Bacterial Culture & Sensitivity for Pets

Your dog has had an ear infection for two months and it is not getting better. A bladder infection never seems to completely clear up and your cat has been on four different antibiotics. The veterinarian told you that the chronic drainage from your poodle's eyes is caused by a bacterial infection, but it does not clear up with any of the ointments you have used. Infections in places such as these are common conditions that veterinarians see in their clinic every day. And in most instances, they should not have developed into a chronic, long-term problem.

Whether you are talking about oral or injectable antibiotics, antibacterial ear or eye preparations, or the various salves and ointments, veterinarians of today have a wide range of products from which to choose. On the shelf in our clinic, we routinely have at least 9 different oral or injectable antibacterial preparations and we can prescribe many more through a pharmacy. With this vast array, you could assume that we would have eliminated every bacterial species from the face of the earth. However, that is not the case. The 'germs' are still winning, both in types and total numbers.

Resistance to antibiotics

Several problems can occur in the treatment of bacterial diseases that may cause them to turn into chronic, long-term disorders. Probably the most common reason is that the bacteria causing the problem (be it an ear, eye, bladder, or other infection) is not affected by the antibiotic being used. That is, they are not sensitive to (killed by) the medication. There are many different antibiotics available for use in human and veterinary medicine. Although there are similarities between them, most have different mechanisms of action, which determines exactly how they kill or destroy bacteria. Simply stated, if a bacterial species is not affected by the particular mechanism of the antibiotic used, it will not be killed, inhibited, or affected at all. It is then said to be resistant to the medication. There is no all-inclusive, broad-spectrum preparation that kills every kind of bacteria. Each one affects a few different types of bacteria, but none is effective against them all.

The second - and related - problem leading to chronicity or recurrence of infections is that during the course of treatment, bacteria have the ability to mutate, or change, and in doing so develop a resistance to the medication. For the first few days or weeks that an antibiotic is used, it may do an excellent job at eliminating the organisms present. The patient starts to return to normal health and we feel we are well on the way to a complete cure. However, after this initial stage, a portion of the bacteria may change and alter their structure or physiology, so that they are no longer affected by the particular mechanism of action utilized by the medication. With these newly altered properties, the disease-causing organisms increase in number and the signs of the disease return. This is what is meant by 'acquired' resistance.

Some microbiologists feel that this is not exactly the way it happens. They believe that in the millions of the same kind of bacterial organisms present during an infection, some may not be exactly alike. All of the people living in an American city are the same species, but genetically, they vary greatly from individual to individual. The same is true within the population of bacteria causing the infection in your pet. These same microbiologists believe that within this number, there are some individual bacteria that already have a natural resistance to the antibiotic being used. As the medication eliminates the rest of the bacteria, those individuals that are not affected will greatly increase in number, and the signs of the disease will return. For practical purposes, however, it makes little difference whether the disease-producing organisms previously had or later gained the ability to function in the presence of the medication. In either case, the results are the same. The patient's illness returns or never goes away.

Antibacterials only kill bacteria

Antibacterials Only Kill Bacteria - NOT viruses, fungi, or yeast.

The third most common reason for the failure of an antibacterial medication is that it is not being used against bacteria. Remember that antibiotics and other antibacterials only affect bacteria they do not affect viruses, yeasts, fungi, or allergies. When dealing with a case of pneumonia, treating it with amoxicillin will do no good, if the pneumonia is caused by the current strain of the flu virus. Putting drops of Gentocin Otic TM in an ear that is sore, red, and draining, will not even slow down a yeast which is a common cause of ear infections. And if fungus has decided to reside in your dog's toenails or bronchi, penicillin will help about as much as something used by the Wicked Witch of the West. And last but not least, when your favorite dog chews on his feet and scratches his sides because of an allergy, do not try to put out the fire with an antibiotic salve because it may just make the whole thing worse. If the primary problem being treated is anything else, do not expect a cure.

