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:

1. IDENTIFY THE SPECIES OF BACTERIA.
• This is done with various techniques, including examination of lawn characteristics (color, texture, growth pattern, etc.) gram-staining, microscopic examination, metabolic requirement "footprints" and even DNA sequencing.
  
• Bacterial species commonly isolated from rabbit infections include Pasteurella multocida, Pseudomonas aeruginosa, Bordetella bronchiseptica, Staphylococcus aureus, and several others, though just about anything might turn up, depending on the location and cause of the infection.

2. DETERMINE THE BACTERIAL POPULATIONS SENSITIVITY TO A RANGE OF ANTIBIOTICS.

• This can be done by placing small disks of filter paper or agar impregnated with various types of antibiotics onto the bacterial lawn. The bacteria are allowed to incubate for a day or two, and then the plate is examined to see whether the bacterial growth is inhibited (or not) by the antibiotics on each disk.

  • SENSITIVE: In this case, a clear, circular "halo" (technically known as a "plaque," or zone of inhibition) will appear around the antibiotic disk, indicating an absence of bacteria. The antibiotic has inhibited their growth and/or killed them, meaning that this particular antibiotic should be effective against the infection your rabbit has.

  • INTERMEDIATE: A somewhat cloudy plaque indicates that not all the bacteria in the area around the disk have been killed. This means that there are some members of the bacterial population that are sensitive to this particular antibiotic, but others that are genetically immune to its effects. If an antibiotic to which the bacteria show "intermediate" sensitivity is used, it is likely that the sensitive members of the bacterial population will be killed, and the resistant ones will survive, resulting in the selection of a population resistant to that particular antibiotic.

  • RESISTANT: In this case, the filter paper will have no discernable plaque around it, meaning that the bacteria are growing normally, even in the presence of the antibiotic. An antibiotic producing no plaque will most likely be ineffective against the bacteria causing your bunny's infection.

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

• give the full dose
• not miss any doses
• and continue the treatment for the full time period your vet prescribes, even if the symptoms subside.

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.