Isolation and characterization of a novel virulent phage (phiLdb) of Lactobacillus delbrueckii

A new virulent phage (phiLdb) of Lactobacillus delbrueckii subsp. bulgaricus was isolated from a Chinese yogurt sample showing slow acidification. It belonged to the Siphoviridae family with an icosahedral capsid of 47.7 ± 0.9 nm in diameter and a long tail of 129.8 ± 2 nm. The genome of phage phiLdb was estimated to be approximately 41 kbp, and did not contain cohesive ends. One-step growth kinetics of its lytic development revealed latent and burst periods of 45 min and 75 min, respectively, with a burst size of 56 ± 2 phage particles per infected cell. Phage phiLdb was highly specific for Lactobacillus delbrueckii subsp. bulgaricus. The presence of calcium or magnesium ions was necessary to accelerate cell lysis and improve plaque formation. Phage phiLdb was able to survive in a pH range between 2 and 10, and resist ethanol and isopropanol. However, a treatment of 90 °C for 40 min was observed to inactive phage phiLdb thoroughly. Calcium ions, pH as well as temperature did not show significant influence on phage adsorption, and the adsorption kinetics were similar on viable and nonviable cells. The characterization of this novel phage was helpful to establish a basis for adopting the most effective phage control strategies in industrial plants.