2.2.1. Salmonella

This genus is composed of two species; Salmonella enterica and Salmonella bongori. S. enterica, subdivided into hundreds of serovars, is the causal agent of many human, animal, and bird diseases worldwide, including gastroenteritis and typhoid fever. It is the pathogen most frequently linked to consumption of fruit and vegetables. Despite its clear adaptation to survival in human hosts, Salmonella has also been detected in the rhizosphere of several crop plants. For a long time, Salmonella was assumed to survive on plants after a more or less accidental infection. However, this notion has recently been challenged. Salmonella bacteria are able to persist in soil. Studies on native Conzattia multiflora, a legume tree indigenous to Mexico, revealed the existence of Salmonella plant-borne lines isolated from nodule-like structures. S. enterica ssp. enterica serovar Typhimurium (S. typhimurium) has been found in the rhizosphere of several crop plants including: wheat (Triticum sativum), oilseed rape (Brassica napus), and strawberry (Fragaria ananassa). S. enterica levels on wild tomato (Solanum pimpinellifolium) were lower than on domesticated tomato cultivars (Solanum lycopersicum). An additional infection route could be water, which was in contact with human or animal waste. Contaminated water is a well-known means of dissemination of numerous pathogens, including Salmonella. In this way, bacteria can move through the water supply system and subsequently reach crop fields where the contaminated water is used for irrigation. S. enterica populations were bigger in the phyllosphere of tomato plants irrigated with contaminated water than in plants grown from seeds in pre-infested soil. Infection of plants with Salmonella may also occur with the help of other organisms. Co-inoculation with the nematode Caenorhabditis elegans and Salmonella Newport of lettuce, strawberries or carrots placed on top of animal or composted manure resulted in plant infection with these bacteria; conversely, the plant tissues were not infected with the pathogen in the absence of the nematode. Salmonella can also be transported via insects, e.g. by the pharaoh ant (Monomorium pharaonis), moving from indoor facilities to the environment, and therefore coming in contact with a range of possible hosts, including plants. The interaction between Salmonella and arbuscular mycorrhzal fungi (AMF) has demonstrated a higher persistence of Salmonella in plants colonized by AMF, which indicates another layer of interactions in the rhizosphere.

The susceptibility to contamination by S. enterica via soil differs among agricultural crops. Members of the Brassicaceae family presented higher bacterial populations than tomato and lettuce. However, lettuce was shown to have a more highly contaminated phyllosphere, suggesting that other contamination routes, such as irrigation water, are also effective. Another noteworthy possibility is that the plant itself might contribute to infection. A study performed on alfalfa sprouts demonstrated that, during seed germination, the endosperm breakdown causes the release of reducing sugars and other organic molecules, which can be found in the irrigation water and, in consequence, produce a growth medium for S. enterica. Since bacteria metabolize these molecules as a source of nutrients, plant exudates seem to be a suitable nutrient source. The high multiplication rates of some Salmonella strains in effluxes of germinating seeds has prompted the suggestion of saprophytic growth. Furthermore, other plant pathogenic bacteria may contribute to the infection with Salmonella. Pectinolytic bacterial pathogens, which cause soft rot and in this way mobilize nutrients, are frequently associated with infection of fruits and vegetables by S. enterica. Salmonella is not only capable of colonizing the phyllosphere; it has been shown to infect the plant and cause death of plant organs. Inoculation of Salmonella in Arabidopsis via shoot or root tissues resulted in chlorosis, wilting, and eventually death of the infected tissues within seven days.