Intact plant surfaces, especially those of leaves, are relatively inhospitable environments for microorganisms, providing limited sites for attachment, surface retention of water, and nutrients. Nevertheless, many microorganisms have developed mechanisms to attach, survive, or grow in microniches on different plants. The micrograph shown in Figure 2.3A demonstrates the localization and high density of epiphytic bacteria on a lettuce leaf. Both Gram-positive and Gram-negative bacteria are interacting in aggregates and possibly competing for the limited nutrients available in the microniche at the junction of epidermal cells where cuticular waxes are less dense, water accumulates, and nutrients are more available than in other sites. Although biofilms with classic structures described in recent studies are rarely found on plants, thick three-dimensional biofilms have been observed on sprouts sampled from a commercial sprout facility (Figure 2.3E). The image reveals the potential for complex interactions to occur between Gram-positive and Gram-negative resident bacteria under ideal conditions of plentiful water, exuded nutrients, and warm temperatures during food production or processing. A thick mat of mostly aggregated bacterial cells was detected on the root hairs of the sprouts (Figure 2.3E, "Ep"). Although plant tissue was likely present within the biofilm (Figure 2.3E, arrow), it appears that multiple layers of cells compose the biofilm, and that the presence of EPS at the surface of, or within, the biofilm is possible. Similar aggregates of bacteria have been observed using SEM on roots of alfalfa, broccoli, clover, sunflower, and mung bean sprouts [176,177]. Following attachment of bacteria as individual cells on leaf surfaces, aggregation is crucial as a strategy to ensure survival under environmental stresses such as water or nutrient depletion, UV irradiation, unfavorable temperatures, or predation [178,179]. In many enteric bacteria, fimbriae composed of curli protein interact with a cellulose poly-saccharide resulting in aggregation and either pellicle formation or biofilms (Table 2.2) . T3SS-encoded proteins in other bacteria (Table 2.1 and Table 2.2) are analogous to curli. In a recent report, T3SS-encoded protein in Echr was shown to interact with p-glucanlike (noncellulose) carbohydrates, and this interaction was crucial for pellicle and biofilm formation in vitro . Thus, T3SS proteins, and possibly type 1 pili, conjugative pili, and curli (fimbriae), are important in aggregation leading to biofilm formation.
Biofilm formation is thought to be a major reason for the persistence of microorganisms, including pathogens, for long periods in food processing environments . Bacteria, filamentous fungi, yeasts, and even viruses may be represented within biofilms on a plant surface. Therefore, the mechanisms of initiating bacterial autoaggregation and mixed-species aggregation, and the attachment of bacteria singly or as aggregates to plant surfaces or to microorganisms/EPS in preexisting biofilms on plant surfaces, could involve attachment factors such as those described in this review (Table 2.1 and Table 2.2). Understanding the mechanisms could yield intervention strategies for decontamination of produce.
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