Injury and death of various Salmonella serotypes due to acidic conditions

Acid injury could prevent detection of Salmonella in feed and feed-type samples. A previous study showed that after incubation in commonly used pre-enrichment media, mixed feeds and feed ingredients reached a pH (4.0 to 5.0), capable of injuring or killing Salmonella. Approximately 105 colony forming units (CFU) of S. Enteritidis (SE), S. Heidelberg (SH), S. Kentucky (SK), or S. Typhimurium (ST) were individually placed into 50 mL of a citrate buffer at pH of 4, 4.5, or 5.0 for 6 or 24 h at 37 degrees C. After holding at 37 degrees C, each serotype was serially diluted in sterile physiological saline and plated onto xylose lysine tergitol 4 [for injury] and nutrient agar (NA) [for death]. These plates were enumerated and percent injury and/or death determined. The injury and death confirmed that pH 4.0 was detrimental to these Salmonella serotypes. After 24 h at pH 4.0, 60% or more of all 4 serotypes were killed with 100% of S. Kentucky killed. At pH 5.0, 65 to 75% of these organisms were uninjured, with death between 15 to 20%. Researchers testing feed/feed ingredients for Salmonella may not be aware of the acidic nature of the pre-enrichment step and the subsequent injury or death of any Salmonella present, whether healthy or stressed. Current protocols that are considered the gold standard may not be detecting Salmonella in samples containing fermentable substrates and extraneous microorganisms, which prevent the accurate detection of Salmonella. Injury or death of a few Salmonella cells could reduce the chance of recovery from any sample. More research is needed with additional Salmonella serotypes and strains to fully understand the challenges of isolating Salmonella from feed.