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Taken from newsletter of summer 2004
Culture and sensitivity testing
for bacterial infections
At some point, your rabbit may develop an illness or
infection for which you will need to
consult your veterinarian for a culture and sensitivity test. Culture
and sensitivity testing is performed if your veterinarian suspects a serious
bacteria infection. The signs of such an infection may be an unrelenting
sneeze, an infected abscess, or a recurring cough. Your veterinarian may
also perform this test if a bacteria infection has not been resolved in
response to initial antibiotic therapy. The goal of this article is to
shed a little light on what a bacteria infection is, how it is diagnosed
and treated by your veterinarian, and the role played by the laboratory
technicians who assess the bacteria.
Determining Antibiotic Sensitivity
and Resistance
Your veterinarian may use a swab to collect a sample (for example, from
nasal or
ocular secretions/discharge) or may collect other samples (for example,
a urine sample
or discharge from a skin abscess). The sample is then submitted to a bacteriology
laboratory for incubation in an attempt to grow the bacteria contained
in the sample. Once growth is achieved, the bacteria are isolated using
a variety of culture media and growth conditions that allow the laboratory
to identify the organisms in the sample. Individual bacteria are then
exposed to different antibiotics applied directly to the culture
plate to determine antibiotic resistance. The laboratory assesses the
bacteria's ability to grow in the presence of these antibiotics and determines
a profile of antibiotic sensitivity or resistance. This information is
submitted to your veterinarian and is used to determine the appropriate
antibiotic to fight the bacterial infection.
Interestingly, the antibiotic testing that occurs at
the laboratory uses a standardized battery of antibacterial drugs (although
your veterinarian can request that the organisms be challenged with a
specific antibacterial drug). These antibacterial drugs can include drugs
that may not be appropriate for use in rabbits. It is your veterinarian's
responsibility to make the most clinically appropriate antibiotic choice
for your rabbit. Your veterinarian must consider species-specific concerns,
the bacteria's sensitivity profile, and the overall clinical picture (for
example, the site or organ that is infected,
pharmacology of drug options, and several other concerns).
Aerobic and Anaerobic Bacteria
There are two types of bacteria that can affect your rabbit: aerobic and
anaerobic bacteria. Aerobic bacteria are bacteria that require oxygen
to grow and thrive, while
anaerobic bacteria are able to grow and thrive without oxygen. Some bacteria
are able to grow under either aerobic or anaerobic conditions (facultative
anaerobes) while others cannot thrive in the presence of oxygen (obligate
anaerobes). If your veterinarian suspects that anaerobic bacteria are
present, samples are submitted in both aerobic and special anaerobic transport
media. Culture conditions at the lab will then ensure the specific aerobic
or anaerobic conditions are maintained throughout the culture process.
Pathogen or Normal Flora?
Certain normal bacteria are part of a healthy rabbit's biological makeup.
As part of an antibiotic treatment, your veterinarian and the bacteriology
laboratory will try to distinguish between true pathogens and the normal
flora as part of the decision process.
This will help minimize the suppression of your rabbit's normal flora
and maximize the
impact on the pathogen.
The laboratory will often isolate several organisms from
one culture, which will be used to determine whether the suspected organism
is a pathogen or part of your rabbit's normal flora. Determining which
organisms represent pathogens, contaminants, or normal flora is a complex
task and is often a matter of experience on the part of your veterinarian.
Reference materials do exist to assist your veterinarian in making such
judgments, and many veterinary bacteriology labs often employ veterinary
microbiologists who can assist in making such determinations should questions
arise in response to a culture profile.
Bacteria Resistance
Throughout this article we have explored how your veterinarian determines
whether a bacterial infection is present in your rabbit and determines
the appropriate treatment options. It is important to keep in mind that
despite determining a bacteria's sensitivity, treatment may not always
be effective in resolving the infection due to certain key factors. It
is essential that the antibiotic reach the infection site and that adequate
blood circulation be present at the site. This allows adequate penetration
and distribution of the antibiotic. When blood circulation to the site
is inadequate, and depending on the conditions of the site, antibiotic
treatment may not always be successful.
While the biochemical mechanisms that allow bacteria
to develop antibiotic resistance are many, inappropriate antibiotic use
(for example, frequency of administration, dose, and duration) is the
biggest issue that causes resistance. This fact alone stresses the importance
of working with a knowledgeable veterinarian you trust, who takes the
time
to explain your rabbit's condition and the treatment plan.
The information contained in this article was provided
by Dr. Keith Johnson from the
Carling Animal Hospital in Ottawa, Ontario
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