Characterization of a T7-like lytic bacteriophage (phiSG-JL2) of Salmonella enterica serovar gallinarum biovar gallinarum

Abstract

PhiSG-JL2 is a newly discovered lytic bacteriophage infecting Salmonella enterica serovar Gallinarum biovar Gallinarum but is nonlytic to a rough vaccine strain of serovar Gallinarum biovar Gallinarum (SG-9R), S. enterica serovar Enteritidis, S. enterica serovar Typhimurium, and S. enterica serovar Gallinarum biovar Pullorum. The phiSG-JL2 genome is 38,815 bp in length (GC content, 50.9%; 230-bp-long direct terminal repeats), and 55 putative genes may be transcribed from the same strand. Functions were assigned to 30 genes based on high amino acid similarity to known proteins. Most of the expected proteins except tail fiber (31.9%) and the overall organization of the genomes were similar to those of yersiniophage phiYeO3-12. phiSG-JL2 could be classified as a new T7-like virus and represents the first serovar Gallinarum biovar Gallinarum phage genome to be sequenced. On the basis of intraspecific ratios of nonsynonymous to synonymous nucleotide changes (Pi[a]/Pi[s]), gene 2 encoding the host RNA polymerase inhibitor displayed Darwinian positive selection. Pretreatment of chickens with phiSG-JL2 before intratracheal challenge with wild-type serovar Gallinarum biovar Gallinarum protected most birds from fowl typhoid. Therefore, phiSG-JL2 may be useful for the differentiation of serovar Gallinarum biovar Gallinarum from other Salmonella serotypes, prophylactic application in fowl typhoid control, and understanding of the vertical evolution of T7-like viruses.

FIG. 1.

Comparison of φSG-JL2 and φYe-O3-12 promoters. The 15 putative promoter sequences of φSG-JL2 are aligned with those of φYeO3-12. The positions of the first nucleotides of the promoter sequences in the phage genome are given. Homologous nucleotides are represented by dashes.

FIG. 2.

Putative genome organization of φSG-JL2. The locations of putative regulatory elements, host (A0 to A3) and phage (φL to φR) promoters, RNase III recognition sites (R0.3 to R18.5), terminators (T_E and T_φ), and replication origin (Ori) are represented at the top, and the predicted ORFs are numbered and arranged according to reading frame (1, 2, and 3). A point on the scale represents 0.5 kb.

FIG. 3.

Survival curves of φSG-JL2-treated and untreated groups. Eighty 13-day-old commercial male brown layer chicks were challenged with a field strain of serovar Gallinarum biovar Gallinarum (SG101) directly or after it was mixed with φSG-JL2 (MOIs, 0.1, 1, and 10) for 4 h at room temperature, and mortality was observed for 15 days.