Casamino acids and oxyrase enhance growth for Listeria monocytogenes in muulti-pathogen enrichments

Authors: Gehring, Andrew; Paoli, George; Reed, Sue; Tu, Shu I; Lindsay, James

Submitted to: Food Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/26/2013
Publication Date: 12/1/2013
Citation: Gehring, A.G., Paoli, G., Reed, S.A., Tu, S., Lindsay, J.A. 2013. Casamino acids and oxyrase enhance growth for Listeria monocytogenes in muulti-pathogen enrichments. Food Control. 40:93-99.

Interpretive Summary: Tests have been developed for the rapid detection of harmful bacteria in foods. However, most rapid tests are not able to sense the presence of very low numbers of bacterial organisms (cells) in food products. Therefore, the food samples are often placed into a nutrient-containing liquid and any bacteria are allowed to grow for a fixed time period (often overnight). This growth technique, referred to as culture enrichment, often results in multiple contaminant bacteria being grown to detectable concentration levels. Unfortunately, not all bacteria grow at the same speed under the same culture enrichment conditions (nutrients, temperature, time, etc.). This investigation compares the growth of four major harmful bacteria (Escherichia coli O157:H7, Listeria monocytogenes, Salmonella enterica serovar Typhimurium, and Yersinia enterocolitica O:8) known to be associated with contamination of food. Different growth nutrients were tested with or without the additives of casamino acids (from degraded milk protein)and/or Oxyrase (an oxygen controlling substance) for their ability to grow a mixture of the four bacteria in a potential food product reservoir (ground pork). Ultimately, the results suggest a significant increase in the concentration of the slowest growing microorganism of the four: Listeria monocytogenes. This new growth technique can be employed by those interested in successfully testing foods with contemporary rapid methods.

Technical Abstract: Tests have been developed for the rapid, multiplex detection of pathogenic bacteria in foods. However, since most rapid methods suffer from not having sufficient sensitivity needed to detect very low levels of pathogens in foods, culture enrichment is often employed to increase numbers of target analyte prior to detection. In particular, it is a great desire to enrich as many different contaminant microorganisms as possible in a timely manner. This investigation compares the growth of four major foodborne pathogens (Escherichia coli O157:H7, Listeria monocytogenes, Salmonella enterica serovar Typhimurium, and Yersinia enterocolitica O:8) inoculated into pristine media or a potential food product reservoir (ground pork) and enriched in various culture media. Initial results suggested additional growth (1-log increase) of the slowest-growing pathogen (L. monocytogenes) in Universal Preenrichment Broth containing added Casamino acids and/or Oxyrase. Growth of L. monocytogenes in broth cultured ground pork was even more dramatic indicating up to a 2-log increase for the pathogen in all of the tested growth media containing either Casamino acids or Casamino acids and Oxyrase. Ultimately, an overnight culture of the inoculated samples in any of the selected media containing both Casamino acids and Oxyrase was observed to yield target bacterial concentrations that were at sufficient levels (between 10e5-10e6 CFU/mL) for detection by an overwhelming majority of existing rapid methods.