Previously uncharacterized Salmonella enterica genes required for swarming play a role in seedling colonization

Abstract

Incidences of bacterial foodborne illness caused by ingestion of fresh produce are rising. Instead of this being due to incidental contamination, the animal pathogen Salmonella enterica utilizes specific molecular mechanisms to attach to and colonize plants. This work characterizes two S. enterica genes of unknown function: a putative periplasmic protein, STM0278, and a putative protein with a hydrolase in the C-terminus, STM0650. STM0278 and STM0650 are important for seedling colonization but appear to have different roles during the process of colonization. Mutants of either STM0278 or STM0650 showed reduced colonization of alfalfa seedlings at 24 h, and the STM0278 mutant also showed reduced colonization at 48 h. Both genes were expressed in planta at 4 h following inoculation of 3-day-old seedlings and at 72 h after seed inoculation. This suggests that the role of STM0650 in seedling colonization is less important later in the process or is duplicated by other mechanisms. Mutants of STM0278 and STM0650 were defective in swarming. The STM0278 mutant failed to swarm in 24 h, while swarming of the STM0650 mutant was delayed. Addition of surfactant restored swarming of the STM0278 mutant, suggesting that STM0278 is involved in surfactant or osmotic agent production or deployment. Alfalfa seed exudates as the sole nutrient source were capable of perpetuating S. enterica swarming. Sequence analysis revealed sequences homologous to STM0278 and STM0650 in plant-associated bacteria, but none in Escherichia coli. Phylogenetic analysis of STM0650 showed similar sequences from diverse classes of plant-associated bacteria. Bacteria that preferentially colonize roots, including S. enterica, may use a similar hydrolase for swarming or biofilm production on plants. Multicellular behaviours by S. enterica appear central to plant colonization. S. enterica genes involved in plant colonization and survival outside of a host are most likely among the 'function unknown' genes of this bacterium.

Requirement of Siderophore Biosynthesis for Plant Colonization by Salmonella enterica

“…Swarm induction was verified by the presence of hyperflagellated cells. The number of flagella per swarm cell was determined following flagellar staining as described previously (2). Tryptophan and ferrous sulfate were added to final concentrations of 100 M and 10 M, respectively.…”

“…The motility of S. Typhimurium strains was tested using swarming or swimming plates, as described previously, with modification (2). Overnight bacterial cultures grown on LB agar were suspended in sterile water to reach an OD 600 of ϳ1.5 or ϳ0.5 for swarming and swimming assays, respectively.…”

Invasion and Survival of Salmonella in the Environment: The Role of Biofilms

“…This appears to be the result of S. enterica serovar Typhimurium attaching to and colonizing plants, rather than incidental contamination. S. enterica serovar Typhimurium that preferentially colonize roots use a hydrolase for swarming or biofilm production on plants; this multicellular behavior of S. enterica serovar Typhimurium has emerged as central to plant colonization (Barak et al, 2009).…”

Legume seed exudates and Physcomitrella patens extracts influence swarming behavior in Rhizobium leguminosarum

“…During rhizosphere colonization Pseudomonas fluorescens F113 produces hyperflagellated variants with enhanced swarming abilities that may allow preferential bacterial colonization of distal root structures (Sánchez-Contreras et al 2002). Barak et al (2009) correlated swarming and colonization of alfalfa seedlings in Salmonella enterica by identifying a putative periplasmic protein and a putative hydrolase that are required for both swarming motility and colonization of alfalfa seedlings. Furthermore, the surface movement of differentiated swarmer cells of S. enterica was maintained using alfalfa root exudates as the sole nutrient source (Barak et al 2009).…”

“…Barak et al (2009) correlated swarming and colonization of alfalfa seedlings in Salmonella enterica by identifying a putative periplasmic protein and a putative hydrolase that are required for both swarming motility and colonization of alfalfa seedlings. Furthermore, the surface movement of differentiated swarmer cells of S. enterica was maintained using alfalfa root exudates as the sole nutrient source (Barak et al 2009). …”