GROWTH RESPONSE AND RECOVERY IN SELECTIVE MEDIA OF A LYSINE AUXOTROPH ESCHERICHIA COLI FOR A RAPID MICROBIOLOGICAL ASSAY

Abstract Human foods and animal feeds vary in their amino acid availability based upon the nature of the protein source and subsequent processing treatments to which the source may have been subjected during manufacture. In this study, growth and recovery of an Escherichia coli lysine auxotroph assay organism was tested in the presence of an antibiotic and antifungal supplemented medium previously developed. Overall growth rate comparisons in amended liquid minimal media showed that addition of antistatic agents did not alter the growth rate of the indicator strain and that it is independent of lysine concentration. Six different animal feeds were studied to determine the potential background contribution of indigenous feed Escherichia coli and whether the selective medium would suppress these organisms. Recovery of the indicator strain used for the rapid bacterial lysine assay was above 94% in all feed suspensions. In addition to this, indigenous microflora of the animal feeds was unable to grow in the presence of the antistatic agents selected. Microbial growth measured as agar plate colonies from short (1 week) and long term storage (6 months) feeds were completely suppressed on the antibiotic supplemented plates after 24 h of incubation. This result confirms that the amendments will suppress the growth of indigenous feed E. coli populations during the time frame typically used to conduct the rapid bacterial lysine assay with the E. coli lysine auxotroph without altering the growth rate response of the auxotroph.