Human pathogen contamination of produce was the leading cause of food borne illnesses and outbreaks associated with a single-ingredient commodity between 2004 and 2013 . Lack of visual evidence that indicates the presence of contamination on produce contributes to the estimated 9.4 million cases of food borne illness in the United States annually . Various pathogen groups and toxins can be causal agents of food borne illness associated with produce; however, non-typhoidal Salmonella ranks as the second leading cause of all illnesses associated with consumption of produce . In a pre-harvest setting, enteric pathogenic bacteria are introduced to fresh produce through many routes, including low quality irrigation water, use of contaminated organic fertilizers, close proximity to livestock operations, wildlife intrusions, improper worker hygiene, or contaminated equipment . Once on the leaf surface, bacteria are faced with harsh conditions, such as UV irradiation, low nutrient and water availability, and unfavorable weather . Bacteria may escape these conditions by attaching to the leaf surface and forming bio-films or by transitioning to an endophytic lifestyle through internalization into the leaf extracellular space via natural pores or wounds . While leaf internalization is likely to confer some protection to the bacteria, it is not without some disadvantages. Plants can detect endophytic bacteria in the apoplast through pattern recognition receptors localized at the cell membrane that perceive conserved microbial molecules known as pathogen- or microbe-associated molecular patterns . PRR-PAMP binding leads to initiation of PAMP-triggered immunity , which functions to prevent further internalization of bacteria and to eradicate those that have already entered the apoplast . This suggests that internalization trades one challenge for another ,black plastic planting pots and only bacteria that can cope with these challenges will be able to colonize leaves successfully.
Previous studies have shown that Salmonella spp. interact with plants in a sophisticated manner, although the exact mechanisms are not fully understood . For instance, similar to some plant pathogens, Salmonella enterica serovar Typhimurium SL1344 can modulate stomatal movement in Arabidopsis leaves, where it induces a transient stomatal closure and re-opening at 4 h post incubation . Stomatal closure can diminish bacterial internalization and subsequent contamination of internal leaf tissues. The mechanism for stomatal re-opening by the phytopathogen Pseudomonas syringae pv. tomato is through the action of coronatine , a polyketide phytotoxin . However, the genomes of STm strains LT2 and 14028s do not encode genes for coronatine synthesis . Furthermore, stomatal re-opening is not a ubiquitous response to human pathogens. For instance, Escherichia coli O157:H7 induces a lasting stomatal closure in lettuce and Arabidopsis for at least 4 and 8 h, respectively . Beyond the ability to modulate stomatal movement, STm SL1344 can survive at a higher titer within the apoplast of Arabidopsis leaves than O157:H7 after surface-inoculation and S. enterica serovar Thompson strain RM1987 can survive at high titers on the surface of romaine lettuce leaves . Therefore, S. enterica may either induce a weak plant immune response or can counteract plant immunity and consequently persist on and in leaves . Internalization and persistence within the apoplast are arguably the most important targets for managing contamination of produce by Salmonella, as endophytic populations cannot be removed through typical washing tactics . Here, we provide details of multiple genomic regions required for internalization and persistence of STm 14028s into lettuce leaves. These genomic regions were identified with a genetic screen of multi-gene deletion mutants of STm 14028s to pinpoint proteins and metabolic pathways responsible for stomatal re-opening and apoplastic persistence. Selected MGD mutants were further characterized regarding their ability to survive in the apoplast, induce hallmark plant defenses, and replicate in apoplastic wash fluid. While all mutants induced a prolonged stomatal closure when applied to the leaf surface, the mutants were found to vary in other aspects of phyllosphere survival. Five-mL of LSLB bacterial cultures were grown in 14.0 mL culture tubes on a rotary shaker to an OD600 of 0.8–1.0. Cells were harvested by centrifugation and resuspended in water to an OD600 of 0.002 . Inoculum was infiltrated with a needleless syringe into leaves of 5-week-old lettuce plants that were kept under the same environmental conditions used for plant growth. Stomatal bioassay was conducted as previously described at 2 and 4 hpi. Mean stomatal aperture widths from three independent leaves and SE were calculated. The difference between the means was compared by Student’s t-test to determine statistical significance.
Apoplastic wash fluid was extracted from 5-week-old lettuce leaves, omitting the cotyledons, using an infiltration centrifugation method as previously described . To ensure that plant cellular contamination did not occur during extraction, AWF was evaluated for cellular contaminants using the Sigma-Aldrich RGlucose-6-Phosphate Dehydrogenase Assay Kit . None of the AWF used exhibited detectable levels of G6PDH . AWF was saved in aliquots to limit freeze-thaw cycles and stored at −20◦C and filter sterilized at the time of use. Bacterial cultures were grown in LSLB liquid medium on an orbital shaker to an OD600 of 0.8–0.1. Cells were harvested by centrifugation and resuspended in water to an OD600 of 0.2 . An aliquot of this inoculum was added to each medium to achieve an initial bacterial culture concentration of 5 × 106 CFU/mL in a 96-well plate format. Growth curves were obtained by growing cultures stationary, except for a 30-s rotation prior to each OD600 reading using a BioTek EPOCH 2 Microplate Spectrophotometer . OD600 readings were obtained every 30 min throughout a 24-h period and blanks were included as a control. This experiment was performed three times with three technical replicates each time. Mean OD600 for each time point of the growth curve was calculated after subtracting the mean blank value and subsequently converted to bacterial cell number per mL of culture. Growth rates in the log-phase of growth were determined using the formula N0 x 2 n = Nf where N0 is the number of bacteria at the first time point of interest, Nf is the number of bacteria at the final time point of interest, and n is the number of generations.We utilized a collection of MGD bacterial mutants derived from STm strain 14028s . We first confirmed that this strain induces a similar stomatal response to that of STm strain SL1344 . We evaluated changes in the stomatal aperture width in leaves of young lettuce plants by floating leaf pieces onto bacterial inoculum as previously reported . Both STm strains induced an initial stomatal closure at 2 h post inoculation followed by re-opening at 4 hpi , suggesting that the MGD library could be useful to identify STm genomic regions required for successful stomatal re-opening at 4 hpi. Second, to ensure that lack of re-opening was due to deletion of genes required for stomatal re-opening by STm 14028s rather than temporal factors, the circadian movement of lettuce stomata was determined. This analysis indicated that the stomatal aperture was widest at 6 h after first light . We therefore, chose to start the stomatal bioassay at 2 hafl to ensure that the 4 hpi time point corresponded to a time with maximum expected stomatal aperture width. A primary screen of 303 MGD strains with a single biological replicate indicated that 177 mutants were unable to re-open stomata, suggesting a high rate of false-positives. Thus, we functionally annotated the predicted deleted genes in these 177 mutants .
