Page 256 - Zoo Animal Learning and Training

240 Tasks for the Veterinary Assistant

the surface of the agar. Each disk is coded with the anti-
biotic it contains. Compare the code on the disk with the
code on the tube to determine the name of the antibi-
otic it contains. The disks are pressed into the agar with
a flamed loop or sterile swab. The dish is inverted,
labeled, and incubated.
The antibiotic seeps out into the agar surrounding
the disk. After incubation, the plate is held to a bright
light and zones of inhibition of bacterial growth are
noted and measured around each disk. If the bacteria
are sensitive to the antibiotic, they will not grow up to the
disk. The diameter sizes of the zones are compared with
the manufacturer’s standardized chart to determine
which antibiotic is the best. Some bacteria will be resis-
FIGURE 12.22 Quadrant method of steaking a blood agar plate for tant to the antibiotics and will grow right up to the disk.
isolating bacteria colonies.
Record the antibiotic zone sensitivities in the patient’s
file and alert the technician or veterinarian that the sen-
result, identification procedures using selected media sitivity results are in. Culture media has to be autoclaved
will be started. before being discarded. When a Petri dish is no longer
needed place it in an autoclavable bag. Seal the bag with
Antibiotic Sensitivity Testing autoclave tape and run it through a general autoclave
cycle to destroy all of the bacteria. Then it can be dis-
Not all antibiotics work equally well on all organisms. posed of in the regular garbage.
Therefore it is advantageous to test the bacteria isolated
from the patient with different antibiotics to determine
the best one to use. The efficacy of an antibiotic depends
on the sensitivity or susceptibility of the pathogen to it. Learning Exercise
Isolated colonies of interest from the blood agar plate If available look at the antibiotic disk dispenser.
are selected and tested for antibiotic sensitivity. Write down the types of antibiotics available, then
Identification of the bacteria should proceed at the same look them up on the internet.
time. The key to successful testing is to choose the most
likely antibiotics to test because there are so many avail-
able. The answer begins with the Gram stain. Some anti-
biotics are effective against Gram‐negative rods, others
against Gram‐positive cocci, and so on. The veterinarian Necropsy: Preparation
will determine which antibiotic disks to use for testing
after having examined the Gram stain. and Follow‐Up
First, however, the bacteria need to be grown in a tube
of broth media. A few colonies of the bacteria are trans- Necropsy is the term used to describe the examination of
ferred into a tube of broth media and the tube is incu- an animal’s body after death. It is conducted in the same
bated until its turbidity matches the turbidity of a manner as a human autopsy. The purposes are to aid the
standard. (Alternate method: if sufficient colonies are veterinarian in making a more complete diagnosis and
present, prepare the broth by adding enough bacteria to to further professional knowledge. Necropsy is per-
match the turbidity of the standard.) A sterile swab is formed as soon as possible after death or euthanasia
dipped into the tube and spread evenly over the entire providing the owner has given permission to do so. A
surface of Mueller Hinton agar, this is a special media delay in the necropsy procedure may result in
used for sensitivity testing. Some organisms like strepto- postmortem autolysis, resulting in inconclusive results.
cocci (Gram positive, cocci‐shaped bacteria) will not Tissue samples taken during necropsy are sent to a
grow well on Mueller–Hinton agar. For these, Mueller– pathologist in a reference laboratory for microscopic
Hinton with 5% blood must be used, and interpretation examination. The packaging and shipping to the labora-
of results adjusted. tory become the responsibilities of the assistant, as does
Small paper disks in plastic tubes, each saturated with the preparation for the necropsy. Reference laboratories
a single antibiotic, are placed in a disk dispenser. Each usually provide containers for submitting tissue samples.
hole in the dispenser holds one tube. The dispenser If there are to be special requests, contact the laboratory
holds multiple tubes. It fits directly over the Petri dish. before the procedure to determine how the specimens
The lever is pushed releasing one disk of each type onto are to be collected, handled, and shipped.