Thank you so much Dr.Holden for the lecture ! 6:50 You mentioned that the black dots present in the salmonella colonies grown on the (XLD) agar are due to the production of hydrogen sulfide " H2S " . Actually hydrogen sulfide " H2S " is a colorless gas with an odor of " rotten eggs " ; which does NOT give that characteristic black dot appearance to the salmonella colonies . That black dot appearance of the colonies is rather due to the precipitation of ferrous sulfide salt " FeS " & that is produced by the reaction between (ferric ammonium citrate) & (sodium thiosulfate ) in what is called " H2S indicator system " . The switch in pH levels ; from acidic pH due to lactic acid production from xylose fermentation to basic pH due to cadaverine production from lysine decarboxylation catalyzes the reactions of the " H2S indicator system " ===> FeS production . 8:00 Finally coliform bacteria favor lactose & sucrose on xylose NOT lysine as an energy source to produce lactic acid as a byproduct .
@@Sandysan11111 According to what Dr. Holden mentioned ; Sucrose & lactose COMPETE with lysine to be taken up by coliform bacteria so that coliforms do NOT decarboxylate lysine ==> preventing them from producing false positive results ! Lysine per se is an amino acid ! The question here is : does lysine use carbohydrate carriers to enter coliform bacteria ?
If salmonella can use lysine then why even have xylose as an ingredient in the first place? Wouldn't lactose, sucrose, lysine and H2S system be enough?
@@vikrambhaskar7976 Salmonella seems to be unique among enterobacteriaceae in utilizing XYLOSE as an energy source , as well as possessing the capacity to decarboxylate LYSINE ! XLD agar is a kind of selective media that selects the growth of a large population of different Gram -ve bacterial species including : Coliform & Non - Coliform bacteria ! Coliforms are of course lactose fermenting bacteria that preferably utilize LACTOSE as an energy source , in contrast Non - coliforms do NOT utilize lactose as an energy source ! Finally Coliforms do NOT necessarily decarboxylate LYSINE as effectively as Salmonella .
Dr Joshua Remedy right now that l have my PHD in biophysics, l can agree with him that seamoss has all the essential elements needed by the humau body, a perfect balance 💕💕
Hot professor strikes again with a super video! I used XLD as a basic trainee biomed without fully knowing the ins and outs of it. Be great to have a whole set of videos based on the agar plates and their uses if at all possible?
Thank you so much Dr.Holden for the lecture !
6:50 You mentioned that the black dots present in the salmonella colonies grown on the (XLD) agar are due to the production of hydrogen sulfide " H2S " .
Actually hydrogen sulfide " H2S " is a colorless gas with an odor of " rotten eggs " ; which does NOT give that characteristic black dot appearance to the salmonella colonies .
That black dot appearance of the colonies is rather due to the precipitation of ferrous sulfide salt " FeS " & that is produced by the reaction between (ferric ammonium citrate) & (sodium thiosulfate ) in what is called " H2S indicator system " .
The switch in pH levels ; from acidic pH due to lactic acid production from xylose fermentation to basic pH due to cadaverine production from lysine decarboxylation catalyzes the reactions of the " H2S indicator system " ===> FeS production .
8:00 Finally coliform bacteria favor lactose & sucrose on xylose NOT lysine as an energy source to produce lactic acid as a byproduct .
why is the reason for the false positive if we do not use lactose and sucrose?
@@Sandysan11111 According to what Dr. Holden mentioned ; Sucrose & lactose COMPETE with lysine to be taken up by coliform bacteria so that coliforms do NOT decarboxylate lysine ==> preventing them from producing false positive results !
Lysine per se is an amino acid ! The question here is : does lysine use carbohydrate carriers to enter coliform bacteria ?
If salmonella can use lysine then why even have xylose as an ingredient in the first place? Wouldn't lactose, sucrose, lysine and H2S system be enough?
@@vikrambhaskar7976 Salmonella seems to be unique among enterobacteriaceae in utilizing XYLOSE as an energy source , as well as possessing the capacity to decarboxylate LYSINE !
XLD agar is a kind of selective media that selects the growth of a large population of different Gram -ve bacterial species including : Coliform & Non - Coliform bacteria !
Coliforms are of course lactose fermenting bacteria that preferably utilize LACTOSE as an energy source , in contrast Non - coliforms do NOT utilize lactose as an energy source !
Finally Coliforms do NOT necessarily decarboxylate LYSINE as effectively as Salmonella .
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Dr Joshua Remedy right now that l have my PHD in biophysics, l can agree with him that seamoss has all the essential elements needed by the humau body, a perfect balance 💕💕
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Clear and understanding video thanks keep it up
Pls make videos on brilliance salmonella agar base
excellent video. thank you!!
Thanks Dr. Holden.
Hot professor strikes again with a super video!
I used XLD as a basic trainee biomed without fully knowing the ins and outs of it.
Be great to have a whole set of videos based on the agar plates and their uses if at all possible?
Something like this? ;) ruclips.net/p/PLjMo-ys5HQIZQ5EYCtqN9abio8LiSwoWl
Even more Micro agars and diagnostic tests here: ruclips.net/p/PLjMo-ys5HQIaVpj8Ohn6jO7-8gm_remoj
@@BiologyProfessor thank you!
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