Use of 2-hydroxy-4-(methylthio)-butanoic acid to inhibit Salmonella and Listeria in raw meat for feline diets and palatability in domestic cats

Abstract

While the raw pet food market continues to grow, the risk of bacterial contamination in these types of diets is a major concern, with Salmonella enterica and Listeria monocytogenes being the most frequently associated pathogens in raw pet food product recalls. dl-Methionine is included in some commercial feline kibble and canned diets to improve protein quality; however, an alternative to this is a liquid methionine supplement, 2-hydroxy-4-(methylthio)-butanoic acid (HMTBa), which is also an organic acid. 2-Hydroxy-4-(methylthio)-butanoic acid has previously demonstrated similar efficacy to formic acid against pathogens in a liquid environment and may be a good candidate to inhibit S. enterica and L. monocytogenes in raw ground meat. First, the minimum inhibitory concentration and minimum bactericidal concentration of HMTBa against these pathogens under laboratory growth conditions were determined by measuring growth of pathogens over 36 h when exposed to 10 concentrations of HMTBa (0.10% to 1.00%) mixed with tryptic soy broth. 2-Hydroxy-4-(methylthio)-butanoic acid included at ≥0.50% was bactericidal to S. enterica and L. monocytogenes (P < 0.05). Next, five levels of HMTBa (0.50% to 1.25%) were included in raw ground meat mixtures inoculated with cocktails of S. enterica or L. monocytogenes, and contamination levels were determined at four timepoints: immediately, and after refrigerated storage (4 °C) at 24, 48, and 72 h after removal from freezer (24 h at -20 °C). 2-Hydroxy-4-(methylthio)-butanoic acid included as 1.25% of the meat mixture reduced S. enterica and L. monocytogenes compared with the control (P < 0.05); however, it did not result in total kill of either of these pathogens. Following this, feeding behaviors of seven domestic cats were assessed when offered a raw chicken diet treated with or without 1.25% HMTBa for 5 d each, after which a 2-d 2-choice preference test was conducted. Cats demonstrated a preference for raw diets without HMTBa, but still readily consumed diets with 1.25% HMTBa, suggesting that such a diet was still palatable to them.

Keywords: Listeria monocytogenes; Salmonella enterica; 2-hydroxy-4-(methylthio)-butanoic acid; domestic cat; palatability; raw meat-based diet.