Abstract
A virulent bacteriophage which causes a decrease in acid production during fermentation of a lactic acid beverage named Yakult with Lactobacillus casei was isolated from the abnormal fermentation tank and named PL-1. L. casei S strain was the exclusive host cell among 18 lactic acid bacteria tested. The plaque was round with an average diameter of about 0.5 mm. It exhibited serological cross-reaction with previously isolated J1 phage. Under an electron microscope, the phage had a spermatozoon shape, with an icosahedral head (63 nm) and a long tail (12.5 by 275 nm) with about 55 striae. The free phage particles were stable at pH 5 to 8. The phage was quite sensitive to ultraviolet irradiation or to heating (60 C, 5 min), and the host was more sensitive than the phage to these treatments. Many kinds of antimicrobial chemicals were also phagocidal. Calcium ion (5 mm) was specifically essential for the phage growth cycle. A one-step growth experiment under optimum conditions (37 C and pH 6.0) showed that the eclipse period was about 75 min, that the latent period was 100 min after the phage infection, and that the average burst size was about 200. The possibility of arresting phage development in lactic acid fermentation is discussed.