Stressed salmonella are exposed to reactive oxygen species from two independent sources during recovery in conventional culture media.

Previously, Stephens et al. [J. Appl. Microbiol. 83 (1997) 445-455] developed a sensitive technique for studying the resuscitation of low levels of stressed Salmonella. Using this technique the influence on recovery performance of the peptone component of buffered peptone water was investigated. Within 12 different peptone types as much as 3.5 log10 cells/ml difference was observed between the best and worst performing formulations. Poor recovery performance was linked to reactive oxygen species (ROS) generation through auto-oxidation of reducing sugars and photo-sensitisation of sensitive components such as riboflavin. Supplementary recovery agents were explored with only Oxyrase, which has both enzymes to degrade ROS and the ability to rapidly turn a medium anaerobic, having any significant effect. It improved the speed of recovery and increased, by up to 100-fold, the number of stressed cells recovered. Stressed cells were further studied by flow cytometry with cell sorting, based on the staining pattern from a novel fluorochrome combination, into good and poor recovery media. It was identified that within a stressed population the removal of all oxygen protected actively respiring cells the most by forcing them to generate energy from anaerobic metabolism thus avoiding any risk from accidental endogenous ROS generation. The recognition of two independent sources of oxidative stress in the routine use of conventional culture media is discussed in relation to pathogen detection and other areas of food microbiology.