mjl1717
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the oxidase+, non lactose fermenter grows on EMB or MacConkey
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| bactitech
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Yes, it does, assuming you're speaking of Pseudomonas aeruginosa. Technically, oxidase testing should be performed on a non-selective medium like Blood agar or chocolate. It is a large spready colony, which is sticky and is supposed to smell like grapes. It has a distinctive odor, but I can't say that grapes is what it smells like. It also can have a metallic sheen. It is the only organism that produces green pigment. I have seen some treated Pseudos that produce a reddish pigment, however.
I would say that the majority of micro labs use MacConkey agar instead of EMB agar nowadays, although there are a few holdouts.
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| malinda
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EMB or macConkeys test what?
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| bactitech
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These media are selective for gram negative rods. MacConkey's is a slightly purple/pink agar. Lactose positive organisms present as purple to pink organisms; lactose negatives are clear. The high bile salt content suppresses the growth of gram positive organisms. It is a basic agar used in the setup of virtually all specimens in most labs. EMB works similarly, but there is a different formulation. E. coli presents on this agar with a distinctive metallic sheen. I don't know if there are many advantages of using it over MacConkey - all labs I've worked in have always used Mac. We used to use EMB back in the 70's as an agar to subculture yeasts onto to look for distinctive hyphae of C. albicans. Some labs still do this, I believe, although I haven't used this methodology since the 70's.
When we report out preliminary reports, we will report as "moderate growth lactose fermenting gram negative rod" or "heavy growth non-lactose fermenting gram negative rod" if we're confident of the lactose reaction. Some bacteria don't give a clear-cut lactose reaction at day 1, so I always hedge on these and just call them gram negative rods until the ID is finished. Acinetobacter baumanii is one of these organisms. This organism is tracked in many hospitals because it has become an infection control issue and has become very very resistant. We had a big problem in our ICU unit with this bug until they moved the ICU unit to a new location and it seems to be pretty much gone for now. Apparently they never found the source and had done environmental cultures on a lot of different sites.
If you do any type of ICU medicine, you will see this organism in your reports.
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