Enteric bacteria Salmonella is the causative agent for gastro enteropathy and enteric (typhoid) fever. Salmonella enterica is a gram-negative, flagellated, rod-shaped bacterium that belongs to the Enterobacteriaceae family. More than 2600 distinct serovars belong to this varied type of bacteria, which are divided into two main categories: (a) Typhoidal Salmonella and (b) Non-typhoidal Salmonella. Salmonella can infect a wide range of hosts from humans to a variety of warm-blooded animals. Salmonella may infect a broad range of hosts, including humans and several warm-blooded animals. It is a common foodborne pathogen that is primarily present in poultry, eggs, and dairy products and it may be transferred by human feces, contaminated food, water, and person-to-person contact. Antibiotics have been used more often in recent years to treat bacterial illnesses, which has led to the rise of germs that are multi-drug resistant and it requires to be addressed with an alternative treatment strategy such as phage therapy. Bacteriophages are viruses that can kill their host bacteria without affecting other microflora and co-survive and evolve with their host bacteria. Therefore, bacteriophages have been proposed as an alternative biocontrol agent for bacterial pathogens. Salmonella enterica is a foodborne pathogen and causative agent for gastroenteritis, diarrhea, and enteric fever in both humans and animals. The genus Salmonella has several serovars, and many of them are recently reported to be resistant to multiple drugs. Therefore, this study aimed to isolate and identify a specific enteric bacteriophage followed by its characterization as a potential biocontrol agent for enteric bacteria. 

In this study, a Salmonella phage STWB21 was isolated from a lake water sample from Kolkata, India, and found to be a novel lytic phage with promising potential against the host bacteria Salmonella typhi. However, some polyvalence was observed in their broad host range. In addition to S. typhi, the phage STWB21 was able to infect S. paratyphi, S. typhimurium, S. enteritidis, and a few other bacterial species such as Sh. flexneri 2a, Sh. flexneri 3a, and ETEC. Phage morphology was visualized with FEI Tecnai 12 BioTwin Transmission Electron Microscope (TEM) operating at 100 kV. TEM study revealed that phage STWB21 belongs to the Siphoviridae family with an icosahedral head (65 ± 3 nm in diameter) and a long flexible, non-contractile tail (113 ± 6 nm in length). The latent period and burst size of phage STWB21 was 25 min and 161 pfu/cell. As a biological control agent, phage STWB21 exhibited a high temperature (4°C-50°C) and pH (4-11) tolerance for different typhoidal and non-typhoidal Salmonella strains. Since Salmonella is a foodborne pathogen, the phage STWB21 was applied to treat a 24 h biofilm formed in onion and milk under laboratory conditions to analyze its antibiofilm property, the phage STWB21 was applied to treat a 24h biofilm formed under controlled laboratory conditions. In both cases, a significant reduction was observed in the bacterial population of S. typhi biofilm.  These findings highlighted phage STWB21 as an anti-biofilm agent for Salmonella spp. and showed its application in food industries. It contained a dsDNA of 112,834 bp in length, and the GC content was 40.37%. The genomic analysis confirmed the presence of lytic genes and the absence of any lysogeny or toxin genes. Further, we investigated the effectiveness of phage STWB21 in preventing Salmonella typhi from invading mouse liver and spleen tissue and in providing a therapeutic advantage for salmonellosis in a mammalian host. After introducing phage treatment, the phage showed reduced colonization in both the treatment and preventive groups. The results of the study revealed that mice treated with phage both before and after Salmonella infection had less tissue inflammation than mice that were not treated, as determined by light microscopy and transmission electron microscopy images. Furthermore, the results suggested that a single STWB21 phage could be efficient in treating Salmonella infection without detectable levels of resistance. Our findings collectively showed that the Salmonella phage STWB21 has considerable potential for further investigation as a therapeutic and preventative option against Salmonella typhi. Overall, the study showed that the novel polyvalent phage STWB21 has a promising ability as a biocontrol agent of Salmonella spp. and proposes its application in food industries and for therapeutic purposes.