The many years of difficult and labor-intensive studies on plant-microbe interactions involved in plant symbiosis and disease have begun to yield fundamental molecular information regarding bacterial attachment to plants (Table 2.1). The attachment factors designated in Table 2.1 can be grouped essentially into five categories: polysaccharides (EPS, CPS, LPS), outer membrane proteins, flagella, pili, and fimbriae. In some systems, bacterial protein factors have been identified that bind to plant carbohydrates (e.g., rhicadhesin), and in others, a bacterial polysaccharide is bound by a plant lectin (e.g., Rhsp EPS/CPS/LPS). It is probable that attachment for some bacteria will involve both strategies ("dual bridge'') simultaneously, or with different hosts and/or in different environments. The attachment factors identified in human pathogens mostly relate to studies with animal cell lines or animal models (Table 2.2). However, it is very likely that flagella, pili, and fimbriae might have roles as attachment factors for human pathogens on plants, considering their prominent outer surface location and length. Absent from Table 2.2 are EPS (e.g., colanic acid), CPS (e.g., K-antigens), and LPS (e.g., O-antigens), all very important complex carbohydrate-containing molecules synthesized by human pathogens; these molecules are surface-expressed and often regulated by environmental cues [183-185]. Surface complex carbohydrates are excellent candidates for possible interactions with plant lectins of the appropriate specificity ; a precedent is the well-defined rhizobiaceae EPS interaction with pea plant lectin (Table 2.1).
The model studies of enteric human pathogens with plants/produce indicate the general fitness of human pathogens in these environments. Similar to plant bacteria, human pathogens appear to possess multiple specific mechanisms of attachment and growth (Table 2.3). The interactions of human pathogens with host plants probably will involve many unique factors depending upon the plant and the pathogen. It is probable that events occur preceding the direct interaction of a human pathogen with a plant that are important for attachment. For example, the environment in which the pathogen has remained viable (water, manure, soil, eukaryotic microorganisms, insects, animals) will dictate what surface molecules are expressed and the metabolic state of the human pathogen prior to interaction with the plant host. Also, the human pathogen may be associated with other microorganisms in aggregates or in a detached biofilm. The plant may release chemicals that are signals and/or chemotaxis factors for some human pathogens. The availability of different types of plant receptors (specific and nonspecific) will determine the efficiency of attachment. After the human pathogen cell or cells make direct contact with the potential host plant, the human pathogen attaches either specifically or nonspecifically by weak or strong interactions depending upon the site of attachment. Flagellated cells may move (e.g., twitching motility) along a surface until an optimal attachment site is recognized. Initial attachment likely occurs by biochemical forces or by human pathogen proteins extended from the surface (pili/fimbriae, flagella), with tighter attachment established later by other surface molecules. Based on other plant-microbe interactions (Table 2.1), a possible strategy for attachment may combine human pathogen protein-plant receptor (e.g., carbohydrate) and plant lectin-human pathogen polysaccharide (e.g., EPS, CPS, LPS) interactions. The human pathogen may then be further secured by human pathogen cell-cell aggregation (possibly involving T3SS) or human pathogen-plant microbe aggregation, both of which likely require expression of different attachment factors.
The presence of putative attachment factors in enteric human pathogens that are similar to those of plant-related bacteria, point to obvious approaches for identifying fundamental mechanisms of human pathogen attachment to produce. Fimbriae, pili, flagella, polysaccharides, and porin proteins are all candidates for direct attachment to and aggregation of human pathogens on plant tissue. Recent advances by researchers in studies of how native microbes attach and interact with the rhizoplane and phylloplane provides inspiration and guidance for researchers studying the biology of human pathogens in similar environments.
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