| Salmonella is a common foodborne zoonotic pathogen that causes various infections in animals and humans.The duck stock and duck meat production in China ranks first in the world.Jiangsu province is an important duck breeding area.Compared to pig and chicken breeding,the duck breeding is a small-scale industry composed mainly of free-range duck farms.The lack of appropriate biosecurity measures makes it highly susceptible to infection by various pathogenic bacteria,including Salmonella spp.Salmonella infection in ducks affects the economic efficiency of the duck breeding industry and leads to the spread of multidrug-resistant bacteria,which can be transmitted to humans through duck meat,duck eggs,and other related products,thereby posing a threat to public health.Therefore,investigating the prevalence of Salmonella in free-range duck farms can provide data for the prevention and control of Salmonellosis and promote the healthy development of duck farming.In this study,an epidemiological investigation of Salmonella in free-range duck farms in Jiangsu province was conducted to reveal the prevalence and characteristics of Salmonella in ducks.We then explored the feasibility of using lytic phages to replace antibiotics to reduce Salmonella infection in ducks.1 Prevalence and Characteristics of Salmonella in Free-range Duck Farms in Jiangsu ProvinceIn this study,1,285 samples were collected from 29 free-range duck farms in seven cities of Jiangsu province for the detection and isolation of Salmonella.A total of 207 Salmonella isolates were obtained with an isolation rate of 16.10%.Eight Salmonella serotypes were identified,with the predominant serotype being S.Typhimurium,accounting for 44.44%(92/207).The second-ranked serotype was S.Derby,accounting for 25.60%(53/207),followed by S.Newport(10.14%,21/207).These isolates were subjected to antimicrobial resistance testing using 15 antimicrobial agents belonging to seven classes.The results showed that 87.90%(182/207)of the isolates were drug-resistant,85.50%(177/207)were resistant to three or more antibiotics,and four isolates(1.9%)had the most serious resistance,showing resistance to 13 antibiotics belonging to six categories.The virulence gene test results showed that the agfA,spiC,lpfA and sivH virulence genes were detected in all the isolates,the hilA was identified in 99.03%(205/207)of these isolates,followed by spvC(19.81%,41/207)and sefA(9.18%,19/207).Additionally,the predominant 43 S.Typhimurium isolates were subjected to PFGE typing.The results showed that some isolates had the same PFGE profile,indicating that these isolates may have identical sources or could spread among different duck farms.2 Biological characteristics of phages and evaluation of the reduction effect of phage SP13 on Salmonella in vitro and in vivoPhages are considered as a promising alternative to antibiotics for the prevention and control of bacterial pathogens.In this study,two lytic phages were isolated from duck droppings and named as SP3 and SP13.After purification,the measured titers of SP3 and SPI13 were 3.1×1011 PFU/mL and 3.5×1011 PFU/mL,respectively.Transmission electron microscopy revealed that both lytic phages belonged to the long-tailed phage family.The optimal MOI was 0.01.The incubation period of SP3 was 30 min,the outbreak period was 30-80 min,it entered the equilibrium phase after 80 min,and the burst amount was 127 PFU/cell.SP13 had an incubation period of 20 min,an outbreak period of 20-70 min,and an equilibrium phase after 70 min,with an explosive amount of 62 PFU/cell.Both phages had good stability at 4℃,25℃,and 37℃ with 4≤pH ≤9,among which the pH tolerance range of SP3 was 4-11 and the pH tolerance range of SP13 was 3-11.SP13 is slightly more stable than SP3 at the same pH.Based on the identification of eight Salmonella serotypes in free-range duck farms in Jiangsu Province,100 isolates were selected to determine the phage lysis spectrum.The results showed that among the different Salmonella serotypes tested,SP3 could lyse the isolates of S.Typhimurium,S.Derby,and S.Anatum.SP13 could lyse the isolates of S.Typhimurium,S.Derby,S.Enteritidis,and 5.Anatum.SP13 was then selected for in vitro and in vivo Salmonella reduction test based on temperature,pH stability,and lysis spectrum test results.To simulate the contamination of drinking water by S.Typhimurium SL1344,three groups of different temperature conditions at 4℃,25℃,and 37℃ were set using an MOI of 0.01-1000 to study the reduction effect of phage SP13 on Salmonella in vitro.The results showed that under the same MOI,the absorption capacity of phage SP13 against S.Typhimurium SL1344 in water at 25℃ and 37℃ was higher than that at 4℃,and the absorption ability of SP13 on Salmonella was significantly enhanced with an increase in MOI.Subsequently,we treated BALB/c mice(6 weeks old)inoculated with an oral infection lethal dose of S.Typhimurium SL1344 with different concentrations of the bacteriophage SP13,and the results showed that SP13 could delay the death of mice and improve their survival ability.In summary,there was a high prevalence of Salmonella in free-range duck farms in Jiangsu province.These isolates displayed severe drug resistance,and most carried virulence genes.Some isolates may have the same source or spread among different duck breeding farms.We also isolated and identified a lytic phage SP13 with good temperature and pH stability,which could reduce S.Typhimurium infection in vitro and in vivo,indicating that the phage has the potential to be developed as a biological agent to replace antibiotics for Salmonella control in ducks. |
PDF Full Text Request