Part 1: What happens to the sample you submit to the NDSU-VDL for culture? Once your sample is received it is plated to culture media. The media chosen varies depending on the species and sample source (urine, feces, respiratory, tissue, etc.). The culture is allowed to grow in an incubator (at least) overnight and then our skilled microbiologists begin the task of identifying what has grown and whether or not it is a pathogen (it is the cause of the infection).
Culture media contains nutrients to support bacterial and fungal growth (figures 1, 2). Organisms growing on the plates have characteristic looks (and odors) that aid the microbiologist in identification. Some medias are selective, meaning they inhibit some bacteria and allow other to grow more prominently (figure 3). Media can also be differential, meaning it can show you what some bacteria can use to grow (sugars, for example) that others cannot, usually by a change in pH that makes the bacteria grow a certain color. MacConkey’s for example (figure 4), is selective because it allows only Gram-negative bacteria to grow and differential because bacteria that use lactose to grow are pink while others appear clear if they do not use lactose.
A lot of information about the sample can be gathered before the culture has even had a chance to grow. A Gram stain of the sample can tell the microbiologist whether there were white blood cells present, how well the sample was collected, and if there are bacteria present, are they in pure culture or are they mixed with other kinds of bacteria.
Microbiologists can use traditional methods to identify the bacteria and fungus they isolate, like which sugars the organism uses (figure 5) or by rapidly testing if the bacteria can produce certain characteristic enzymes or end products when exposed to certain chemicals. However, other methods are now available to greatly speed up the process and add more certainty to the identification of unusual organisms. More to follow….
Kelli Maddock, MLS (ASCP)CM
