Listeriosis is one of the most important food-borne diseases. A variety of culture and rapid methods are available for the detection of Listeria spp. in foods. Although the presence of L. innocua may indicate potential contamination with L. monocytogenes, only the latter species is pathogenic for humans. Therefore, the most adequate tests are those which specifically detect L. monocytogenes. Chromogenic media is currently the most common method used for the presumptive identification of L. monocytogenes. Some tests like those based on antigen detection are fast and easily applied, but only a few may specifically detect L. monocytogenes. Real-time polymerase chain reaction is increasingly applied in food diagnostics for the detection of L. monocytogenes due to the availability of different specific commercial test methods. Microarrays and biosensors are some examples of new technologies that might be used routinely for the detection of L. monocytogenes in foods in the future.
Aims: Formulations of dietary probiotics have to be robust against process conditions and have to maintain a sufficient survival rate during gastric transit. To increase efficiency of the encapsulation process and the viability of applied bacteria, this study aimed at developing spray drying and encapsulation of Lactobacillus reuteri with whey directly from slurry fermentation. Methods and Results: Lactobacillus reuteri was cultivated in watery 20% (w/v) whey solution with or without 0Á5% (w/v) yeast extract supplementation in a submerged slurry fermentation. Growth enhancement with supplement was observed. Whey slurry containing c. 10 9 CFU g À1 bacteria was directly spraydried. Cell counts in achieved products decreased by 2 log cycles after drying and 1 log cycle during 4 weeks of storage. Encapsulated bacteria were distinctively released in intestinal milieu. Survival rate of encapsulated bacteria was 32% higher compared with nonencapsulated ones exposed to artificial digestive juice. Conclusions: Probiotic L. reuteri proliferate in slurry fermentation with yeastsupplemented whey and enable a direct spray drying in whey. The resulting microcapsules remain stable during storage and reveal adequate survival in simulated gastric juices and a distinct release in intestinal juices. Significance and Impact of the Study: Exploiting whey as a bacterial substrate and encapsulation matrix within a coupled fermentation and spray-drying process offers an efficient option for industrial production of vital probiotics.
A SYBR Green I based real-time PCR assay with inlA-specific oligonucleotide primers was developed for easy and rapid detection of Listeria monocytogenes in a model food that usually has a high incidence of contamination with this pathogen. Results with pure cultures and artificially contaminated chicken meat samples indicate that the PCR assay was highly specific and sensitive. The melting point analysis of the 160 bp amplified DNA fragment was different for L. monocytogenes isolates of the two major phylogenetic divisions of the species, 1 and 2. The assay was then used to survey retail ground chicken meat for contamination with L. monocytogenes. Thirty-seven samples were enriched according to the United States Department of Agriculture culture assays to detect L. monocytogenes on meat. The use and efficiency of PCR assay was examined following both primary and secondary enrichments, which were also plated on chromogenic agar for enumeration of L. monocytogenes and nonpathogenic Listeria spp. to investigate the discrepancies between culture and PCR. Overall, L. monocytogenes was detected in 75% of the samples. Primary enrichment yielded detection rates of 70% and 37% for culture and PCR, respectively. The corresponding rates for secondary enrichment were 54% and 70%, respectively. Test sensitivity is therefore influenced by the type of enrichment and is probably related not only to the limited growth of L. monocytogenes in the primary enrichment media (false-negative PCR results), but also to the high populations of nonpathogenic Listeria spp. in the secondary enrichment broths (false-negative culture results). The main challenge of rapid PCR-based detection of L. monocytogenes from food is the poor sensitivity of primary enrichment media. The improvement of enrichment conditions may help increase assay sensitivity.
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