The Salmonella Transcriptome in Lettuce and Cilantro Soft Rot Reveals a Niche Overlap with the Animal Host Intestine

Abstract

bFresh vegetables have been recurrently associated with salmonellosis outbreaks, and Salmonella contamination of retail produce has been correlated positively with the presence of soft rot disease. We observed that population sizes of Salmonella enterica serovar Typhimurium SL1344 increased 56-fold when inoculated alone onto cilantro leaves, versus 2,884-fold when coinoculated with Dickeya dadantii, a prevalent pathogen that macerates plant tissue. A similar trend in S. enterica populations was observed for soft-rotted lettuce leaves. Transcriptome analysis of S. enterica cells that colonized D. dadantii-infected lettuce and cilantro leaves revealed a clear shift toward anaerobic metabolism and catabolism of substrates that are available due to the degradation of plant cells by the pectinolytic pathogen. Twenty-nine percent of the genes that were upregulated in cilantro macerates were also previously observed to have increased expression levels in the chicken intestine. Furthermore, multiple genes induced in soft rot lesions are also involved in the colonization of mouse, pig, and bovine models of host infection. Among those genes, the operons for ethanolamine and propanediol utilization as well as for the synthesis of cobalamin, a cofactor in these pathways, were the most highly upregulated genes in lettuce and cilantro lesions. In S. Typhimurium strain LT2, population sizes of mutants deficient in propanediol utilization or cobalamin synthesis were 10-and 3-fold lower, respectively, than those of the wildtype strain in macerated cilantro (P < 0.0002); in strain SL1344, such mutants behaved similarly to the parental strain. Anaerobic conditions and the utilization of nutrients in macerated plant tissue that are also present in the animal intestine indicate a niche overlap that may explain the high level of adaptation of S. enterica to soft rot lesions, a common postharvest plant disease.T he association of food-borne illness with contaminated produce has prompted numerous investigations into the ability of human enteric pathogens to attach to and survive on fresh fruits and vegetables. Since enteric pathogens are unlikely to land on plants at high densities, it still remains unclear how they achieve the population sizes required to cause human illness at infectious doses in that habitat. Therefore, the factors that drive the growth of enteric pathogens on plants still need to be explored. We have previously demonstrated that Salmonella enterica and Escherichia coli serovar O157:H7 can multiply in the cilantro and lettuce phyllosphere under optimal conditions of warm temperatures and free water on the leaves (1, 2). Based on our observations that enteric pathogens appear less fit in the phyllosphere than plant-associated bacterial species, even under optimal growth conditions (2), and that their growth on middle-aged lettuce leaves is limited by nitrogen availability (1), we hypothesized that these human pathogens have not evolved to utilize the full range of nutrients present on leaf surfaces (3). The occurrenc...

Salmonella Persistence in Tomatoes Requires a Distinct Set of Metabolic Functions Identified by Transposon Insertion Sequencing

“…It involves major changes in the bacterial transcriptome and requires genes involved in amino acid biosynthesis and transport, cellulose production, fimbriae, regulators, and surface structures (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17). Some of the same mechanisms were involved in the colonization of both vegetative and reproductive tissues on different plant species.…”

“…The capacity of S. enterica (and other enteric pathogens) to readily multiply within plant tissues led to the hypothesis that persistence on plants is a part of the Salmonella life cycle, serving as reservoirs prior to reinfection of the preferred animal hosts (2)(3)(4)(5). Several studies have dissected the molecular basis of plant-Salmonella interactions (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16). However, the entire complement of genetic functions required for plant colonization by Salmonella is not yet known.…”

Microbial ecology of fresh vegetables

“…Interestingly, propanediol is a metabolite resulting from ripening or rotting of plant tissues (Bobik et al, 1997;Brandl et al, 2013;Goudeau et al, 2013). Previous studies revealed that populations of mutants deficient in propanediol utilization were several logs lower than wild-type strains grown in cilantro (Goudeau et al, 2013). In the light of S. Saintpaul's documented association with at least Jalapeno and Serrano peppers, these changes may be of significance, representing key adaptive changes among produce-specific Salmonella.…”

“…Surprisingly, these changes have provided a substantial clue to potential areas of adaptation having occurred in genes responsible for propanediol utilization (pduF) and propanediol diffusion facilitator (pudB) genes. Interestingly, propanediol is a metabolite resulting from ripening or rotting of plant tissues (Bobik et al, 1997;Brandl et al, 2013;Goudeau et al, 2013). Previous studies revealed that populations of mutants deficient in propanediol utilization were several logs lower than wild-type strains grown in cilantro (Goudeau et al, 2013).…”

Identification of novel members of the bacterial azoreductase family in Pseudomonas aeruginosa

“…To date their role has been tied to their participation in the detoxification of cytotoxic quinones produced as antibacterial compounds by many plants [1]. A transcriptome study in Salmonella typhimurium has shown >100-fold up-regulation of an azoreductases during growth in lesions on cilantro and lettuce leaves versus in vitro growth [10]. A second study also showed significant up-regulation of Escherichia coli azoreductase during infection of radishes [6].…”