The fresh-cut produce industry has been the fastest-growing portion of the food retail market during the past 10 years, providing consumers with convenient and nutritious food. However, fresh-cut fruits and vegetables raise food safety concerns, because exposed tissue may be colonized more easily by pathogenic bacteria than intact produce. This is due to the higher availability of nutrients on cut surfaces and the greater potential for contamination because of the increased amount of handling. We found that applied Listeria monocytogenes populations survived and increased only slightly on fresh-cut Red Delicious apples stored at 10°C but increased significantly on fresh-cut honeydew melons stored at 10°C over 7 days. In addition, we examined the effect of lytic, L. monocytogenes-specific phages via two phage application methods, spraying and pipetting, on L. monocytogenes populations in artificially contaminated fresh-cut melons and apples. The phage mixture reduced L. monocytogenes populations by 2.0 to 4.6 log units over the control on honeydew melons. On apples, the reduction was below 0.4 log units. In combination with nisin (a bacteriocin), the phage mixture reduced L. monocytogenes populations by up to 5.7 log units on honeydew melon slices and by up to 2.3 log units on apple slices compared to the control. Nisin alone reduced L. monocytogenes populations by up to 3.2 log units on honeydew melon slices and by up to 2.0 log units on apple slices compared to the control. The phage titer was stable on melon slices, but declined rapidly on apple slices. The spray application of the phage and phage plus nisin reduced the bacterial numbers at least as much as the pipette application. The effectiveness of the phage treatment also depended on the initial concentration of L. monocytogenes.

Penicillium expansum, P. digitatum, and P. italicum acidify the ambient environments of apple and citrus fruit during decay development. They use two mechanisms for this: the production of organic acids, mainly citric and gluconic, and NH(4)(+) utilization associated with H(+) efflux. Exposure of P. expansum and P. digitatum hyphae to pH 5.0 increased their citric acid production, compared with the production of organic acids at acidic ambient pH. In decayed fruit, both pathogens produced significant amounts of citric and gluconic acids in the decayed tissue and reduced the host pH by 0.5 to 1.0 units. Ammonium depletion from the growth medium or from the fruit tissue was directly related to ambient pH reduction. Analysis of transcripts encoding the endopolygalacturonase gene, pepg1, from P. expansum accumulated under acidic culture conditions from pH 3.5 to 5.0, suggesting that the acidification process is a pathogenicity enhancing factor of Penicillium spp. This hypothesis was supported by the finding that cultivars with lower pH and citric acid treatments to reduce tissue pH increased P. expansum development, presumably by increasing local pH. However, organic acid treatment could not enhance decay development in naturally acidic apples. Conversely, local alkalinization with NaHCO(3) reduced decay development. The present results further suggest that ambient pH is a regulatory cue for processes linked to pathogenicity of postharvest pathogens, and that specific genes are expressed as a result of the modified host pH created by the pathogens.

The preparation and distribution of fresh-cut produce is a rapidly developing industry that provides the consumer with convenient and nutritious food. However, fresh-cut fruits and vegetables may represent an increased food safety concern because of the absence or damage of peel and rind, which normally help reduce colonization of uncut produce with pathogenic bacteria. In this study, we found that Salmonella Enteritidis populations can (i) survive on fresh-cut melons and apples stored at 5 degrees C, (ii) increase up to 2 log units on fresh-cut fruits stored at 10 degrees C, and (iii) increase up to 5 log units at 20 degrees C during a storage period of 168 h. In addition, we examined the effect of lytic, Salmonella-specific phages on reducing Salmonella numbers in experimentally contaminated fresh-cut melons and apples stored at various temperatures. We found that the phage mixture reduced Salmonella populations by approximately 3.5 logs on honeydew melon slices stored at 5 and 10 degrees C and by approximately 2.5 logs on slices stored at 20 degrees C, which is greater than the maximal amount achieved using chemical sanitizers. However, the phages did not significantly reduce Salmonella populations on the apple slices at any of the three temperatures. The titer of the phage preparation remained relatively stable on melon slices, whereas on apple slices the titer decreased to nondetectable levels in 48 h at all temperatures tested. Inactivation of phages, possibly by the acidic pH of apple slices (pH 4.2 versus pH 5.8 for melon slices), may have contributed to their inability to reduce Salmonella contamination in the apple slices. Higher phage concentrations and/or the use of low-pH-tolerant phage mutants may be required to increase the efficacy of the phage treatment in reducing Salmonella contamination of fresh-cut produce with a low pH.

The phytopathogenic fungus Colletotrichum gloeosporioides produces one pectate lyase (PL) that is a key virulence factor in disease development. During growth of C. gloeosporioides, Colletotrichum acutatum, and Colletotrichum coccodes in acidified yeast extract medium, the fungus secreted ammonia and increased the medium pH. Ammonia accumulation and the consequent pH change increased as a function of initial pH and buffer capacity of the medium. PL secretion by C. gloeosporioides correspondingly increased as the pH of the medium increased. The C. gloeosporioides pelB gene-disrupted mutant was able to increase ammonia accumulation and pH of the media similarly to the wild-type isolate. C. gloeosporioides in avocado, C. coccodes in tomato, and C. acutatum in apple showed ammonia accumulation in the infected area where pH increased to 7.5 to 8 and PL activity is optima. In nonhost interactions where C. gloeosporioides was inoculated in apples, the addition of ammonia-releasing compounds significantly enhanced pathogenicity to levels similar to those caused by the compatible C. acutatum-apple interaction. The results therefore suggest the importance of ammonia secretion as a virulence factor, enhancing environmental pH and pathogenicity of the Colletotrichum species.