Interactions Between Softrot And Human Pathogens On Fresh Produce

Despite the lack of a known mechanism for attacking plants, the gastrointestinal human pathogens including salmonella, E. coli O157:H7, and L. monocytogenes can survive and even grow on fruits or vegetables over a long period of time [99]. Their survival and growth can be affected by the indigenous microflora and by the storage conditions [4]. The dynamics of the interactions between native microflora, spoilage bacteria, and human pathogens, especially under modified atmospheres, have been investigated [5]. The results obtained thus far indicate that the effect of spoilage or saprophytic microorganisms on the proliferation of human pathogens on fresh produce could be either synergistic or antagonistic, largely depending on the type of pathogens, fresh produce, and storage conditions examined.

5.5.1 Synergistic Interactions

The interactions between soft-rot and human pathogens on fresh produce began to catch the attention of public health officials when Wells and Butterfield [15] reported that the rotted plant tissue more often harbored salmonella than the healthy counterpart. They also demonstrated that the population of salmonella increased by 5- to 10-fold on potato or carrot slices that were co-inoculated with soft-rotting E. carotovora or P. viridiflava. Carlin et al. [100] later showed a positive correlation between the number of L. monocytogenes and the extent of soft rot observed with endive leaves. These studies indicate that the rotted plant tissues may provide extra nutrients to enhance the growth of human pathogens. Contaminated plant tissues can then serve as a reservoir or vehicle for the dissemination of clinically important pathogens in farms or food processing facilities.

Surveys of salmonella contamination on rotted fruits and vegetables induced by molds or fungi have also been reported [16]. The incidence of salmonella contamination on rotted tissue induced by molds or fungi was about one tenth of that induced by soft-rotting bacteria. Nevertheless, the fungi-induced rotted tissues are three times more likely to contain salmonella than the healthy counterpart. In spite of this, the investigators [16] concluded that rotted tissues pose little or no greater safety risk than the healthy tissues. Gastrointestinal pathogens including salmonella and L. monocytogenes usually do not grow, or grow very poorly, on acidic fruits (pH < 4.0) such as apple and orange [101]. Conway et al. [102] demonstrated that these bacterial pathogens were able to multiply in rotted tissues induced by specific groups of fungal pathogens such as Glomerella cingulata but not in rotted tissues induced by other groups of fungal pathogens such as Penicillium expansum. They found that the population of L. monocytogenes increased in apple fruits infected with G. cingulata but declined in fruits infected with P. expansum. Conway et al. [102] revealed that the pH in P. expansum-induced rotted apples decreased from 4.7 to 3.7 as opposed to the increase in pH from 4.6 to 7.7 in G. cingulata-induced rotted fruits. Riordan et al. [103] also showed that the population of E. coli O157:H7 increased 1 to 3 logs on apple infected with G. cingulata but continued to decrease in rotted tissues infected with P. expansum. The pH change in rotted tissues induced by different groups of fungi thus plays a critical role in the fate of human pathogens on fresh produce.

5.5.2 Antagonistic Interactions

In contrast to the synergistic effect, a number of studies have shown that the growth of human pathogens on fresh produce could be suppressed by the presence of postharvest rot pathogens. For examples, the growth of L. mono-cytogenes on potato slices [104], spinach [30], and endive [105] could be markedly reduced by diverse strains of fluorescent pseudomonads. The inhibition was thought to be caused by the production of iron-chelating fluorescent siderophores or antimicrobials by the pseudomonads [106]. Carlin et al. [105]

reported that more growth of L. monocytogenes was detected on endive leaves that were rinsed with disinfectants than those rinsed with water. Two pseudomonad antagonists possibly responsible for inhibiting the growth of L. monocytogenes on endive leaves have been identified [105]. Additional strains of fluorescent pseudomonads capable of inhibiting the growth of L. monocytogenes or L. innocua on different types of produce including carrot, lettuce, bell pepper, and sprouting seeds have been isolated [106,107].

L. monocytogenes is in general more susceptible than salmonella or E. coli O157:H7 to the antagonists naturally present on the surfaces of fresh produce [106]. In addition to the saprophytic antagonists, postharvest rot pathogens including P. fluorescens and P. expansum can also inhibit the growth of human pathogens such as E. coli O157:H7 and L. monocytogenes [102-104]. It has been suggested that elimination of native microflora (including bacterial and fungal rot pathogens) from fruits and vegetables may create a less competitive environment for the proliferation of human pathogens on fresh produce [4].

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