Growth control of listeria monocytogenes in experimental cheese samples by lactobacillus casei RN 78 and its bacteriocin

Anti-listerial effect of freeze dried fractions of partially purified bacteriocin (Fd-PPL) and the producer strain Lactobacillus casei RN 78 strain was evaluated in experimental cheese samples during storage at two different temperatures (4 and 35 degrees C). With the addition of 6400 AU/g of Fd-PPL fractions to the cheese samples of the initial concentration of 4.81 +/- 0.06 log CFU/ml of Listeria monocytogenes was reduced up to 0.91 +/- 0.01 log CFU/ml. Whilst, in the presence of 10(7) CFU/ml live bacterial cells of L. casei RN 78 of the viability of the sensitive cells decreased sharply (0.71 log CFU/ml). The effect was more pronounced after 24 h of incubation and high levels of antibacterial activity (12800 AU/ml) was seen in these samples. An enhanced reduction in the pH was recorded (4.8 - 5.0) in the cheese samples inoculated with live producer cells compared to control samples without any enrichment. A synergistic bactericidal effect of Lactocin RN 78 in combination with 3% Sodium chloride in cheese samples was observed, and L. monocytogenes population were reduced to 0.69 log CFU/ml within 90 days of incubation, at 35 degrees C. In contrast to the producer strain, the antibacterial effect of Lactocin was more pronounced in cheese samples stored at 4 degrees C. The texture of the experimental cheese samples including odour, colour and consistency in different batches were also recorded through out the study.