Partial characterization of two ruminal bacteriophages with similar restriction patterns and different capsids morphology

Abstract. Two bacteriophages (F5 and F6) were isolated from lysogenic Streptococcus bovis strains 46/2 and 59/2, respectively, after spontaneous induction. Both bacteriophages produced clear and small plaques (0.1–0.2 mm in diameter) on two host Streptococcus bovis strains (47/3 and 4/1) out of 12 tested. After UV-light induction, clear plaques of 1.0–1.2 mm in diameter were observed. Both bacteriophages were relatively resistant to chloroform, however, their survival in sterilised rumen fluid was not long because their complete inactivation was observed after 18 hours of cultivation in sterilised rumen fluid. Transmission electron microscopy showed great differences in the morphology of particles of these two bacteriophages. The F5 phage had an elongated slightly angular head, of a mean length of 94 nm and width of 50 nm, as well as a 180 nm long and 10–15 nm wide noncontractile flexible tail without a sheath. The bacteriophage F6 had an isometric head of icosahedral shape, 43 nm in diameter, and its noncontractile flexible tail without sheath was about 180–200 nm long and 10–12 nm wide. According to Ackermann's classification both phages belong to Siphoviridae. The genome sizes of both phages, determined by summing the molecular sizes of fragments generated by digestions with EcoRI, PstI, BglII and XbaI, were similar (about 41–43 kb). Restriction analysis of the genomes of these 2 bacteriophages by 8 restriction enzymes showed that SalI, BamHI and BspRI did not cleave the DNAs of these phages. Cleavage of the DNAs with the other five restriction enzymes showed a similar restriction pattern, however, with small differences. It is a curiosity to find two bacteriophages with the same genome size and relatively similar DNA restriction patterns having a different morphology of their capsids.