Biocontrol of Pathogenic Bacteria in Egyptian Cheeses by Bacteriophages
Kandil, S.T.K. (2023) Biocontrol of Pathogenic Bacteria in Egyptian Cheeses by Bacteriophages. Doctoral thesis, Harper Adams University.
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Sherif Kandil PhD Thesis in Food Safety (with final corrections). Harper Adams University.pdf Download (4MB) | Preview |
Abstract
Raw milk cheeses such as Karish, Domiati and Ras are among the most consumed cheeses in Egypt; however, they have been a vehicle for transmission of Escherichia coli and Staphylococcus aureus pathogens. Improving safety without compromising the natural raw milk microbiota and sensorial characteristics requires an alternative to chemical antimicrobials. Bacteriophages are a promising natural tool that can specifically kill bacterial pathogens without affecting the beneficial microbiota; however, far too little attention has been paid to testing phage efficacy against E. coli and S. aureus strains at more challenging physicochemical conditions and in raw milk cheeses. The main objectives of this study were to evaluate the effect of raw milk cheese physicochemical conditions, non-targeted milk-associated microbiota and cow’s milk fat separation on E. coli and S. aureus phages role as biocontrol agent, in order to inhibit E. coli and S. aureus growth in Karish, Domiati and Ras cheeses. Among four E. coli and four S. aureus tested phages, E. coli T4, S. aureus EBHT and S. aureus K2 phages were selected based on their high lytic ability at 105 PFU/ml in liquid medium, host range and high biocontrol activity under Egyptian raw milk cheeses physicochemical conditions: temperature (4, 24, 37 and 45 °C), pH (4.2, 5.1, 5.9 and 6.7) and salt concentration (0.5, 4, 8 and12 %). E. coli phage T4 and S. aureus phage cocktail (1:1 EBHT and K2) were able to reduce the initial host contamination below the detection limit (≤ 10 CFU/ml) after 2 h when they were initially independently tested against 104 CFU/ml host strain in sterilized whole and skimmed milk. However, higher phage concentration at 108 PFU/ml was required to reach the complete elimination of host in raw milk, due to the effect of present non-target milk components. Although there were no observed differences in phage lytic ability in sterilized whole and skimmed milk, E. coli phage T4 showed higher ability to reduce (p ≤ 0.05) inoculated and indigenous E. coli in raw skimmed milk (3.6 log CFU/ml) compared to raw whole milk (4.6 log CFU/ml) after 24 h. Similar observation was found with S. aureus phage cocktail; however, due to their wider host range, inoculated and indigenous S. aureus were not detectable after 2 h in raw skimmed milk, while in raw whole milk inhibition was only observed after 24 h. Finally, E. coli phage T4 and S. aureus phage cocktail (1:1 EBHT and K2) were applied in situ as biocontrol agents in Karish, Domiati and Ras cheeses manufactured from temperature-abused raw skimmed (Karish) and raw whole (Domiati, Ras) cows’ milk, with initial microbial populations of 5 to 6 log CFU/ml. E. coli phage T4 was able to decrease inoculated and naturally present E. coli to ≤ 2 log CFU/g after 15, 1 and 90 days in Karish, Domiati and Ras, respectively. Interestingly, S. aureus phage cocktail showed a high ability to reduce S. aureus (inoculated and indigenous) and Staphylococcus spp. (indigenous) below the detection limit (≤ 10 CFU/g) after 1, 1 and 60 days in Karish, Domiati and Ras, respectively. This research shows that selected phages can be applied as effective biocontrol agents against E. coli and S. aureus in raw milk cheeses without affecting the cheese fermentation process.
| Item Type: | Thesis (Doctoral) |
|---|---|
| Divisions: | Harper Adams Business School |
| Depositing User: | Mrs Terésa-Marie Brown |
| Date Deposited: | 14 Nov 2024 16:46 |
| Last Modified: | 14 Nov 2024 16:46 |
| URI: | https://hau.repository.guildhe.ac.uk/id/eprint/18143 |
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