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Nitrate reduction is currently considered to be a key characteristic for the determi- native grouping of strains of the Propionibacterium but many controversies exist regarding nitrate reduction by members of this genus. The aim was thus to determine the influence of dif- ferent media, nitrate concentrations and pH values under aerobie and anoxie conditions, using the conventional technique and the diazotization coupling method, on the nitrate reducing capability of propionibacteria. The low concentrations of nitrite formed during nitrate reduction by the propionibacteria could not be accurately determined using the conventional technique. This method was found to be reliable only when the initial KN03 concentration exceeded 1.0 mmol/L. ln contrast, with high concentrations of nitrite, the diazotization coupling method was found to be too sensitive. Nitrate reduction in the yeast extract lactate (Y EL) medium with added glu- cose and KN03, was not effective as only low concentrations of nitrite were produced. Good nitrate reduction was obtained on both the YEL and yeast extract (YE) media with or without the addi- tion of KN03 (pH 8.0) to the test media. The presence of nitrite in the YEL culture media of the five Propionibacterium type strains, un der aerobie conditions (pH 8.0) with 20 mmol/L KN03, was determined using the diazotization coupling method. The type strains of P. acidipropionici, P. acnes and P.freudenreichii subsp.freudenreichii were able to reduce nitrate. In contrast, the P. jensenii and P. thoenii type strains were not able to reduce nitrate under these conditions. Furthermore, P. acidipropionici and P. acnes were both able to further reduce the formed nitrite to gaseous nitrogen. It was found that nitrate reduction was strongly influenced by environmental factors such as oxygen, nitrate concentration, pH, media composition, incubation period and the presence of glucose. Data are presented which permit a more standardized determination of nitrate reduction of propionibacteria. © Inra/Elsevier, Paris.

Optimized standard conditions for determination of nitrate reduction in propionibacteria