Just to keep the record straight, there is nothing wrong with using antibacterials at times when the patient is suffering from something other than a bacterial disease. As a matter of fact, it may be good medicine. Secondary bacterial infections frequently occur when animals or people are afflicted with any of the above disease-causing critters, or weakened due to stress, injury, or shock. Catching the individual in a compromised state, it is easy for bacteria to gain a hold and initiate their own set of problems. Therefore, antibiotics are used to protect the patient and allow them to concentrate on the primary disorder.

What does a 'culture and sensitivity' tell us?

A 'culture and sensitivity' is the name given to the laboratory test that determines what bacteria or yeast may be present in a sample, and what drug will kill that bacteria/yeast. To use a common example, although the same principles apply with any possible infection, let us say we are confronted with a 70-pound Labrador that loves to swim daily and now has an inflamed ear. If we really want to get all the information we would like, the only way is to do a 'culture and sensitivity.' It will tell us:

  1. Exactly which organism is causing the problem.

  2. What the best medication is to use.

When your physician or veterinarian prescribes a medication for an infection without performing a culture or sensitivity, he or she is just making a "best guess" as to the type of infection and proper treatment.

How is a 'culture and sensitivity' performed?

Culture samples


Samples are taken from the infected area with wire loops or cotton swabs that have been sterilized. They are then spread across the surface of a jelly-like medium called agar, which is contained in a petri dish. The latter is nothing more than a glass 3-inch dish with a cover. The agar and petri plate together are often called a 'culture plate.' The organisms grow on the surface of the agar and after 24 hours or so, each organism has multiplied so many times that the large numbers of organisms actually form a small bump or 'colony' on the agar. Each colony came from one organism. Sometimes, there is more than one kind or type of organism present. These can be separated by appearance and then re-inoculated (spread) onto another plate. After a single kind or type has been isolated, it can be tested to determine what type of organism it is.

In our example, we are talking about the discharge from an ear, but it could easily be urine, feces, or material from an infected tissue or wound. We would wipe a sterile cotton swab in the ear, collecting some of the discharge. We would then gently streak (rub) the swab across one or two different kinds of culture plates which contain the agar. The plate is then immediately covered and placed in an incubator that is heated to resemble the conditions of the body. Soon thereafter, probably the following day, the surface of the medium is examined to check for the growth of organisms. If colonies are present, there will be small bumps on the surface of the medium similar to the little plastic dots you find on some gardening gloves. Each type of colony is put through a separate battery of tests to determine exactly what kind of bacteria, yeast, or fungi is present.

Culture and sensitivity with growth


For the sensitivity portion of the test, the organisms will be spread across the plate so the whole plate is covered uniformly. Then small paper discs are dropped on its surface. Each of these discs contains a minute quantity of an antibacterial medication. Usually, several different discs are placed on the medium, therefore testing several different antibiotics. The following day, this plate is checked to see if the presence of any of the antibiotic-containing discs retarded or prevented growth of the organism. Around some of the discs, the bacteria will not be affected and will be seen growing right up against or very close to the disc. Around others, there will be an area in which no organisms have grown. In these examples, the medication has leached out of the disc into the agar medium and has prevented growth in that area. The organism, is therefore, said to be sensitive (versus resistant) to this antibacterial preparation.

How are the results used?

Armed with the results the kind of organism and the list of medications that would be appropriate to use against it the veterinarian determines the method of treatment. Not all antibiotics work equally well in all areas of the body. Some antibiotics, if taken orally, are never absorbed by the gastrointestinal tract and are therefore excreted in the stool. Others are metabolized and broken down before they get to the target area. Some cannot be used topically, as they are irritating. How they are used, either by injection, orally, or topically, also affects where and how well they work. The trick is, therefore, to match where the infection is located with the best method of administration and the best medication for the bacteria present.

Once a course of treatment is initiated with a particular medication, we must still be ready to make a change, if there is no response, or if the effectiveness decreases over a period of time. Ask your veterinarian what to expect and over what time frame it should occur. Any time the treatment your veterinarian prescribed is not doing the job, or the clinical signs seem to be coming back, it is the owner's responsibility to contact their veterinarian immediately.

Article by: Race Foster, DVM

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