Inactivation of Salmonella enterica on post-harvest cantaloupe and lettuce by a lytic bacteriophage cocktail

Abstract

Salmonella enterica ( S. enterica ) is a causative agent of multiple outbreaks of foodborne illness associated with fresh produce, including pre-cut melon and leafy vegetables. Current industrial antimicrobial interventions have been shown to reduce microbial populations by <90%. Consequently, bacteriophages have been suggested as an alternative to chemical sanitizers. Seven S. enterica strains from four serovars (10 5 CFU/mL) were separately inoculated onto excised pieces of Romaine lettuce leaf and cantaloupe flesh treated with a five-strain bacteriophage cocktail 24 h before S. enterica inoculation. S. enterica , total aerobic populations and water activity were measured immediately after inoculation and after 1 and 2 days of incubation at 8 °C. The efficacy of the bacteriophage cocktail varied between strains. Populations of S. enterica Enteritidis strain S3, S. Javiana S203, S. Javiana S200 were reduced by > 3 log CFU/g and S. Newport S2 by 1 log CFU/g on both lettuce and cantaloupe tissues at all sampling times. In contrast, populations of strains S. Thompson S193 and S194 were reduced by 2 log CFU/g on day 0 on lettuce, but were not significantly different (P > 0.05) from the controls thereafter, S. Newport S195 populations were reduced on lettuce by 1 log CFU/g on day 0 and no reductions were found on cantaloupe tissue. Both aerobic populations and water activity were higher on cantaloupe than on lettuce. The water activity of lettuce decreased significantly (P < 0.05) from 0.845 ± 0.027 on day 0–0.494 ± 0.022 on day 1, but that of cantaloupe remained between 0.977 and 0.993 from day 0–2. The results of this study showed that bacteriophages can reduce S. enterica populations on lettuce and cantaloupe tissues but that the magnitude of the effect was strain-dependent.

Isolation and Characterization of a Novel Salmonella Phage vB_SalP_TR2

“…Similar to previous studies on phage-based biocontrol, bacterial regrowth was also observed during bacteriolytic experiment, likely due to the emergence of bacterial resistance mechanisms, including restricted modification systems, spontaneous mutations, and adaptive immunity through the CRISPR-Cas system (Oechslin, 2018). Several solutions have been proposed to delay bacterial regeneration, such as the use of high concentrations of phage suspensions (Guenther et al, 2009) and the use of phage cocktails or phages in combination with other antibacterial agents to minimize the impact (Abhisingha et al, 2020;Alves et al, 2020;Wong et al, 2020). In practice, food is naturally contaminated with a very low pathogen titer (Zhu et al, 2014).…”

Characteristics of a Series of Three Bacteriophages Infecting Salmonella enterica Strains

“…Phylogenetic analysis and sequence similarity searches revealed the phage vB_SenM-1 genome to be similar to previously described phage SE13. This phage has been characterized as being of broad host range among various S. enterica serotypes as well as having been effectively used as part of an experimental phage cocktail against Salmonella infection in food, especially cantaloupe and lettuce [ 34 , 46 ]. Moreover, phage LSE7621, to which phage vB_SenM-1 shows ~95% identity, has also been effectively tested in S. Enteritidis biocontrol on lettuce.…”

Microbes in Our Food, an Ongoing Problem with New Solutions

“…The combination treatment reduced bacteria by 3 log, but was less effective when seeds had been stored for 4 weeks or longer [21]. Most recently, her group is conducting a series of research projects exploring the use of bacteriophages (i.e., viruses that infect bacteria) on inactivating foodborne pathogens on fresh produce and poultry products [22]. This group works on improving food processing and safe food handling for long term food security.…”