Optimizing Salmonella recovery from commercial poultry environmental samples with selective pre-enrichment.

Abstract

The current, culture-based methods for detecting Salmonella are time and resource intensive, as it can take between three to five days with pre-enrichment and selective enrichment steps. Previous work by our group has shortened this process by combining novobiocin and selective ingredients from Rappaport-Vassiliadis (RV) (malachite green; 0.1 g/L) and tetrathionate (TT) (bile salts; 1 g/L) to BPW in parallel, creating an all-encompassing selective pre-enrichment step. In this study, we sought to validate the use of selective pre-enrichment on commercial poultry live production samples, as the increased presence of background bacteria may limit Salmonella recovery. Two pairs of boot sock samples were collected from 35 houses, representing 17 different commercial broiler or breeder farms (n = 70 samples). The samples were cultured under selective pre-enrichment conditions in parallel with standard non-selective pre-enrichment (BPW) followed by selective enrichment (RV, TT). Additionally, molecular enumeration was performed to quantify the amount of Salmonella present in each sample. Overall, Salmonella was found in 74% (52/70) of samples collected, and selective pre-enrichment and selective enrichment conditions each recovered Salmonella in 14/17 farms. The average quantity per sample was greater in those recovered with selective pre-enrichment (5.2 log₁₀ CFU/sample) than those that were not (3.0 log₁₀ CFU/sample; p = 0.01, Welch two sample t-test). CRISPR-SeroSeq was used to determine the relative frequency of Salmonella serovars in each sample and culture condition. The proportion of multiserovar populations observed in the selective pre-enrichment conditions (53%, 29/55) was not significantly different from those in selective enrichment conditions (56%, 39/70; p = 0.1, McNemar's chi-squared test). These findings suggest that increasing the selectivity of the Salmonella pre-enrichment step could eliminate the need for a subsequent selective enrichment, thus reducing the time to Salmonella isolation by 24 hours.