CHALK CULTURE METHOD FOR PRESERVING BACTERIA CULTURES AND ITS APPLICATIONS1

Abstract

A simple and efficient chalk culture method was developed to preserve bacterial cultures in this study. The stability of Bacillus cereus, Enterobacter aerogenes ATCC 13048, Escherichia coli, Escherichia coli O157:H7 ATCC 43895, Listeria monocytogenes, Pseudomonas aeruginosa ATCC 25619, Salmonella Typhimurium and Staphylococcus aureus chalk cultures at 25C and 4C was evaluated at 0, 1, 2, 3 and 9 months on Tryptic Soy Agar and a specific agar for each organism. During drying process, B. cereus, E. coli O157:H7 and S. aureus chalk cultures showed less than 1.0 log CFU/g reduction, while E. aerogenes, E. coli, L. monocytogenes, P. aeruginosa and S. Typhimurium showed between 1 to 3 log CFU/g reduction. In the shelf‐life study of chalk cultures, B. cereus and E. coli O157:H7 were stable (< 1 log CFU/g reduction) at both 25C and 4C during the first 3 months. E. aerogenes, E. coli, L. monocytogenes, P. aeruginosa and S. aureus had better stability at 4C (<1 log CFU/g reduction) than 25C (<3 log CFU/g reduction) (P<0.05) during the first 3 months. S. Typhimurium was not stable at 25C and 4C during the first three months (P<0.05). From the third month to the ninth month, B. cereus, E. coli O157:H7, L. monocytogenes and S. Typhimurium were stable at both 4C and 25C (<1 log CFU/g reduction), while E. aerogenes, E. coli, P. aeruginosa and S. aureus were more stable at 4C than at 25C.E. coli O157:H7, L. monocytogenes, S. Typhimurium and S. aureus chalk cultures were inoculated into milk, ground beef and dry pet food, and retained constant (< 1 log CFU/mL or g reduction) after 5 days storage indicating that chalk cultures are stable in these foods. For the heat treatment (55C for 5 min) study, the counts of chalk cultures in milk and ground beef reduced approximately 2 log CFU/mL or g, and about 1 log CFU/g in pet food after heating. The difference was statistically significant compared to the unheated‐inoculated sample (P<0.05). The chalk cultures are easy to prepare, stable in storage and convenient to use for challenge studies especially for dry solid food samples.