Page 293 - The Veterinary Laboratory and Field Manual 3rd Edition

262 Susan C. Cork and Roy Halliwell

Introduction

Traditional approaches to clinical microbiol-
ogy revolve around the detection and isolation
of microorganisms and the analysis of pheno-
typic characteristics such as, bacterial colony
morphology, viral cytopathic effects, serology,
biochemical and drug resistance markers. These
characteristics are used to identify species and
strains and to inform treatment and epidemio-
logical investigations. These methods however
do have some drawbacks. Many organisms are
Figure 4.29 Influenza A and B viruses can be differ- either non-culturable, low in number or too dif-
entiated based on rapid antigen ELISA test conducted ficult or slow to grow. In addition, cultures are
on a membrane. The figure shows five Directigen™ subject to contamination and the sample type
Flu A+B test membranes. Nasopharyngeal swabs submitted to the laboratory may carry com-
can be screened using this antigen detection test. ponents that are inhibitory in culture media.
This is a rapid (takes about 15 min) and qualita- Preserving the quality (including viability) of
tive test. Negative results in suspect clinical cases field-derived specimens, particularly when trans-
should be confirmed using other means. In positive port over long distances is required, can also
samples (that is, those containing antigen), the anti- pose problems that impact the successful isola-
gen-antibody reaction is determined by visual colour tion and culture of microorganisms for further
development, that is, samples 1, 4 and 5 are nega- characterization. In addition, some microor-
tive for both influenza A and B antigens. Samples ganisms are highly dangerous pathogens and
2 and 3 are positive for influenza B virus and influ- culture requires specialized facilities with high
enza A virus respectively. Photo: Davor Ojkic, Animal levels of biosecurity and biosafety. These fac-
Health Laboratory, University of Guelph, Canada. tors can make conventional microbiology both
time-consuming and expensive and may require
considerable technical expertise.
In more recent times, the rapidly evolving
4.8 Molecular microbiology and its
application as a diagnostic tool field of molecular biology (Figure 4.30) has sig-
nificantly broadened the repertoire of diagnostic
Julie Collins-Emerson and M. Faizal tools available to laboratories. In particular, the
Abdul-Careem speed, sensitivity and specificity of many of
these techniques have helped circumvent the
In the following section, some of the current difficulties and limitations presented by some
molecular techniques used in diagnostic micro- of the traditional phenotypic methods. While
biology are discussed. Many of these techniques the initial costs for establishing some molecular
are applicable for the detection of a range of techniques in the laboratories can be consid-
potential pathogens including bacteria, viruses, erable, the routine running costs and the high
fungi and multi-cellular parasites, however, the throughput possibilities for some processes can
main focus here is on bacteria and viruses.
make the techniques economical. The market
place changes rapidly with increasing numbers
of commercially produced, molecular based

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