Culture and Sensitivity Testing

by Dana Krempels, Ph.D.
University of Miami Biology Department
House Rabbit Society of Miami

If your bunny has an infection of any kind--from an upper respiratory infection, to a jaw abscess to a urinary tract infection--it's critical to know which antibiotics will be effective against the particular pathogen (i.e., disease-causing agent) causing the problem. This means that (1) the species (and strain) of bacteria (or other pathogen) must be identified and (2) the drugs most effective at inhibiting their growth must be determined. The only reliable way this can be done is a culture and sensitivity test.

In modern laboratories, bacteria are usually identified by characterization of the genome: identifying the characteristics of the DNA and RNA of a sample species. This type of testing is generally considered more reliable (and soon, less expensive) than actually growing bacterial cultures and exposing them to various types of antibiotics to see which drugs kill or inhibit the bacterial growth. But if more than identification is required, and if an antibiotic that usually works against a particular bacterial strain is ineffective, then it may be necessary to actually grow the bacteria and perform an "old fashioned" culture and sensitivity test.

How is a Culture and Sensitivity Test Done?
Your rabbit-experienced vet will take a sample of infected tissue or discharge from the infected area (the capsule of an abscess is the best location from which to take a sample, as the internal pus often contains only dead bacteria that will not grow in culture), and send it in a special culture tube to a licensed laboratory for testing.

In the lab, technicians will spread a sample of the infective material onto a plate of nutrient substance (usually agar, a type of gel made from algae) and allow to grow whatever species of bacteria were in the bunny's infected area.

With a sufficient population of bacteria grown on the plate in the form of a "lawn", the technicians will perform two main operations:

The Petri dish in the image above (shamelessly borrowed from the University of Wisconsin at Madison online Textbook of Bacteriology, which includes a more detailed explanation of the appearance of the halos used in bacterial identification), shows bacteria being strongly and moderately inhibited by most of the antibiotics (impregnated on circles of filter paper), but unaffected by the antibiotics on the disks located at 5 o'clock and 9 o'clock on the dish.

In three to seven days after the sample is taken, your vet will receive the results from the lab, including the species of bacteria and the range of antibiotics to which the bacteria are sensitive (S), resistant (R) and intermediate (I). Again, "sensitive" means that the bacteria were inhibited or killed by that particular antibiotic, and this is what you want to hear.

Choosing and Using the Appropriate Antibiotic
Note that not all antibiotics are safe for rabbits! Any oral penicillins (e.g., amoxycillin, ampicillin, penicillin) or lincosamines (e.g., clindamycin) should be avoided, as they can cause fatal cecal dysbiosis by killing normal, beneficial intestinal microorganisms and allowing dangerous ones to proliferate. Be sure your bunny is seen by a veterinarian who is familiar with the special medical needs of rabbits. If you don't already have such a vet, you can find one via the list linked at the House Rabbit Society's Veterinary Referral Page.

Commonly used antibiotics that are safe for rabbits include the fluoroquinolones (e.g., Baytril and ciprofloxacin), sulfas of various types, chloramphenicol, aminoglycosides (e.g., gentamycin, tobramycin, amikacin--though these are not a first choice as they can be toxic to the kidneys), and injectible Penicillin-G Procaine. It's critical that the appropriate rabbit-safe antibiotic for the particular infection be prescribed and administered for a course long enough to allow the bunny's immune system to conquer the infection (with a little bit of help from the antibiotics).

It can take several weeks of antibiotics (sometimes a combination of two different ones!) to get the problem under control. Don't delay having your bunny properly diagnosed and treated. Almost any infection can develop into a much worse problem if left to its own devices.

It is also extremely important to

If you stop treatment early, or give too low a dose, you will risk breeding resistant strains of bacteria by killing off only those most sensitive to the drug(s) you are using, and leaving only the more resistant individuals behind to be the progenitors of the next generation, and to share their genetic resistance with the sensitive members of the bacterial population. It's not hard to see that misuse of antibiotics can cause real problems.

Why Bother with a Culture and Sensitivity Test?
One cautionary note. Some veterinarians who are not as experienced with rabbits as they are with cats and dogs will take one look at a rabbit with "snuffles" or other infection and proclaim that the problem is caused by Pasteruella multocida. Although this bacterial species is not uncommonly carried by rabbits, please do not let anyone convince you that your rabbit's problem is caused by Pasteurella unless that diagnosis is confirmed via culture and sensitivity test! Not only are some strains of Pasteurella resistant to commonly prescribed antibiotics such as Trimethoprim sulfa, Baytril (enrofloxacin) and even ciprofloxacin, but infections in rabbits also can be caused by even more resilient strains of bacteria, such as Pseudomonas aeruginosa, Bordetella bronchiseptica, Staphylococcus aureus, and others. Without a culture and sensitivity test to positively I.D. the pathogen, you could not only delay your rabbit's return to good health, but also be throwing good money after bad by treating with an antibiotic that is not effective against the particular strain of bacteria your bunny has.

If no bacteria grow at all, then it's possible that the bunny has a fungal infection. If this is the case, antibiotics will likely make the problem worse, not better. Hence, it's very important to check for fungal species if the culture and sensitivity test comes back negative for bacteria. Completely different medications are needed to control infections caused by fungi.

Followup: Backtracking to the Cause
Once an infection is under control, it's wise to do a bit of detective work and seek possible causes, especially if the condition is chronic. For example, runny eyes and nose or jaw abscesses can be caused by dental problems such as molar spurs or molar roots extending into the sinuses. This is much more common in older rabbits, but all rabbits should routinely have their molars checked for spurs, which are not only painful, but potentially dangerous. A tear duct flush will sometimes temporarily stop runny eye problems, but ultimately it is best to do a complete check for molar problems including visual inspection for spurs and even radiographs to detect molar root infections.

Good care, healthy diet, a happy, calm environment, and your constant vigilance for problems are your bunny's best bets for a long, healthy, infection-free life. But when even those things fail, it's good to know there are medications that can help, as long as they're used wisely, appropriately, and always under the supervision of a good, rabbit-savvy veterinarian.

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