Factors Affecting The Survival Of Softrot Bacteria In Nature

5.3.1 Plant Vegetation

Soft-rot erwinia and pseudomonas are widespread in nature and can be readily isolated from decayed tissue, plant debris, rhizosphere soil, and weeds [25,26,38,41,42]. De Boer [41] reported that Eca was isolated more often from soil in which potatoes had been grown in previous years than from soils in which other types of crops had been grown. Thus, survival and over-wintering of erwinia in soil can be greatly affected by the type of crop grown in the previous season. Soft-rot erwinia is rarely found on the surfaces of leafy vegetables and true seeds and survives poorly in sterilized soil [22]. Plant vegetation appears to be important for long-term survival of erwinia and pseudomonas in soil [26,28,41]. Because of the widespread distribution in plant and nonplant environments, it is impossible to eliminate completely soft-rot erwinia and pseudomonas from propagation materials, irrigation water, or soils in the field.

5.3.2 Temperature and Atmospheric Conditions

Refrigeration is the most convenient and effective means to maintain the organoleptic properties, to reduce the spoilage, and to extend the shelf life of fresh produce. The International Fresh-Cut Produce Association (IFPA) [42] recommends minimally processed produce be stored at 1 to 4° C to maintain the quality and safety. Refrigeration of fresh produce at between 4 and 10°C is commonly used by the industry to extend the shelf life and to prevent the soft rot caused by bacteria (such as Ecc and Ech) and fungi. At this temperature range, the development of soft rot by Eca and PF pseudomonads will occur. The minimum temperature for growth of Eca has been estimated to be between 3 and 6° C [7] and the minimal temperature for growth of PF pseudomonads estimated to be 4°C or below [21].

The seven genera of soft-rot bacteria mentioned above require somewhat different optimal atmospheric conditions for growth and induction of spoilage. For instance, Clostridium spp. are strictly anaerobic and PF pseudomonads (with the exception of nitrate-denitrifying strains) are strictly aerobic. Induction of soft rot in potatoes by erwinia and clostridium is greatly enhanced by the depletion of oxygen [16]. Reduction in oxygen concentration or increase in carbon dioxide concentration in the atmosphere reduced the growth of PF pseudomonads [43] and their ability to induce soft rot on fresh produce [44].

5.3.3 Latent Infection and Internalization

Although the internal parts of plant organs are generally considered sterile [6], many different types of bacteria including soft-rot erwinia, pseudomonas, and serratia can be detected within apparently healthy tomatoes [45] and cucumber fruits [46]. These bacteria presumably exist in a commensalistic or quiescent state, which can be activated only when the stressed conditions in fruits are removed. A large proportion of storage rot of fruits is due to external contamination by soft-rotting microorganisms and a small proportion of them may be caused by the activation of latent bacteria inside the fruits. The route by which soft-rot erwinia penetrates into the internal parts of apparently healthy tomato fruits is unclear but possibly may be through the connective tissue at the stem end of the fruits [47]. Bartz and Kelman [48] also reported that the bacterial soft-rot potential in potato tubers was affected by difference in temperature between tubers and suspensions of erwinia at the time of inoculation by immersion. A series of laboratory experiments have conclusively demonstrated that human pathogenic bacteria including E. coli O157:H7 and salmonella can be infiltrated into apple [49,50], orange [51], tomatoes [51], and lettuce [52-54] if the temperature of bacterial suspension is lower than that of fruits. Surface cleaning and sanitization treatments are not expected to eliminate completely the undesirable bacteria that become internalized [55] and those attached to the surfaces of intact or injured fruits [56,57].

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