Salmonella is a natural inhabitant of the gastrointestinal tract of animals and abundant in animal feces, various waste effluents typically provide the best source for phage isolation [15]. It is reported that the separation rate of sewage samples (47.2%) was much higher than stool samples (26.1%) among 1,100 samples in Jiangsu [16]. In this study, swine manure and sewage samples were collected to isolate the lytic phage against a selected group of Salmonella. However, only one lytic phage of Salmonella was isolated from the sewage sample. The overall isolation rate (2.5%) of among swine manure and sewage samples in this study was considerably lower than that reported in other study (47.2%; 26.1%)[16], and it was slightly lower than that reported in other study(6.6%)[17] and (7.1%)[18]. The difference can be explained that samples collected from different isolation sources and different regions. It is worth noting that the difference of host strains is the significant reason for the isolation rate.
The phage PSH-1 has a wide host range. It can be seen that the phage PSH-1 has a strong ability to lyse clinically isolated multi-drug resistant Salmonella stains, and not affected by the resistance genes carried by the host strain. It is well known that the phage has a strong inherent specificity in the lysis ability to the strains. In general, every phage can lyse different strains from the same genus. However, the phage PSH-1 can also lyse some clinical isolates of E. coli in this study. Although the lysis rate is only 30%, this is different from the strong host specificity of bacteriophages reported in other studies. It’s a interesting phenomenon and it deserves people's attention how to make the best use of this character to produce a wide spectrum of bacteriophage preparation.
In this study, the titer of purified phage PSH-1 was 1.15x1010 PFU/mL, which indicated that PSH-1 was a high-titer phage [10]. The optimal multiplicity of infection of phage PSH-1 was 0.01, and it meant that when the bacteriophage PSH-1 infected the host bacteria, only needed a small dose can cause great damage, this is also the advantage of bacteriophages over antibiotics [14]. According to the one-step growth curve, its latency time was 20 min, the burst time was 80 min, and the burst was 495 particles. Compared with φst1 (its latency time was 50 min, the burst time was 70 min, and the burst was 22 particles), it could be seen that the incubation period of bacteriophage PSH-1 was short, but the burst time was long, and the burst was much larger than that of bacteriophage φst1, it indicated that bacteriophage PSH-1 may exerted a greater bactericidal effect in clinical applications [19] .The optimum pH and temperature of PSH-1 were in the range of pH (3–11) and temperature (< 60 ℃). It had reported that the lytic phage of Salmonella enterica, SaFB14 grew between pH 3–10 [20], and a strain of Salmonella typhimurium ΦBLCC8-0050-3[18] could maintain activity under the conditions of temperature (30–50 ℃) and pH (5–10), which suggested that PSH-1 could survive in more hostile environments than SaFB14 and ΦBLCC8-0050-3[21]. In short, physical and chemical stability are the basis for phages to be used as clinical antimicrobial agents[22]. These indicate that phage PSH-1 has potential application value in rapid diagnosis and treatment of clinical infections.
Genomic studies are beneficial to a deeper understanding of salmonella phages at the genetic level. The genome of PSH-1 consists of double-stranded DNA with a total length of 48,466 bp and a G + C content of 45.33%. It has strong cracking property to Campylobacter jejuni. The phage has a total of 85 ORFs and 3 trnas, of which 28 ORFs are functional. In addition, phylogenetic trees constructed with whole genome sequences showed that PSH-1 was closely related to Salmonella phage KFS_SE2. In addition, after the whole genome BLAST comparison of PSH-1, we found that the genome of PSH-1 was very similar to that of E. coli phage C1 and vB_EcoS_swi2, which were 88.65 and 94.82%, respectively, and the comparison coverage was about 60%. Moreover, certain affinity was also reflected in the phylogenetic tree, which was an interesting phenomenon. At the same time, this phenomenon also confirmed that PSH-1 of Salmonella phage had certain lytic ability to E. coli during the determination of the host range of phages, and it was also worthy of our further exploration.
Foods involved in outbreaks of salmonellosis are mainly eggs, poultry and other meats, raw milk, etc. By tests, the use of phage could succeed in controlling the artificial salmonella contamination in milk at a range of temperatures. These showed a well effectiveness of the isolated phages for controlling salmonella contamination in some salmonellosis outbreak-associated food vehicles regardless at cool or warm temperatures. Additionally, in these tests, phages were used with pre-addition instead of post-treatment, this might differ from previous studies; however, our results have demonstrated the feasibility of phages as ‘‘food additives’’ for controlling contamination in some foods.
The biological characteristics (temperature, PH, growth characteristic and MOI) of the phage are necessary conditions for producing a new bacteriophage preparation. At present, there are certain basics in producing a new bacteriophage preparation, the safety of phages was also determined through genomic analysis, however, it still faces many difficulties and challenges. Can the number of infective phage particles in the preparation exist stably? Can the titer of bacteriophage remain stable in the phage PSH-1 solution treatment or storage? Resolving these issues was essential in producing a new bacteriophage preparation. In addition, further research will be required to determine the dosage form, administration dosage, interval and method of administration.