Here we are back in the laboratory with your pet’s doctor. Today we’ll be discussing the test called bacterial culture and sensitivity.As the name implies, it’s a two-part test: The culture part of the test tells you that something is going to be grown. In fact, laboratory technicians will take the specimen the doctor sends in and attempt to grow any and all bacteria present in the specimen.
“Now, how in the world do they do that?”, you might ask. It is truly part of the best testing that modern diagnostics has to offer.
Let’s assume that the sample we’re trying to grow bacteria from is a urine specimen. The doctor collects the specimen from the patient in a sterile fashion, meaning he does his best not to allow any contamination from other parts of the patient’s body, from his own hands or from surfaces such as the table the patient is sitting on. A urine specimen is usually collected into a sealed tube or bottle.
When the container arrives at the lab, the lab technician is also careful not to contaminate the specimen as she opens it and transfers it to a culturing device, a tube divided into several sections which contain nutrients for bacteria to grow in. Down through the middle of the tube the sample is introduced, inoculating each section. Different types of bacteria will grow in each different section and what-grows-where, and the effect the growing bacteria have on the medium in which they’re growing actually tells the technician what kind of bacteria is causing the patient’s infection!
All bacteria have names, so when the test is finished, the technician can call your veterinarian and tell him what the name of the infection is, and, almost as important, whether there are a lot or a few of them.
At this point, the test is only half finished. The sensitivity portion of the test remains to be performed. But, if the doctor has begun a medication for infection of the urinary tract, and now, armed with the information that the infection is being caused by such-and-such bacteria, he can change the medication if necessary while he awaits the sensitivity test.
The first step in the sensitivity test is to obtain some of the bacteria that grew in the culture. Now, feed them well, put a light on them, keep them warm and allow them to grow again.
Now that she has a good stock of the bacteria causing the infection in our patient, the technician will subject those bacteria to antibiotics to try to kill them. All of this is done in a single Petri dish with antibiotic sensitivity discs. These are small pieces of special paper, about the size your hole puncher makes, saturated with a known amount of antibiotic. Ten or so discs are placed in each Petri dish. The choice of antibiotic discs is based on previous knowledge of what antibiotics usually kill the bacteria identified in the culture step.
In a few days, the technician will examine the dish for zones of inhibition. The size of the area of dead bacteria around the antibiotic disc tells us which antibiotics are working and how well. Now, a report is generated and faxed to the clinician so that he can choose an antibiotic known to kill the bacterial infection in his patient.
What good is this? Before bacterial culture and sensitivity, all a doctor could do for a known infection is say, “Let’s try this medicine and see if it works.” If it didn’t he tried another medicine, hoping it would work. If it didn’t, he kept trying medicines until he hit on the right one.
Wow, sounds like a perfect test, doesn’t it? Are there any drawbacks? Well, yes.
One, it’s labor-intensive and uses high technology devices. That means it isn’t cheap. And if the culture yields two or more types of bacteria, there is a charge for each sensitivity that follows the identification. Ear infections may grow three or more organisms, and that extra cost adds up.
Two, sometimes your pet’s doctor takes a specimen, handles it carefully, sends it to the lab and it doesn’t grow. Some bacteria are just so fastidious that they won’t grow in an artificial environment, but that doesn’t keep them from growing in your pet’s body! Likewise, urine cultures in our feline patients can be frustrating because of a natural ingredient in cats’ urine that prevents bacterial growth. Again, it may not keep the infection from growing in your pet, but the infection may die at the lab before it can be identified.
It is also important to understand that a specimen for bacterial culture and sensitivity is different from a biopsy specimen and testing. MMCULT