Page 229 - The Veterinary Laboratory and Field Manual 3rd Edition

chapter 4

Microbiology

Susan C. Cork and Roy Halliwell

4.1 General sample collection, for microbiology testing should be kept chilled
preparation and handling (but not frozen). Owing to the difficulty in get-
ting suitable diagnostic samples to the laboratory
The discipline of microbiology includes bacteriol- from the field, many of the samples submitted
ogy, virology and the study of fungi (mycology). to a microbiology unit may be of limited value
Most regional veterinary laboratories and small and a cautious approach must be taken when
district laboratories are equipped to perform interpreting culture results. To assist laboratory
basic bacteriology and some mycology work. staff in the selection of suitable media and culture
Traditional virology is more specialized requir- conditions for microbial growth it is important
ing cell culture and controlled conditions for that the submitter completes a laboratory sub-
viral growth. However, many viral diseases can mission form to accompany the samples. On
initially be diagnosed based on clinical and epi- the submission form, the submitter should out-
demiological characteristics and confirmed using line the nature of the disease observed (clinical
serological or antigen capture tests (see Chapter signs/epidemiology) and note the species, age
6). The emergence of commercial ‘kits’ for and sex of the animal(s) sampled as well as the
molecular testing (see Table 4.3) has also made type of sample (skin, faeces, milk, tissue swab,
targeted molecular screening for microorganisms urine and so on). The submitter’s name and con-
more feasible with some regional laboratories tact details should be clearly noted along with
now able to offer a limited range of molecular the date of sample collection. Where possible
tests. The type of samples submitted for microbi- the submitting veterinarian or extension officer
ological examination is varied and includes faecal should list the likely differential diagnoses on
samples, milk, urine, tissues, exudates, transu- the form to ensure that the pathogens suspected
dates, pus and mucus. In stained preparations can be specifically tested for, that is, some bacte-
and wet smears submitted for microbiological ria will only grow under anaerobic (oxygen free)
examination, protozoal organisms may also be conditions (for example, Clostridium spp.) and
present, for example, Coccidia and Cryptosporidia others require special nutrients in the media for
spp. (see Chapter 3, section 3.5). growth (for example, Leptospira spp.). If suitable
The correct collection and handling of micro- culture media or conditions are not provided cul-
biology samples prior to laboratory testing is ture will be unsuccessful even in cases where the
important because poor technique may lead to pathogen was initially present in the specimen.
contamination and overgrowth of the initial This chapter outlines the basic principles
causative organism. Where possible, specimens of microbiology and is divided into separate

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