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SHORT REPORT

Characterization of Streptococcus thermophilus Host Range Phage Mutants

Martin Duplessis,^1,2 Céline M. Lévesque,^2, and Sylvain Moineau^1,2,3*

Département de Biochimie et de Microbiologie, Faculté des Sciences et de Génie,¹ Groupe de Recherche en Écologie Buccale (GREB), Faculté de Médecine Dentaire,² Félix d'Hérelle Reference Center for Bacterial Viruses, Université Laval, Quebec City, Quebec, Canada G1K 7P4³

Received 25 July 2005/ Accepted 12 January 2006

To investigate phage-host interactions in Streptococcus thermophilus, a phage-resistant derivative (SMQ-301R) was obtained by challenging a Tn917 library of phage-sensitive strain S. thermophilus SMQ-301 with virulent phage DT1. Mutants of phages DT1 and MD2 capable of infecting SMQ-301 and SMQ-301R were isolated at a frequency of 10^–6. Four host range phage mutants were analyzed further and compared to the two wild-type phages. Altogether, three genes (orf15, orf17, and orf18) contained point mutations leading to amino acid substitutions and were responsible for the expanded host range. These three proteins were also identified in both phages by N-terminal sequencing and/or matrix-assisted laser desorption ionization-time-of-flight mass spectrometry. The results suggest that at least three phage structural proteins may be involved in phage-host interactions in S. thermophilus.

Present address: Department of Oral Microbiology, Faculty of Dentistry, University of Toronto, 124 Edward St., Toronto, Ontario, Canada M5G 1G6.

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