Physiological and Molecular Responses of Lactuca sativa to Colonization by Salmonella enterica Serovar Dublin

Abstract

This paper describes the physiological and molecular interactions between the human-pathogenic organism Salmonella enterica serovar Dublin and the commercially available mini Roman lettuce cv. Tamburo. The association of S. enterica serovar Dublin with lettuce plants was first determined, which indicated the presence of significant populations outside and inside the plants. The latter was evidenced from significant residual concentrations after highly efficient surface disinfection (99.81%) and fluorescence microscopy of S. enterica serovar Dublin in cross sections of lettuce at the root-shoot transition region. The plant biomass was reduced significantly compared to that of noncolonized plants upon colonization with S. enterica serovar Dublin. In addition to the physiological response, transcriptome analysis by cDNA amplified fragment length polymorphism analysis also provided clear differential gene expression profiles between noncolonized and colonized lettuce plants. From these, generally and differentially expressed genes were selected and identified by sequence analysis, followed by reverse transcription-PCR displaying the specific gene expression profiles in time. Functional grouping of the expressed genes indicated a correlation between colonization of the plants and an increase in expressed pathogenicity-related genes. This study indicates that lettuce plants respond to the presence of S. enterica serovar Dublin at physiological and molecular levels, as shown by the reduction in growth and the concurrent expression of pathogenicity-related genes. In addition, it was confirmed that Salmonella spp. can colonize the interior of lettuce plants, thus potentially imposing a human health risk when processed and consumed.

Concomitant uptake of antimicrobials and Salmonella in soil and into lettuce following wastewater irrigation

“…When lettuce was grown in soil contaminated by 7 log CFU Salmonella per g soil, the internalization frequency of S. Dublin, S. Enteritidis, and S. Typhimurium in lettuce was 10%, 0% and 0% (28 biological replicates), respectively (Klerks et al, 2007a). In another study by Klerks et al (2007b), the internalization frequency was 5% (3 out of 56) in the S. Dublin-lettuce system. Similarly, very low internalization frequency (<0.3%) of S. Enteritidis or Escherichia coli O157: H7 into lettuce was reported for soil concentrations between 3 and 6 log CFU/g soil (Erickson et al, 2013;Zhang et al, 2009aZhang et al, , 2009b.…”

“…Human pathogens can internalize in plants through root or leaf uptake. Salmonella enterica serovars Cubana and Dublin accumulate inside hydroponically grown alfalfa and lettuce at the level of 4 log CFU/g fresh weight through root uptake (Dong et al, 2003;Klerks et al, 2007b). Internalization of human pathogens can also occur through root uptake when the pathogens are introduced by contaminated soil or irrigation water ( Franz et al, 2007, Hintz et al, 2010, Hora et al, 2005, Klerks et al, 2007aand Wachtel et al, 2002.…”

Differential interaction of Salmonella enterica serovars with lettuce cultivars and plant-microbe factors influencing the colonization efficiency

“…After 7 days of incubation the shoots were harvested by removing the roots at the transition region. The shoot tissue of each plant was weighed and subsequently surface disinfected by washing for 10 s in 70% ethanol and rinsing twice with sterile water (Klerks et al, 2007). Then, leaf tissue was thoroughly ground in cold BPW.…”

“…Each plant of the first set of plants was ground in 1 ml of BPW. From the second set of plants each shoot was surface disinfected in 70% ethanol and washed twice in sterile water prior to grinding in 1 ml of BPW (Klerks et al, 2007). Of each suspension with ground plant material 40 ml was plated on Hektoen enteric agar, in duplicate.…”

“…The induction of salicylic acid plant defense pathway (salicylic acid resistance) appeared correlated to the expression of bacterial genes for TTSS-SPI effector proteins, whereas the presence of flagella only induced the SA-independent plant defense (induced systemic resistance). A recent study involved the molecular response of axenically grown lettuce to colonization by S. enterica serovar Dublin, from which a differential expression was indicated of various virulence and pathogenicity-related genes of lettuce, over time (Klerks et al, 2007).…”