Microbes are crucial to plant life, and, therefore, to the successful production of produce as a commodity. Plant microbes can be beneficial as symbionts [1-3], competitors of plant pathogens for biocontrol [4,5], and for promoting plant growth [6-8]. Indeed, most of the fundamental knowledge of the biology, microbial ecology, and genetics of plants has been obtained as a result of studies to understand and prevent plant disease. However, plants also are vulnerable during growth to microbial pathogens from the environment (e.g., soil, water, air, amendments). The links between fresh produce/produce dishes with more than 300 outbreaks in the U.S. since 1990 [9,10], and the obvious vulnerability of preharvest produce to pathogens in the production environment, have stimulated similar basic studies of the biology of enteric pathogens on produce.
The sources of the microorganisms exposed to plant surfaces may be from the plant seed itself [11-13], and through the initial contact with soil, irrigation water and air. The microbial communities of the rhizosphere (roots and the part of the soil affected by contact with roots) and the phyllosphere (leaves and the environment in contact with leaves, e.g., water, air) of produce are in constant change due to factors that affect microbes, such as humidity, temperature, nutrients, UV radiation, insects, and wild animals. Plant tissues are in close contact with potentially thousands of different species of bacteria, viruses, and other microorganisms . Fruit and vegetable crops also have a rich microbial flora, including in many cases coliforms and fecal coliforms that are unavoidable considering the presence of domestic and wild animals near production environments [15-18]. Indeed, 190 produce-associated outbreaks have been documented in the U.S. for the years 1973-1997 . Plant bacteria have evolved multiple mechanisms suitable for initiating interactions essential for successful colonization of plants [3,20,21]. However, a major interest of those working on microbial food safety of produce is whether there are equivalent, or similar, mechanisms of attachment used by human pathogens that contaminate produce commodities.
Considering the analogous or common secretion systems, outer surface proteins, and polysaccharides among the plant and human pathogens, and similarities among disease-associated genes of humans and plants (e.g., Arabidopsis thaliana), this area of investigation perhaps offers more promise than many plant and animal pathogen researchers perceived initially . An ultimate goal of studies of attachment of human pathogens to plants is the development of intervention methods to minimize attachment and survival of human pathogens.
Our goal in this chapter is to review current knowledge of perhaps the most important event that initiates the association between most microorganisms and plants: attachment. We focus our attention on studies that provide insight into fundamental molecular plant-microbe interactions. The most definitive knowledge on plant-microbe interactions involving attachment has been provided in two areas: (1) mechanisms of disease of bacterial plant pathogens, and (2) molecular mechanisms involved in the symbiotic relationship between nitrogen-fixing bacteria and plants. These studies provide a context for assessing the potential mechanisms of attachment of enteric human pathogens to produce.
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