Colonization of tomato plants by Salmonella enterica is cultivar dependent, and type 1 trichomes are preferred colonization sites

Abstract

Nontyphoid salmonellosis caused by Salmonella enterica is the most common bacterial food-borne illness in humans, and fresh produce, including tomatoes, is a common vehicle. Accumulating data indicate that human enteric pathogenic bacteria, including S. enterica, interact actively with plants. Tomato plants were inoculated with S. enterica to evaluate plausible contamination routes and to determine if the tomato cultivar affects S. enterica colonization. S. enterica population levels on tomato leaves were cultivar dependent. S. enterica levels on Solanum pimpinellifolium (West Virginia 700 [WVa700]) were lower than on S. lycopersicum cultivars. S. enterica preferentially colonized type 1 trichomes and rarely interacted with stomata, unlike what has been reported for cut lettuce leaves. Early S. enterica leaf colonization led to contamination of all fruit, with levels as high as 10(5) CFU per fruit. Reduced bacterial speck lesion formation correlated with reduced S. enterica populations in the phyllosphere. Tomato pedicels and calyxes also harbored large S. enterica populations following inoculation via contaminated water postharvest. WVa700 green fruit harbored significantly smaller S. enterica populations than did red fruit or S. lycopersicum fruit. We found that plants irrigated with contaminated water had larger S. enterica populations than plants grown from seeds planted in infested soil. However, both routes of contamination resulted in detectable S. enterica populations in the phyllosphere. Phyllosphere S. enterica populations pose a risk of fruit contamination and subsequent human disease. Restricting S. enterica phyllosphere populations may result in reduced fruit contamination. We have identified WVa700 as a tomato cultivar that can restrict S. enterica survival in the phyllosphere.

FIG. 1.

Mean S. enterica phyllosphere populations on tomato plants of different cultivars. Two-week-old tomato plants of S. pimpinellifolium cultivar WVa700 and S. lycopersicum cultivars H7996, Yellow Pear, and Nyagous were dip inoculated with an S. enterica cocktail. Plants were sampled at 2, 4, 6, and 8 dpi. Bacterial counts were log transformed, and populations were calculated as log CFU/cm² across four sampling dates. The bars show the means across the 4 days, and the error bars represent the standard deviation of the mean.

FIG. 2.

Mean S. enterica phyllosphere populations of tomato plants. Four-week-old tomato plants of S. pimpinellifolium cultivar WVa700 and S. lycopersicum cultivar H7996 were dip inoculated with an S. enterica cocktail. Plants were sampled at 7 and 14 dpi. Bacterial counts were log transformed, and populations were calculated as log CFU/cm². Error bars represent the standard deviation of the mean.

FIG. 3.

Preferential S. enterica colonization of type 1 trichomes. (A to C) Micrographs of S. enterica(pKT-kan) colonizing the type 1 trichomes of S. lycopersicum cultivar Money Maker. The scale bars represent 100 μm. (D) Mean S. enterica populations on the wild type (LA2838A and LA0337) and mutants LA3172 (h), LA3556 (hl), and LA1049 (af). LA2838A is labeled WTa, as it is the genetic background of trichome mutants with accession numbers LA3172 and LA3556; LA0337 is labeled WTb, as it is the genetic background of the trichome mutant with accession number LA1049. Two-week-old S. lycopersicum tomato plants were dip inoculated with an S. enterica cocktail. Plants were sampled at 7 dpi. Populations were calculated as CFU/mm². Error bars represent the standard deviation of the mean. Different letters represent significant differences between mean S. enterica population sizes.

FIG. 4.

Mean bacterial speck lesions on two compound leaves of tomato plants. Three-week-old S. pimpinellifolium cultivar WVa700 and S. lycopersicum cultivar H7996, Yellow Pear, Money Maker, and Nyagous tomato plants were dip inoculated with a P. syringae pathovar tomato suspension. Bacterial speck lesions were enumerated at 5 to 7 dpi. Error bars represent the standard deviation of the mean.