Evaluation of antimicrobial compounds to inhibit growth of select gram-positive pathogenic or antimicrobial resistant bacteria in air-exposed silage

Authors: ONTIVEROS-MAGADAN, MARINA - Universidad Autonoma De Chihuahua; Anderson, Robin; RUIZ-BARRERA, OSCAR - Universidad Autonoma De Chihuahua; ARZOLA-ALVAREZ, CLAUDIO - Universidad Autonoma De Chihuahua; SALINAS-CHAVIRA, JAIME - University Of Tamaulipas; Hume, Michael; SCHOLLJEGERDES, ERIC - New Mexico State University; Harvey, Roger; Nisbet, David; CASTILLO-CASTILLO, YAMICELA - Universidad Autonoma De Chihuahua

Submitted to: Canadian Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/2021
Publication Date: 3/1/2022
Citation: Ontiveros-Magadan, M., Anderson, R.C., Ruiz-Barrera, O., Arzola-Alvarez, C., Salinas-Chavira, J., Hume, M.E., Scholljegerdes, E.J., Harvey, R.B., Nisbet, D.J., Castillo-Castillo, Y. 2022. Evaluation of antimicrobial compounds to inhibit growth of select gram-positive pathogenic or antimicrobial resistant bacteria in air-exposed silage. Canadian Journal of Animal Science. 102:75-84. https://doi.org/10.1139/cjas-2021-0061.
DOI: https://doi.org/10.1139/cjas-2021-0061

Interpretive Summary: Spoiled feedstuffs can harbor pathogenic as well as antimicrobial resistant microbes which can risk infection of food-producing animals and humans. Livestock producers and farmers are interested in finding effective, yet environmentally friendly interventions that may be used to rescue the nutritive and economical value of spoiled feedstuffs while enhancing their microbiological safety. Two naturally occurring medium chain fatty acids called lauric or myristic acid, often found in high amounts in coconut oil, and a related compound called monolaurin are known to exhibit potent antimicrobial activity against many Gram-positive bacteria. When we tested these fatty acid compounds against pure cultures in the laboratory, we observed they exhibited appreciable inhibitory activity against Listeria monocytogenes as well as Enterococcus faecalis. The short-chain nitrocompound, 2-nitropropanol, exhibited modest antimicrobial activity against Enterococcus faecalis yet was a potent inhibitor of L. monocytogenes and another bacterium named Staphylococcus aureus. These inhibitors exhibited antimicrobial effects against the test bacteria when tested in spoiled feedstuffs, albeit to a more modest effect than when tested against the bacteria in individual culture. When added to incubations of gut microbes from a cow, lauric acid, monolaurin, and the 2-nitrocompound were all found to be potent inhibitors of methane production, a beneficial effect that may help reduce economic and environmental costs of this potent greenhouse gas emitted by cows. These results may lead to the development of improved and cost-effective strategies to preserve the nutritive value of feeds fed to animals while reducing risks of their contamination with foodborne pathogens; thereby helping farmers produce more wholesome meat and milk at less cost for the American consumer.

Technical Abstract: Spoiled silages can harbor pathogenic as well as antimicrobial-resistant microbes, with risk of infection of food-producing animals and humans. Livestock producers and farmers are interested in finding effective, yet environmentally friendly interventions that may be used to rescue the biological and economical value of these fermented feedstuffs. The medium chain fatty acid, lauric acid, and its glycerol monoester, monolaurin, exhibit potent antimicrobial activities against many Gram-positive bacteria and when supplemented to pure cultures (each at 5 mg/mL), revealed appreciable inhibitory activities, causing decreases of 2 to 7 Log10 colony forming units (CFU/mL) against Listeria monocytogenes and Enterococcus faecalis, yet 10-fold higher treatment amounts were needed to achieve near equivalent decreases with staphylococci. The short-chain nitrocompound 2-nitropropanol (at 1 mg/mL) exhibited modest antimicrobial activity against E. faecalis yet was a potent inhibitor of L. monocytogenes, causing between 2.4 to 5.9 Log10 CFU/mL decrease in bacterial numbers. These medium chain fatty acids, when administered individually at 5 mg/mL, as well as 2-nitropropanol at 0.5 mg/mL, exhibited more modest inhibition (achieving at most 2 log10 CFU/mL decrease) when tested with mixed ruminal microbe populations in silage than when tested against the pure cultures. Laurate, monolaurin (each at 5 mg/mL), and 2-nitrocompound (1 mg/mL) were all found to be potent inhibitors of methanogenesis, causing > 50% decrease in methane accumulations after 24 h in vitro incubation of mixed populations of ruminal microbes. When added to mixed cultures of ruminal microbes, laurate, and monolaurin (each at 5 mg/mL) were more inhibitory against L. monocytogenes Scott A and wildtype enterococci, achieving decreases of 1.4 to 3 Log10 CFU/mL compared to untreated controls, than were cultures of mixed rumen microbes treated with 1.4 mg 2-nitropropanol/mL, the latter achieving at most a 1.4 Log10 CFU/mL decrease in wildtype enterococci. These results indicated that differential antimicrobial activities exhibited by these chemically dissimilar compounds may yield opportunities to combine treatments to overcome individual limitations and broaden their antimicrobial activities against certain pathogenic and antimicrobial-resistant microbes.