Bacterial wilt is one of the most destructive diseases of cucurbits in the Midwestern and Northeastern United States. Although the disease has been studied since 1900, host colonization dynamics remain unclear. Cucumis- and Cucurbita-derived strains exhibit host preference for the cucurbit genus from which they were isolated. We constructed a bioluminescent strain of Erwinia tracheiphila (TedCu10-BL#9) and colonization of different cucurbit hosts was monitored. At the second-true-leaf stage, Cucumis melo plants were inoculated with TedCu10-BL#9 via wounded leaves, stems, and roots. Daily monitoring of colonization showed bioluminescent bacteria in the inoculated leaf and petiole beginning 1 day postinoculation (DPI). The bacteria spread to roots via the stem by 2 DPI, reached the plant extremities 4 DPI, and the plant wilted 6 DPI. However, Cucurbita plants inoculated with TedCu10-BL#9 did not wilt, even at 35 DPI. Bioluminescent bacteria were detected 6 DPI in the main stem of squash and pumpkin plants, which harbored approximately 10(4) and 10(1) CFU/g, respectively, of TedCu10-BL#9 without symptoms. Although significantly less systemic plant colonization was observed in nonpreferred host Cucurbita plants compared with preferred hosts, the mechanism of tolerance of Cucurbita plants to E. tracheiphila strains from Cucumis remains unknown.
Little is known about the abiotic factors contributing to the preharvest persistence of Salmonella in tomato tissues. Therefore, we investigated the effects of specific environmental conditions and contamination methods on the persistence and dissemination of Salmonella enterica subsp. enterica serotype Typhimurium (JSG626) in tomato plants. When plants were sprayed on the leaves with a JSG626-contaminated solution, JSG626 persistence in the phyllosphere (bacteria located on the surface of the inoculated foliage and stem tissues) was lower at higher temperatures (30°C day/25°C night) than at lower temperatures (20°C day/15°C night). However, wounding cotyledons with contaminated tools improved JSG626 persistence and the internalization rate (2.27%) in planta compared to spray inoculation (0.004%). The systemic dissemination of JSG626 to other tissues increased when contaminated plants were grown under low relative humidity (Ͻ40%); however, JSG626 was only detected in the root systems at later sampling times (between 21 and 98 days postinoculation [dpi]). Further, after tomato scions were grafted onto rootstocks using contaminated cutting tools, dissemination of JSG626 was preferentially basipetal and occasionally acropetal in the plants, with higher persistence rates and loads of JSG626 in root systems compared to foliar tissues. JSG626 was detected in the grafting point and root systems up to 242 dpi; however, none of the fruits harvested from contaminated plants between 90 and 137 dpi were positive for JSG626. This study demonstrates that environmental temperature and relative humidity could be good indicators for estimating the persistence of Salmonella enterica in tomato plants. Further, root systems may represent a risk for long-term persistence of Salmonella enterica in tomato plants. IMPORTANCE Tomatoes are one of the most widely produced vegetables around the world; however, fresh tomatoes have been connected to multiple wide-scale salmonellosis outbreaks over the past decades. Salmonella is commonly found in the environment and can persist in hostile conditions for several weeks before being internalized into plant tissues, where it is protected from conventional sanitation methods. In addition to biotic factors (host, inoculum size, and phytobiome), abiotic factors (environmental conditions) may affect the persistence of Salmonella in crop production. This study demonstrates that specific environmental conditions, the inoculation method, and the inoculum density affect the persistence and dissemination of JSG626 in tomato plant tissues. Our findings enhance the understanding of interactions between Salmonella enterica and fresh produce and may lead to the development of novel management practices on farms.
The control of Salmonella from farm to fork is challenging because of the lack of symptoms on contaminated fresh produce and the limited effectiveness of control methods before and after harvest. Therefore, the identification of new antimicrobials effective against Salmonella is needed to improve food safety. In this study, we investigated the activity of four anti-Salmonella small molecules (SMs; SM1, SM3, SM4, and SM5) on tomato tissues contaminated with Salmonella enterica subsp. enterica serotype Typhimurium (JSG626). No toxicity was detected on tomato seeds, seedlings, and fruits when tissues were treated with 200 μM SM. Moreover, SM1 and SM4 reduced (79 and 64%, respectively) the Salmonella Typhimurium population in tomato seedlings when sprayed on the leaves with two times the minimal bactericidal concentration (SM1 = 100 μM, SM3 and SM5 = 50 μM, and SM = 20 μM) at −1 and 3 days postinfection. The four SMs (200 μM) that have antagonistic effects on Salmonella had limited effect on the growth of beneficial plant bacteria (Bacillus amyloliquefaciens strain BA1 and Bacillus subtilis strain BS1, and Enterobacter sp. strain SM1796). SM1 and SM4, when combined with BA1 or SM1796, reduced the Salmonella Typhimurium population by at least 98.2% in tomato plants. The four SMs (at two times the minimal bactericidal concentration) also reduced the Salmonella Typhimurium population on the surface and inside contaminated green mature and ripe tomato fruits up to 95% after spraying or soaking the fruits in cold water for 30 min. The real-time in vivo imaging data showed that the stem abscission zone in fruits represents a major source of internalization and persistence of Salmonella Typhimurium against antimicrobials. The four SMs also displayed bactericidal effect on phytopathogens at 200 μM. This study demonstrates that these novel SMs could represent safe bactericides for control of Salmonella and phytopathogens in fresh produce and are compatible for use in combination with biocontrol agents. HIGHLIGHTS
Plant pathogenic bacteria in the genus Erwinia cause economically important diseases, including bacterial wilt of cucurbits caused by Erwinia tracheiphila. Conventional bactericides are insufficient to control this disease. Using high-throughput screening, 464 small molecules (SMs) with either cidal or static activity at 100 µM against a cucumber strain of E. tracheiphila were identified. Among them, 20 SMs (SM1 to SM20), composed of nine distinct chemical moiety structures, were cidal to multiple E. tracheiphila strains at 100 µM. These lead SMs had low toxicity to human cells and honey bees at 100 µM. No phytotoxicity was observed on melon plants at 100 µM, except when SM12 was either mixed with Silwet L-77 and foliar sprayed or when delivered through the roots. Lead SMs did not inhibit the growth of beneficial Pseudomonas and Enterobacter species but inhibited the growth of Bacillus species. Nineteen SMs were cidal to Xanthomonas cucurbitae and showed >50% growth inhibition against Pseudomonas syringae pv. lachrymans. In addition, 19 SMs were cidal or static against Erwinia amylovora in vitro. Five SMs demonstrated potential to suppress E. tracheiphila when foliar sprayed on melon plants at 2× the minimum bactericidal concentration. Thirteen SMs reduced Et load in melon plants when delivered via roots. Temperature and light did not affect the activity of SMs. In vitro cidal activity was observed after 3 to 10 h of exposure to these five SMs. Here, we report 19 SMs that provide chemical scaffolds for future development of bactericides against plant pathogenic bacterial species.
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