Bacteriophages of lactobacilli.

Lactobacilli are members of the bacterial flora of lactic starter cultures used to generate lactic acid fermentation in a number of animal or plant products used as human or animals foods. They can be affected by phage outbreaks, which can result in faulty and depreciated products. Two groups of phages specific of Lactobacillus casei have been thoroughly studied. 1. The first group is represented by phage PL-1. This phage behaves as lytic in its usual host L. casei ATCC 27092, but can lysogenize another strain, L. casei ATCC 334. Bacterial receptors of this phage are located in a cell-wall polysaccharide and rhamnose is the main component of the receptors. Ca2+ and adenosine triphosphate (ATP) are indispensable to ensure the injection of the phage DNA into the bacterial cell. The phage DNA is double-stranded, mostly linear, but with cohesive ends which enables it to be circularized. The vegetative growth of PL-1 proceeds according to the classical mode. Cell lysis is produced by an N-acetyl-muramidase at the end of vegetative growth. 2. The second group is represented by the temperate phage phi FSW of L. casei ATCC27139. It has been shown how virulent phages originate from this temperate phage in Japanese dairy plants. The lysogenic state of phi FSW can be altered either by point mutations or by the insertion of a mobile genetic element called ISL 1, which comes from the bacterial chromosome. This is the first transposable element that has been described in lactobacilli. Lysogeny appears to be widespread among lactobacilli since one study showed that 27% of 148 strains studied, representing 15 species, produced phage particles after induction by mitomycin C. Similarly, 23 out of 30 strains of Lactobacillus salivarius are lysogenic and produce, after induction by mitomycin C, temperate phages, killer particles, or defective phages. Temperate phages have also been found in 10 out of 105 strains of Lactobacillus bulgaricus or Lactobacillus lactis after induction by mitomycin C. Phages so far studied of the latter 2 and closely related lactobacilli, either temperate or isolated as lytic, may be divided into 4 unrelated groups called a, b, c and d. Most of these phages are found in group a and an unquestionable relationship has already been shown between lytic phages and temperate phages that belong to this group. Lytic phage LL-H of L. lactis LL 23, isolated in Finland, is one of the most representative of those of group a and has been extensively studied on the molecular level.