Isolation and characterization of Lactobacillus paracasei LPC and phage ΦT25 from fermented milk

Infections by bacteriophages (phages) cause great economic losses in the dairy industry. Most dairy plants have encountered severe phage contamination, resulting in fermentation failure. This study was the first to isolate and characterize a lactic acid bacteria (LAB) phage from abnormal fermented milk in a Thailand factory. The results showed that only 1 LAB isolate was obtained. This LAB was identified by API 50 CHL, 16S rDNA sequence analysis and PCR using species-specific primers that were designed to differentiate the Lactobacillus casei group. From the above evidence, this isolate was identified as Lactobacillus paracasei. Small plaques were formed on double layer agar plates, and the Lb. paracasei phage was designated as ΦT25. Transmission electron micrographs showed that the phage ΦT25 had a hexagonal head (55 ± 4.6 nm in diameter) and a long noncontractile tail (184 ± 7.8 nm in length), indicating that it belongs to Bradley's group B or the Siphoviridae family. The one-step growth curve revealed a latent period of 55 min, with a burst size of approximately 38 phage particles per infected cell. Our host-range determination study demonstrated that this phage was incapable of cross-infecting with other genera or species of LAB tested. The genome size of the phage ΦT25 was estimated to be 41 kb. This phage did not survive heating to 72 and 90 °C and pH 2. The most effective chemical biocides were 75% ethanol, 800 ppm hypochlorite, 0.15%(v/v) peracetic acid, 1% (w/v) Virkon^®, 5% Dettol^® and 2-, 5% Decon^® 90. This study has provided relevant information about the Lb. paracasei phage and recommendations for phage control in dairy plants.