Considering the current knowledge of STm epiphytic behavior , we reasoned that genes involved in secretion,black plastic pots for plants perception of environmental signals, signaling, and regulatory functions could be involved in opening of the stomatal pore. Thus, we selected 51 MGD mutants based on their functional annotation for retesting with at least three biological replicates. The primary functional units missing in these 51 mutants are described in Supplementary Table S2. From this confirmation screen, only eight mutants were unable to reopen lettuce stomata consistently and they were selected for further characterization. Furthermore, previous results indicated that mutants for the Salmonella Pathogenicity Island 1 and 2 were unable to open lettuce stomatal pores . Thus, we also analyzed two MGD strains from our collection that have a predicted deletion of these regions in addition to a few adjacent genes . To confirm that the lack of stomatal re-opening using leaf pieces floating on bacterial inoculum was a reproducible response that can also be observed in leaves still attached to the plant, we designed a stomatal bio-assay that included infiltration of mature lettuce leaves with STm 14028s, Mut3, or Mut9 bacterium suspensions. In this assay, bacteria are placed in the leaf apoplast, including the sub-stomatal chamber, where they can be in contact with the guard cells. All three strains induced a strong stomatal closure at 2 hpi , similar to observations made using surface inoculation of mature, whole plants . Furthermore, the wild type strain STm 14028s, but not the mutant strains, induced stomatal reopening at 4 hpi , suggesting that this response is robust. To rule out the possibility that the infiltration procedure induced an unpredictable stomatal movement, we assessed the circadian stomatal movement in untreated lettuce leaves as well as leaves infiltrated with water , STm 14028s,or Mut9. Mock-treated and untreated leaves showed an almost identical movement pattern, stomata of Mut9-infifiltrated leaves remained closed throughout the daylight period, and STm 14028s-infiltrated leaves showed a transient reduction in stomatal aperture width at 2 hpi that corresponded to 4 hafl. Upon completion of this screening procedure, the genome position of the deleted region for each mutant was identified at the nucleotide level by whole genome sequencing of the mutant strains. This procedure, which was readily available in a time and cost-effective matter, allowed us to predict the genotype and the functional units missing in each mutant using the available STm 14028s and LT2 genome annotations . Furthermore, each mutant, except Mut5, was able to swim and swarm , confirming the predicted genotype of Mut5 is missing genes involved in flagellar biosynthesis and chemotaxis . Movement and chemotaxis have previously been associated with STm SL1344 internalization through the stomatal pore . Thus, the identification of Mut5 during the Salmonella genetic screening validates our procedure, which identified known and novel features associated with bacterial epiphytic behavior. The 10MGD strains were further tested for phenotypic traits required for colonization of leaves as described below. As all selected mutants were unable to stimulate stomatal re-opening at 4 hpi , we sought to determine whether each mutation also affected the population dynamics in the lettuce leaf apoplast. To characterize each mutant’s ability to survive within the apoplast, leaves were infiltrated with bacterial inoculum. This allowed for direct analysis of population titer changes due to apoplastic interactions and eliminated confounding factors, such as failure to survive on the leaf surface and/or lack of internalization through stomata.The wild type bacterium STm 14028s population declined significantly in lettuce leaves , while variable population titers were observed among the mutants . For instance, seven mutants, Mut1/2/4/5/7/8/10, had significantly greater population titers than that of the wild type bacterium at 21 dpi, whereas Mut3 and Mut6 apoplastic persistence did not differ from that of the wild type at 21 dpi . Interestingly, only Mut9 showed significantly impaired endophytic survival . This finding indicates that genes missing in this mutant, including the SPI-2 and the suf, ynh, lpp, and ttr operons , may be required for the bacterium to cope with or overcome plant defenses and/or the ability of the bacterium to obtain nutrients from the apoplastic environment necessary to maintain its population. To test for these possibilities, we performed a callose deposit assay and a bacterial growth rate assay using lettuce apoplastic wash fluid . Callose deposition is a hallmark plant defense response that is induced upon biotic stress . Thus, we determined the average number of callose deposits in lettuce leaves inoculated with STm 14028s, Mut3, and Mut9. We observed that all three bacteria induced similar numbers of callose deposits that were significantly higher than those seen in the water control . Because all three STm strains induced a relatively low number of callose deposits , we also inoculated Arabidopsis with the virulent phytopathogen Pst DC3000 for comparison with this well-established system.