StockCISSN
Forum Newbie
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Hello. I need some help with my unknown organism for my Microbiology class. I think I have Salmonella. I wasn't able to do all tests and they were incubated for only 24 hrs. Here is what I know for sure:
Gram -, Sugar Fermentation's Lactose was -, Citrate+, motile.
Why I'm not sure...remember 24 hr incubation:
Gelatin testing turned soild in 10 minutes, Methyl Red -, Hs2- from TSI test.
Also worth noting: Organism was green in test tube before plate streaking.
HELP!
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| bactitech
Forum Elite
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Posts: 484
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If the H2S was negative on TSI chances are it's not Salmonella. H2S negative Salmonellas are known to exist, but I've never seen one in 30 years. Was the TSI K/A with H2S? That's what Salmonella should be. Also, as I try to recall my ImVic reactions, if it was MR neg, it should be VP pos. I don't think Salmonella's are VP pos.
If you have a NLF (nonlactose fermenter) the VERY FIRST TEST you should do on it, from a nonselective medium like blood agar (fresh overnight growth, please), is OXIDASE. If it is oxidase positive, chances are you are NOT dealing with enterobacteriaceae. If you were provided this organism on a slant like Nutrient or BHI without blood, AND it was green, there is only ONE organism that produces green pigment (pyocyanin). Your bug should be oxidase positive and it is Ps. aeruginosa. Your TSI will be K/K (no acid or H2S produced). It is motile and citrate positive.
You should read all your reactions at 24 hours for fermenters, and 48 hours AT THE MOST for nonfermenters. Anything that needs reagents added (indole, VP, MR, etc.) needs to be done after 24 hours incubation. You need fresh readings for proper identification. If Micro techs worldwide didn't read reactions at the proper intervals, nothing would get identified correctly. You also need to streak out your bug and get fresh growth before you proceed to perform your ID tests.
It is important that you inoculate a battery of media from a fresh plate. Do your oxidase and your homework. I doubt that they would have you messing with Salmonella in a student laboratory.
When you sub your unknown from your original slant, make sure you streak your plates for isolation. Without proper isolation you won't know if your isolate is pure or mixed. When in doubt, always deal with a ONE COLONY SUBCULTURE. Many identification problems can be solved once you realize you've been working with a mixed culture the whole time. One colony subcultures avoid this problem. When we have messy cultures at work, the first thing we do is pick one colony and isolate to a fresh plate. This takes an extra day but you must do this in order to get enough pure growth to perform identification tests correctly.
Good luck.
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Clinical Microbiology since 1974
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