Evaluation of Lactose-Sulfite-Phenol Red-Rifampicin (LSPR) Agar for the Co-Enumeration of Salmonella enterica and Shiga-toxigenic Escherichia coli in sausage batter

Accurate enumeration of Salmonella enterica and Shiga-toxigenic Escherichia coli (STEC) is critical for validating pathogen control in dry fermented sausage production. Lactose-Sulfite-Phenol Red-Rifampicin (LSPR) agar is a selective medium designed to recover and enumerate these pathogens under environmental stress. S. enterica produces black colonies on LSPR, while STEC produces cream-colored colonies, enabling visual differentiation. This study evaluates LSPR’s performance for recovering S. enterica and STEC after lactic acid (pH 3.0) and salt (35.9 g NaCl/100 mL) stress and its suitability for co-enumeration in sausage batter.

Cocktails of seven rifampicin-resistant serotypes of S. enterica and STEC were stressed for 0, 30, 60, and 120 seconds, then plated on BHI, LSPR, and modified LSPR (MLSPR). Initial bacterial loads averaged 7.5 log CFU/mL. After 120 seconds of acid stress, STEC declined by 4.2 log CFU/mL, while S. enterica declined by 3.1 log CFU/mL (p < 0.05). Under salt stress, reductions were 2.8 and 2.1 log CFU/mL, respectively (p > 0.05). Two-way ANOVA revealed that exposure time significantly affected bacterial survival (p = 0.0027), with no significant differences in recovery between media types (p = 0.0776). MLSPR did not enhance recovery over LSPR (p = 0.238).

The interaction between media and exposure time was not statistically significant for either acid (p = 0.2380) or salt stress (p = 0.9031), indicating consistent bacterial reductions across media.

The findings indicate that media type had no statistically significant effect on bacterial counts, as LSPR, BHI, and MLSPR supported similar levels of pathogen recovery showing the suitability of the media for co-enumeration of Salmonella and STEC in sausage batter. However, exposure time to stress solution showed a significant impact on bacterial survival, with longer exposure times resulting in greater reductions in STEC and S. enterica counts.