Introduction

consumers' awareness of the health benefit expected from this popular diet [1]. More than 300 fresh and fresh-cut produce items are available for sale at supermarkets throughout the U.S. [2], largely due to the advanced postharvest technologies, improved crop varieties, and efficient distribution systems. To meet the market demand, new strategies are required to increase the production of fresh produce in farms and to reduce the postharvest losses caused by biotic and abiotic factors. It has been estimated that between 10 and 30% of fresh fruits and vegetables produced in the U.S. are wasted, mainly due to three factors: mechanical injuries, physiological decays, and microbial spoilage [3].

Microbial spoilage accounts for a substantial proportion of postharvest losses of fresh produce, which can be caused by a wide variety of microorganisms including bacteria, fungi, or yeasts [4,5]. In general, the spoilage of acidic fruits such as apple, orange, and berries is caused by molds, lactic acid bacteria, or yeasts. The spoilage of fresh produce with neutral pH such as salad vegetables and edible roots or tubers is caused by bacteria capable of producing pectolytic enzymes required for degradation of plant cell walls. Bacterial spoilage of fresh produce is usually found in the form of soft rot, which is characterized by water-soaking and total disintegration of plant tissues [6,7]. As reported in the literature, bacterial soft rot has been identified as the leading cause of disorders in many types of produce, including potato [8], lettuce [9,10], bell pepper [11], cucumber [12], and tomato [13]. This disorder can cost the fresh produce industry and consumers hundreds of millions of dollars annually [3].

In addition to its economic impact, soft-rotted plant tissue may serve as a carrier or reservoir for foodborne human pathogens and pose a potential threat to the safe supply of fresh produce. Wells and Butterfield [14,15] reported that the rotted plant tissues were more likely to harbor salmonella than the apparently healthy counterparts. They found a 5- to 10-fold increase in the population of Salmonella Typhimurium in potato slices co-inoculated with soft-rot bacteria [14]. Therefore, an integrated approach to control the proliferation of both soft-rot bacteria and foodborne human pathogens on fresh produce is required.

In this chapter, the diversity of soft-rot bacteria associated with postharvest losses of horticultural commodities and the factors affecting their survival in nature are discussed. In addition, the enzyme and molecular genetic mechanism by which soft-rot bacteria (especially fluorescent pseudomonas) cause maceration of plant tissues is reviewed. Furthermore, the synergistic and antagonistic interactions between spoilage microorganisms and human pathogens on fresh produce are discussed. Farm practices that are useful for controlling the dissemination and proliferation of both soft-rot and human pathogens on fruit and vegetable crops are presented.

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