Diversity Of Softrot Bacteria

Soft rot of fresh produce can be caused by diverse groups of bacteria including erwinia, pseudomonas, xanthomonas, clostridium, bacillus, and cytophaga [4].

The characteristics of these bacteria and their association with the spoilage of fresh produce under different conditions have been briefly described [7]. As strict anaerobes, Clostridium spp. cause soft rot of potatoes under oxygen-depleted conditions, especially when a more aggressive plant pathogen such as erwinia is present [16,17]. Pectolytic Clostridium spp. also play a role in the spoilage of fresh-cut produce that is packaged using an impermeable film [4]. Pectolytic bacillus including Bacillus polymyxa and B. subtilis have also been shown to be associated with soft rot in a wide variety of crops including potatoes, tomatoes, carrot, onion, and cucumber grown at elevated temperatures from ambient to 37°C [18]. Like clostridium and bacillus, pectolytic cytophaga [19] and xanthomonas [20] are generally considered the secondary pathogens which invade plants following the attack of a more aggressive pathogen such as erwinia or pseudomonas. Based on a series of studies previously conducted in our laboratory [21], erwinia and pseudomonas combined account for over 90% of soft rot of fresh produce while in storage or at markets. Less than 10% of soft rot of fresh produce found at the markets could be caused by xanthomonas, cytophaga, bacillus, or other unidentified genera [4,21].

5.2.1 Pectolytic Erwinia spp.

The soft-rot erwinia group, consisting of three species or subspecies, E. carotovora subsp. carotovora (Ecc), E. carotovora subsp. atroseptica (Eca), and E. chrysanthemi (Ech), is the major single cause of microbial spoilage of vegetables. The losses due to soft-rot erwinia cost tens or hundreds of millions of dollars yearly [3,7]. Ecc has the broadest host range causing diseases in almost every species of vegetable crops grown in temperate and subtropical regions [7]. Eca is present at cooler regions and is more often associated with black leg of potatoes [22] in the field than with soft rot of fresh produce after harvesting. In contrast, Ech causes diseases of crops grown in subtropical or tropical regions [23,24]. Both Ecc and Ech grow poorly and fail to induce soft rot of fresh produce at 10°C or below. At 20°C or higher, Ecc is considered the most destructive soft-rotting pathogen of fruits and vegetables. The soft-rot erwinia group is widespread in nature and very closely associated with plant vegetation and can be readily isolated from weed, plant debris, rhizosphere soil, and lenticels of potato tubers [25,26]. However, soft-rot erwinia is rarely detected on the surfaces of plant leaves or true seeds [22]. In addition to erwinia, other enteric bacteria including enterobacter, klebsiella, and serratia are commonly present on the surfaces of many different types of vegetable crops [4,5]. A vast majority of enteric bacteria are nonpectolytic and not expected to cause the spoilage of fresh produce. However, they may play a critical role in maintaining the quality and safety of fresh produce by enhancing or suppressing the growth of spoilage and pathogenic microorganisms on the surfaces of plants.

5.2.2 Pectolytic Fluorescent (PF) Pseudomonads

For fruits and vegetables that are stored at refrigeration temperatures, pectolytic fluorescent (PF) pseudomonads are responsible for a substantial proportion of soft-rot disorder observed in markets. PF pseudomonads as a group are physiologically and taxonomically heterogeneous, mainly consisting of P. viridiflava and five biovars of P. fluoresceins [21,27-29]. The latter was often designated as P. marginalis in the plant pathology literatures [21,28,29]. These pseudomonads are widespread in nature and can be isolated from diverse ecological niches including soils, irrigation water, rhizosphere, and surfaces of fruits and vegetables [27-29]. PF pseudomonads account for over 40% of total bacterial rot found at retail and wholesale produce markets [21]. They are especially abundant on the surfaces of leafy or salad vegetables including spinach [30], lettuce [31-33], cabbage [34], potato lenticels [25], tomatoes [35,36], and bell pepper [37]. On salad vegetables including lettuce, cabbage, and spinach, PF pseudomonads account for over 30% of total native bacteria recovered. Because of their prevalence in nature, PF pseudomonads are expected to play an important role in maintaining the safety and quality of refrigerated or ready-to-eat vegetables or fruits. They could be readily isolated from very diverse ecological niches including soil [38], rhizosphere [39], surfaces of fresh vegetables [30-37], and wash water from produce processing plants [38].

The importance of PF pseudomonads as the leading cause of spoilage of refrigerated fresh produce is primarily due to their psychrotrophic nature, nutritional versatility, and predominant presence on the surfaces of fresh produce. PF pseudomonads are responsible for a very large proportion of decay of fresh fruits and vegetables stored at low temperatures [21]. In addition, some P. marginalis and P. viridiflava strains can also cause soft-rot disease of horticultural crops in the field, e.g., the "pink eye'' of potato tubers [39]. A few reports also showed that other fluorescent pseudomonads including P. aeruginosa [40], P. tolasii [7], and P. chorii [9] were involved in postharvest spoilage of vegetables or mushrooms.

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