Reblogging all these CLS things is making me want to talk about my unknowns so far this semester.
So. In my bacteriology class, we have... 6 or 7 unknowns this semester? Something like that.
The first one covered Staphylococcus and Micrococcus species. My lab partner and I were handed our unknown plate and when I turned it over, I was 95% sure I knew what it was. Very yellow colonies = Micrococcus. We ran a catalase test, a microdase test, and a bacitracin (taxo A) susceptibility test, as well as Gram staining it. There might have been another test but I don't have the unknown sheet and can't remember. Basically, that one was easy.
The second one covered Streptococcal species, including Viridans Strep. Our plate had no hemolysis, and the Gram stain revealed clusters, so I started assuming an Enterococcus species. I set a bile esculin plate and inoculated a salt broth. Then I looked at the colony morphology we had seen in the lab where we tested the Streps, and what was on the plate didn't match up with Enterococcus, but S. agalactiae did. I also remembered that the S. agalactiae strain we had been using wasn't beta hemolytic for some reason (it should have been beta), so I decided to make a CAMP plate and do the hippurate hydrolysis test. By the time I put everything into the incubator, I was fairly sure we had S. agalactiae. And the results confirmed it. That one was also pretty easy.
This week we were assigned the third unknown, and it covered the Gram positive rods we had seen, like Listeria monocytogenes, Corynebacterium diphetheriae, Bacillus, etc. The plates we got were chocolate this time instead of blood, which was meant to make it harder for us, I guess. Anyway, the colonies looked familiar, but I couldn't remember which organism it was. We subcultured to blood, and I did the Gram stain. I was seeing a mix of positive and negative rods, and some even looked like cocci in chains. I called the professor over and she said I decolorized it a little too long, but that it should be G+ rods. So my partner and I were trying to figure out what it was. I considered Bacillus, but when I checked the blood plate, it wasn't characteristic for either Bacillus species we would have been given. Basically it came down to L. monocytogenes and C. diphtheriae. I did a nitrate test, which came out negative for reducing nitrate to nitrite. Then I streaked a Tinsdale agar plate, thinking it could be C. diphtheriae. I got growth, but there were no halos characteristic of C. diphtheriae. I had done two different motility tubes, testing for Listeria. The one we put in the incubator had great motility, but the one we left out at room temperature didn't show umbrella motility when I checked it, which made me doubt the L. monocytogenes assumption. The bile esculin test and the glucose fermentation test I did came back positive (which were also indicative of L. monocytogenes). Finally, we did a CAMP test and a hippurate hydrolysis test, and both of those came back indicating L. monocytogenes. I guess this strain didn't reduce nitrate for some reason.
This last one was kind of a doozy, lol.
