Phage Resistance in a Phage-Insensitive Strain of Streptococcus lactis: Temperature-Dependent Phage Development and Host-Controlled Phage Replication †

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Streptococcus lactis ME2 is a dairy starter strain that is insensitive to a variety of phage, including φ18. The efficiency of plating of φ18 on ME2 and N1 could be increased from <1 × 10−9 to 5.0 × 10−2 and from 7.6 × 10−7 to 2.1 × 10−2, respectively, when the host strains were subcultured at 40°C before plating the phage and the phage assay plates were incubated at 40°C. Host-dependent replication was demonstrated in N1 at 30°C and in N1 and ME2 at 40°C, suggesting the operation of a temperature-sensitive restriction and modification system in ME2 and N1. The increased sensitivity of ME2 and N1 to φ18 at 40°C was also demonstrated by lysis of broth cultures and increased plaque size. ME2 grown at 40°C showed an increased ability to adsorb φ18, indicating a second target for temperature-dependent phage sensitivity in ME2. Challenge of N1 with a φ18 preparation that had been previously modified for growth on N1 indicated that at 40°C phage development was characterized by a shorter latent period and larger burst size than at 30°C. The evidence presented suggests that the high degree of phage insensitivity expressed by ME2 consists of a variety of temperature-sensitive mechanisms, including (i) the prevention of phage adsorption, (ii) host-controlled restriction of phage, and (iii) suppression of phage development. At 30°C these factors appear to act cooperatively to prevent the successful emergence of lytic phage active against S. lactis ME2.

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