The unique physical and biochemical qualities of each plant surface as a result of the plant genotype and of responses to environmental stimuli (light, temperature, humidity, atmosphere, pH, soil) are major determinants of the plant microbial community. Large differences in the types and numbers of bacteria can occur on different plants, leaf-to-leaf of the same plant , and leaf-to-leaf seasonally and even daily [32,37]. Bacteria usually colonize and/or are present in areas of a leaf that retain water and are protected from UV light .
Early studies to quantify the populations of bacteria on vegetables reported that >2 x 106 predominantly Gram-negative CFU/g could be isolated from the outer leaves of cabbage, in contrast to 4 x 103 bacterial CFU/g that were isolated from inner leaves . The results of numerous studies have been in agreement with these findings, with viable aerobic bacteria ranging between 32 CFU/g on inner leaves of lettuce to 107 CFU/g on spinach or peas in warm, humid conditions . Washing vegetables in water usually will decrease the number of bacteria present only marginally (2.5- to 3-fold), reflecting the relatively tight attachment of bacteria to the surface. Viable bacteria have been detected in the interior tissue of cucumbers and tomatoes, locations that had been assumed to be sterile . Thus, bacteria interact with plants by mechanisms that probably involve significant movement of bacteria on tissues, and subsequent attachment, and in some cases entry into tissues (possibly endophytic). These events are in contrast to the infiltration of bacteria into cut or damaged plant tissue, or into natural openings in fruits and vegetables like pores, stems, and calyx during food processing . The internalization of human pathogens in produce is of great concern, regardless of when it occurs (pre/postharvest), because it would decrease the effectiveness of any disinfection steps to minimize contamination .
Many microorganisms are present in the plant rhizosphere and phyllo-sphere, including bacteria, fungi, protozoa, nematodes, and viruses. In addition, many different bacterial genera have been isolated from fruits and vegetables (Table 2.1) . Although most of these organisms probably interact by specific and different mechanisms with plants, little is known about the interactions involved. As noted, most of the fundamental and definitive studies of plant-microbe interactions involve Gram-negative plant epiphytic or pathogenic bacteria. A review of the literature related to this subject provides a context for assessing observations obtained from very preliminary studies of human pathogens in similar environments. In addition, it is probable that some of the human pathogens use similar and/or modified mechanisms of attachment. Thus, attention to any similarities in human pathogens and plant bacteria location on plants, biochemistry of bacterial outer surface structures, and possible adhesins, could provide clues for more fundamental studies.
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