Nontyphoidal Salmonella are estimated to cause 94 million cases of gastroenteritis and 115,000 deaths globally each year. With human salmonellosis outbreaks, having been linked to contaminated animal feeds, Salmonella detection methods for animal feed and feedstuffs are critical.

Research now shows that commonly used pre-enrichment media (lactose broth and buffered peptone) for feeds/ingredients can reach an acidic pH during incubation that may kill or injure Salmonella serotypes and potentially confound their detection in the laboratory.

The challenge is addressed in two collaborative studies conducted by US Department of Agriculture (USDA) and pathogen control specialist Anitox. Results were unveiled at the International Scientific Poultry Forum at 2017 IPPE. This research investigated how and why pre-enrichment media are inhibiting Salmonella detection, and identifies a new triple buffered peptone broth (TBP) that offers significantly improved recovery.

Anitox Chief Science Officer Dr. Kurt Richardson explains, “Our research has shown that the commonly used pre-enrichment media such as lactose broth and buffered peptone can reach an acidic pH during incubation, sufficient to kill or injure Salmonella present in the feed during the laboratory process, before they can be detected.

“Levels of acidity in the pre-enrichment media injure Salmonella to varying degrees depending on the serotype of Salmonella and stress status. We looked at eight different cultures of Salmonella: S. typhimurium, S.infantis, S. enteritidis, S. heidelberg, S. montevideo, S. senftenberg, S. tennessee and S. swartzengrund in a non-stressed and dry-stress state,” Dr. Richardson says, “In the case of non-stressed Salmonella, S. typhimurium was the most acid tolerant and S. tennessee the most acid sensitive. In the case of dry-stressed Salmonella, S. senftenberg was the most acid tolerant and S. tennessee the most acid sensitive. This suggests the pH of the pre-enrichment media not only influence the recovery, but it can also influence the serotype of Salmonella isolated.

“In our second study, we confirmed the pH sensitivity of four non-stressed strains of Salmonella: S. enteritidis, S. heidelberg, S. kentucky and S. typhimurium. We also evaluated the buffering capacity of a new triple buffered peptone (TBP) pre-enrichment broth compared to the traditional medias,” he says. “All four strains of Salmonella were adversely impacted by acidic conditions, suffering death rates of between 77.5% and 100% at pH ranging from 4.0 to 5.0. S. typhimurium was the most sensitive serotype and S. Heidelberg was the most acid resistant in this study. The new TBP media had more buffering capacity compared to lactose broth and buffered phosphate and shows promise in improving the detection rate of Salmonella from feed/ingredients.

Dr. Richardson concludes, “This work highlights the risk that pre-enrichment media is impacting the accuracy of Salmonella detection, affecting detection rates overall, and even influencing the serotype of Salmonella recovered from feed. On a positive note, the new TBP medium tested shows great promise as a means of maintaining near neutral pH in feed samples during the pre-enrichment step, allowing for accurate detection of Salmonella.”