Page 253 - The Veterinary Laboratory and Field Manual 3rd Edition

222 Susan C. Cork and Roy Halliwell

for this reason the traditional methods are still
required for most samples submitted for diag-
nostic purposes.
Much clinical microbiology now relies heav-
ily on rapid, miniaturized systems which can
be used to identify microorganisms in 4–24 h.
Test results can be read and interpreted by eye
or the microbiologist can use computer aided
interpretation. Key advantages of using API kits
include the use of small amounts of media (in
small cupules or chambers), simple inoculation
and incubation techniques, and easy and quick
interpretation of test results based on colour
changes. However, the kits can be expensive to
maintain and, if shelf life is a constraint, use may
not be justified unless there is a high throughput
of samples.
At present, the most commonly used com-
mercial identification system in veterinary
diagnostic microbiology are the API (Analytical
Profile Index) systems, of which there are a
Figure 4.11c Simple biochemical screening test (for number designed to identity different groups
example triple sugar iron agar slopes) can be used or species of bacteria (Figure 4.11d). For exam-
to determine the species of selected bacteria dur- ple, specific kits are used for ‘anaerobes’ and
ing survey work but in most cases a series of 20–30 ‘enterobacteria’ (Analytical Profile Index 20
tests will be required. Illustrated are the Urease test, enterobacteria or API 20E). Clear instructions
lysine iron agar (LIA) and triple sugar iron agar (TSI) for the use are provided with each kit. A range
slopes used to help identify Enterobacteriaceae. of available kit tests are outlined in Table 4.3.
See also Plate 9. Criteria to be considered when selecting a
bacterial identification system for a particular
involves time-consuming preparation of media laboratory include reliability, versatility, the time
and inoculation procedures. In addition, con- required for preparation of reagents, incubation
siderable expertise is needed to interpret the time, relative difficulty in determining positive
results. Although miniaturized ‘kit’ systems are and negative reactions, safety factors for labo-
now available it is still necessary to do a lot of ratory personnel, shelf life of the test kits and
preliminary work (primary and secondary cul- the price of the system. Owing to cost factors,
ture, Gram stain, +/- selective media/growth the use of commercial identification kits is not
conditions, preparation of ‘purity’ plates) and sustainable for most of the smaller district labo-
to ensure good technique. In some laboratories, ratories.
traditional methods of bacterial identification
have been superseded by molecular techniques. Serological typing
However, the latter are less useful when deal-
ing with uncommon microorganism and where a A limited capacity for ‘sero-typing’ strains of bac-
clear list of differential diagnoses is not available, teria might be developed in a regional veterinary

Vet Lab.indb 222 26/03/2019 10:25