Primarily the usage of XLD agar is for the detection of Salmonella species in food analysis. Some sources say that e.coli also can be detected by the presence of yellow colonies. Theoretically yellow colonies observed for the presence of other Enterobacteria which maybe also coliforms. Any suggestion for the testing methods to justify this yellow colonies? Thank you.
Hi, you are correct, mostly XLD Agar is used for detection of Salmonella species (some extent Shigella species included). Not only that, colour colonies development is based on the acid production and enzyme activity in the selective media. As per BIS procedure, other than XLD Agar, Macconkey Agar and EMB Agar also used for detection of E.coli in food samples.while we can defferentiate the coliform also.
I hope this will useful for you and thank you.
Hi, Besides, you may use BGLB(Brilliant green Lactose broth with an inverted durham tube filled with the broth ) first, and if it gives you positive test (broth turns yellow with gas production (gas accumulated within the Durham tube, by replacing the borh, which can be visually observed), then confirm by EMB Agar plate stricking. Good luck.
Hi, Just as you have said that XLD agar is not a selective media for E. coli, if you are able to see the yellow isolates on XLD, they maybe presumptive E. coli, you can plate those yellow isolates on MacConkey agar (appears as pinkish lactose fermenter) or Eosin Methylene Blue agar (appears as bluish green metallic sheen), then you can test them for citrate. Hope this help. Wish you well.
Good day everyone, thanks for the wonderful responses.
Following up on all responses put together, I would like to know about the pre-treatment effects for this particular step. The recommended pretreatment or pre-enrichment for XLD will be from RVS broth incubation. Technically it does say that can help in growth promotion of salmonella as well as e.coli. If we look from EMB agar's selective confirmation, it is recommended to use EC broth as for pre-enrichment. Hence is it compulsory for EC broth to be involved for the analysis involving e.coli and by projection in application of EMB agar?
Hi Ugenthira,
I will advice that you should try and follow the right protocols for the isolations of Salmonella and E. coli. The protocol for Salmonella includes an initial pre-enrichment in buffered peptone water (which is the same for E. coli too). However selective enrichment for salmonella differs from E.coli. Salmonella; selective enrichment is done using modified semi-solid rapparport vassiliadis agar or some people use rapparport vassiliadis broth, then you can culture on XLD, or Brilliant green (BGA) agar, then you purified using nutrient agar. Then you can store the isolates at -80 degree celcius in brucella-broth for further analysis.
For selective isolation of E.coli, from the buffer peptone water pre-enrichment, you have to carry out selective enrichment in EC broth ,then culture on either MacConkey or EMB agar. Then you purified either blood agar or nutrient agar.
I will advise you keep the two protocols separate if you are interested in both E.coli and Salmonella. Cheers
Hi Awosile Babafela,
Thanks for the insightful explanation. Warm regards, cheers.
Most of XLDe Agar is usd for detection of Salmonella species, XLD agar is not a selective media for E. coli, . if you are able to see the yellow isolates on XLD,You can use MacConkey Agar and EMB Agar for detection of E.coli in food
Thanks for the wonderful responses everyone. Appreciate the help. Cheers.
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www.microbiologyinfo.com/xylose-lysine-deoxycholate-xld-agar-pr...
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