Thread: Microbiology Unknown Assistance

I'm doing an assignment of unknowns. Outbreak of patients with abdominal cramping and diarrhea. We were given pictures of unknowns and we have to diagnose the condition. I have put my answers under the description of the unknown and I would just like some confirmation.

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You sure that's chocolate agar and not a standard blood agar? It looks a little too red.

Picture 1, the spore stain: I agree no spores.

Picture 2, MacConkey w/ sorbitol: definitely negative, the bacteria does not ferment sorbitol.

Picture 3, the gram stain: yes they are gram-negative, (No Red Commies/Positively Blue), and they are rod-shape, don't say bacillus shaped because that causes confusion with the bacillus genus.

Picture 4: You are right again, they are lactose fermenting.

Picture 5: Well they grow on chocolate agar, but that looks very red for chocolate agar, you sure it isn't a non-lysed blood agar? If it happens to be a blood agar, your bacterium is gamma-hemolytic.

Right now it looks like you've got some member of Enterobacteriaceae.

Because it is gram negative you can rule out common intestinal pathogens in the Clostridium and Bacillus genera.

Make a chart of what common pathogens can do what biochemical processes, I imagine your instructors insured you conducted all the test needed to narrow down your bug.

Thanks alot for your help. But, quick question: How would the MacConkey look if it did ferment sorbitol? I have been trying to look all over for it but I can't find it. The only thing close I can find is that the agar turns yellowish... Seems like this one has. Obviously I am missing something. And about the chocolate agar, that is how it was presented to us, not as a blood agar.

Macconkey agar has a pH indicator in it that turns red when there is acid present. If the bacteria ferment the sorbitol they should produce acid and the agar should have changed colour. Usually you would see this in the colonies themselves, and in the agar around the colonies. (It's also selective for gram negs, so it confirms your gram stain) I'm not sure how good your picture is, usually you don't want to plate the entire thing, but half so that you can confirm a colour change. Yours looks light pink, but that is normal for MacConkey Agar, it should be strongly red if acidic.

Chocolate agar can be that bright red, but usually a thicker mixture is used and it is kind of brownish (thus its name). Anyway, your bacteria grows on chocolate agar, which doesn't tell you much, since it is a medium for growing fastidious pathogens.

This is actually very useful.

The one is picture 5 comes up all the time in assignments here. Is it 100% blood agar? I always get this one mixed up.

If you have any more assignments questions please do post It really helps with revision. Really worried about upcoming one.

Thanks Guys.