing the NO+ group on the food-spoilage bacterium Clostridium sporogenes

Abstract The chemical and molecular mechanism of toxicity of nitrite towards food-spoilage bacteria such as Clostridium botulinum or Clostridium sporogenes is not well understood. In order to discover the active species and explore its chemistry, a number of compounds related to nitrite were synthesized. Their bacteriocidal effects on C. sporogenes were investigated in Oxoid nutrient broth No. 2 growth medium at pH 7.0. Inhibition of cell growth, expressed as the concentration which causes 50% cell inhibition, was observed with nitrite at 10 mM, whereas [Fe 4 S 3 (NO) 7 ] − (the anion of Roussin's black salt) and (Fe 2 (SCH 2 CH 2 OH) 2 (NO) 4 ] (a water-soluble Roussin's red salt ester) were found to be effective at 0.001 mM and 0.005 mM, respectively, confirming previous reports that iron-sulphur-nitrosyl complexes are much more toxic to these organisms than nitrite itself. The nitroprusside anion, [Fe(CN) 5 NO] 2− was found to be toxic at 0.030 mM and the corresponding chromium species, [Cr(CN) 5 NO] 3− , at 0.1 mM. Therefore, on the basis of the number of NO groups present, the nitrosylcyano complexes are comparable in activity with the iron-sulphur-nitrosyl compounds. These results show that neither iron nor sulphur are essential for the bacteriostatic effect of the Roussin's type compounds. The property that all these compounds have in common is that they contain NO + . It is proposed that this is the active species responsible for the preservative effect of nitrite, and that a relationship may exist between the NO stretching frequency, a measure of the NO + character, and the toxicity of these NO + -containing complexes.