الفهرس 
AcknowledgmentsOnly 14 pages are availabe for public view
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Abstract
Listeria monocytogenes is a Gram-positive, intracellular human pathogen responsible for the disease ‘listeriosis’, which is one of the most severe foodborne diseases in humans. Although the disease has an overall low incidence, it has a high mortality rate (20-30%). A diverse range of food products have been implicated with sporadic and epidemic incidents of listeriosis, linked to the consumption of ready-to-eat (RTE) foods, dairy products, seafood, pre-cooked or frozen meat, chicken and fresh products. The pathogen is a major threat to the food industry since it can survive and proliferate in harsh conditions such as high salinity, acidity and refrigeration temperatures. Further, it has the ability to attach to abiotic surfaces and form biofilms that can frequently contaminate food products and the production environment.
In the present study, samples were collected from dairy shops and prepared meat markets from different areas. The isolation of L.monocytogenes was carried out by using Fraser Broth enrichment as selective enrichment media. The isolated colonies on Oxford agar plates were used for morphological identification and characterization of typical L. monocytogenes colonies.
The Kirby-Bauer disk diffusion method was used to analyze the antibiogram profiles of L. monocytogenes isolates. The bacterial suspension was adjusted to match McFarland tube No. 0.05. Thirteen different antibiotic agents (Oxoid Ltd., Basingstoke, UK) belonging to eight different antibiotic groups were used. Several of these food isolates were resistant to three or more class of antibiotic and the isolates were considered MDR.
DNA extraction of L. monocytogenes isolates was performed using heating method. Conventional PCR was used for the determination of virulence gene (Aad6) responsible for aminoglycosides resistance of the positive multidrug resistant isolates.
Determination of the minimal inhibitory concentration (MIC) of the antibiotics to which positive isolates of L.monocytogenes were resistant was done by the microbroth dilution method using 96-microtitre plates with each of the cathelicidin AMPs (LL-37, WAM-1). Our study confirmed consistent inhibitory activity of WAM-1 and LL-37 antimicrobial peptides against MDR isolates of L.monocytogenes. Furthermore, WAM-1 showed statistically significant synergistic effects with each of the tested antibiotics, even at 0.5X the MIC of WAM-1.
Positive isolates were treated with increasing concentrations of AMP, incubated overnight and stained with crystal violet to determine the AMP’s ability to inhibit biofilm formation WAM-1 was more effective at inhibiting and dispersing biofilms by positive isolates, including MDR isolates. Hence, WAM-1 and LL-37 both inhibited biofilm formation in MDR L.monocytogenes, but LL-37 fails to disperse mature biofilms.