الفهرس 
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Abstract
Tyramine is a phenolic BAwidely found in fermented food especially in dairy products.
The majority of the tyramine in food is generated by the decarboxylation of the amino acid tyrosine through TDCenzyme produced by bacteria present in food.
Tyramine can accumulate in blood due to frequent intake of fermented and pickled food, leading to cheese effects characterized by hypertension, headache, migraine, brain haemorrhage or heart failure.
The aim of the present study was to detect tyramine prevalencein dairy products through isolation and characterization of tyramine-producing bacteria from a variety of dairy product samples collected from the local markets and to suggest methods to decrease tyramine accumulation in these products.
In the currentstudy, a total of fifty samples of different types of dairy products were collected from various local markets in Alexandria, Egypt. Total viable count was determined for each sample. Morphologically different types of bacteria were separated and purified from each sample to collect a total of 101 isolates from the 50 dairy samples.
Total genomic DNA was obtained from each isolate and was used as a template for subsequent PCR amplification using degenerate (DEC5/DEC3) primers.
A total of 40 strains were found to possess tdcgene and were confirmed as tyraminogenic bacteria.
These 40 tyraminogenic isolates were identified using Gram stain which segregated the isolates into 35 Gram positive and 5 Gram negative strains. Further identification to species level was performed by MALDI-TOF-MS which allowed the detection of 20 different species as follows: E. faecalis (4),E. faecium (5),E. durans (1),S. pasteuri (1),S. saprophyticus (2),S. sciuri (1),S. hominis (2),S. succinus (1),S. carnosus (1),M. caseolyticus (1),L. lactis (1),B. cereus (7),B. vallismortis (2),B. subtilis (1),R. pycnus (1), B. pumilis (1),B. thuringiensis (1), K. pneumonia (1),P. mirabilis (2)andE. coli (2).
Searching on database, it was found that sixstrains were not previously reported as tyramine-producing ones.
<These strains, identified as novel tyramine-producing strains, were: M. caseolyticus, B. vallismortis, R. pycnus,B. cereus, S. sciuri, andS. pasteuri. These sixisolates were further subjected to sequencing analysis of the 16S rRNA gene by PCR.
The obtained sequence identities were verified by a BLAST search against available molecular databases with a resultant high similarity level reaching a percentage of 98%.
<Sequences were then submitted to the GenBank databases and accession numbers were obtained.Sequence analysis of the amplified portion of tdcgene for the previously unreported sixtyraminogenic strains confirmed a high percentage of similarity ranging from 97% to 99% to the previously reported tdcgene sequences available on GenBank databases.
The tdc gene fragment sequences obtained in this study were submitted to the GenBankdatabases and the following accession numbers were obtained: PRJNA488559(S. pasteuri), PRJNA488267(B. cereus), PRJNA488264(M. caseolyticus), PRJNA488168(S. sciuri), SAMN09925732 (B. vallismortis), and SAMN09924829 (R. pycnus).HPLC was performed to evaluate qualitatively and quantitatively tyramine production from the previously unreported sixtyraminogenic isolates in addition to two strains which were E.
<Summary, Conclusion, and Recommendations 58faecalisand B. thuringiensis.
<All the previously unreported tyraminogenic strains produced detectable level of tyramine ranging from 1200 to 1448 mg/L.It has been suggested that, probiotics can be used to prevent tyramine accumulation in dairy products. L. rhamnosusLGG®was selected as a probiotic, beneficial and non-tyramine-producing bacteria that may be used in the future to inhibit tyramine production in dairy products.Disc diffusion method was applied to test the inhibitory effect of L. rhamnosusLGG®against the 20 tyraminogenic strains identified in the present work. L. rhamnosusLGG®inhibited the growth of 80% of the studied tyraminogenic strains. Nevertheless, B. vallismortis, S. saprophyticus, L. lactis, and S. pasteuriwere not inhibited by L. rhamnosusLGG®. B. vallismortis, an example of tyraminogenic strain whose growth was not affected by L. rhamnosusLGG®and the bacteria producing the highest concentration of tyramine, was co-inoculated with the probiotic bacteria and then the mixture was subjected to HPLC analysis to quantify the tyramine content of B. vallismortisin the presence of L. rhamnosusLGG®.
Theprobiotic strain L. rhamnosusLGG®completely inhibited the tyramine production of the tested B. vallismortissuggesting that the mechanism of inhibition exerted by L. rhamnosusLGG®was not growth dependent.