Growth response and recovery in selective media of a lysine auxotroph Escherichia coli for a rapid microbiological assay

Human foods and animal feeds vary in their amino acid availability based up on 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 organis m was tested in the presence of an antibiotic and antifungal supplemented m edium previously developed. Overall growth rate comparisons in amended liqu id minimal media showed that addition of antistatic agents did not alter th e growth rate of the indicator strain and that it is independent of lysine concentration. Sir different animal feeds were studied to determine the pot ential background contribution of indigenous feed Escherichia coli and whet her the selective medium would suppress these organisms. Recovery of the in dicator strain used for the rapid bacterial lysine assay was above 94% in a ll feed suspensions. In addition to this, indigenous microflora of the anim al feeds was unable to grow in the presence of the antistatic agents select ed. Microbial growth measured as agar plate colonies from short (1 week) an d long term storage (6 months) feeds were completely suppressed on the anti biotic supplemented plates after 24 h of incubation. This result confirms t hat the amendments will suppress the growth of indigenous feed E. coli popu lations 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.