Oh Salmonella...

My lab assignment for this week is to attempt to culture Salmonella from a fecal sample. I don't know whether or not their actually is Salmonella in the sample, and I probably won't until about Thursday, because apparently Salmonella is very difficult to culture.

Now, you may be asking why I would care about culturing Salmonella. And besides the obvious that I am getting graded on this and it will most likely be a large part of my next lab exam, there is also the fact that everyone reading this has heard of Salmonella. If I decided to talk about culturing Actinobacillus pleuropneumoniae you really wouldn't know what I was talking about. But Salmonella is a very important zoonotic disease; it's why we're told never to eat raw eggs, because chickens can pass Salmonella through their eggs. Salmonella is a common cause of enteritis in humans, which is essentially disease in your intestines, and it is actually the most common food borne zoonosis that causes mortality.

So I guess you could say that Salmonella is a pretty big deal in the infectious diseases world.

Salmonella is a pain to culture because of a couple reasons. One, just because you might have the disease doesn't mean you're actively shedding it. Salmonella sheds intermittently, so a negative culture doesn't guarantee that you're actually negative for Salmonella. You have to have at least 5 negative cultures, each one 48 hours apart, before you can say with 95% certainty that you might be negative for Salmonella. Of course, if it's positive, then you're positive.

Salmonella also takes a really long time to grow. We try to help it out by using a special medium called a Selenite broth, which is a type of enrichment broth. Basically it increases your chances of finding Salmonella present in small numbers by suppressing the growth of other bacteria (Gram+) and enhancing the growth of Salmonella. And then even after you do all this culturing, you only have Salmonella suspects. You have to do additional testing to confirm that what you are seeing is actually Salmonella and not just another Enterobacteriaceae, like E. coli. We do this using slant mediums to test for color changes in the medium to see if it is a fermenting or non-fermenting colony (Salmonella is non-fermenting). Each step of the incubation process takes at least a day, since the plates all need to incubate overnight. And that is why if your vet thinks your animal has Salmonella and wants to culture it, you probably won't hear back for about a week.

These are some positive Salmonella cultures.

Notice how the Salmonella turned black in the one culture? And on the right side it is a pink color on an agar that started out more yellowish-green.

Color change in a slant tube test