Sixty-three virulent bacteriophages of Lactococcus lactis were differentiated by DNA-DNA hybridization. The results, including those of a previous classification of 38 phages of the same bacterial species (P. Relano, M. Mata, M. Bonneau, and P. Ritzenthaler, J. Gen. Microbiol. 133:3053-3063, 1987) show that 48% of the phages analyzed belong to a unique DNA homology group (group III). Phages of this most abundant group had small isometric heads. Group I comprised 29% of the phages analyzed and was characterized by a small phage genome (19 to 22 kilobases) and a particular morphology with a prolate head. Like group III, this group contained representative phages of other classifications. Group II (21%) included virulent and temperate phages with small isometric heads. Two large isometric-headed phages, +109 and 4111, were not related to the three DNA homology groups I, II, and III. The genome of +111 was unusually large (134 kilobases) and revealed partial DNA homology with another large isometric phage, 1289, described by Jarvis (type e) (A. W.

Comparisons of 23 virulent phages of Streptococcus salivarius subsp. thermophilus were made based on morphology, host-range, structural protein analysis, DNA restriction patterns and DNA homology. All the phages had isometric heads (diameters 55-58 nm) and striated tails (lengths 221-275 nm). The genome sizes ranged from 37 to 43 kb. The electrophoretic profiles of the structural proteins were related, with at least two major bands of about 23 and 29 kDa in common for all the phages except three, $80, $96 and $99. These latter phages, which had a particular protein composition (main proteins of 29 and 40 kDa) and high DNA homology between each other, showed only a low DNA homology with the other phages. Extensive DNA homologies were demonstrated by Southern hybridization for all the other phages. All these results suggest a close evolutionary relationship between the phages.

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A 7.275‐kb DNA fragment which encodes resistance by abortive infection (Abi+) to bacteriophage was cloned from Lactococcus lactis subsp. cremoris S114. The genetic determinant for abortive infection was subcloned from this fragment. This gene was found to confer a reduction in efficiency of plating and plaque size for prolate‐headed bacteriophage φ53 (group I homology) and for small isometric‐headed bacteriophage φ59 (group III homology). This new gene, termed abiN, is predicted to encode a polypeptide of 178 amino acid residues with a deduced molecular mass of 20 461 Da and an isoelectric point of 4.63. No homology with any previously described genes was found. A probe was used to determine the presence of this gene only in S114 from 31 strains tested.

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