Page 256 - The Veterinary Laboratory and Field Manual 3rd Edition

Microbiology 225

Figure 4.12 The dilution technique can be used to count bacteria (see text). Serial dilutions are made from
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2
6
the original sample or a broth culture. Aliquots of selected dilutions (that is, 10– , 10– , 10– ) are plated
out using a spread technique and cultured for 24 h at 37°C. (A) Preparing a serial dilution (ten-fold). (B)
Incubated culture plates after surface spread technique.
methods use plating out of specific aliquots of a methods used in quality control procedures for
sample or estimation of the turbidity of a sample public health services use enzyme linked immu-
or an inoculated broth in a spectrophotometer. nosorbent assay (ELISA) technology and antigen
In the latter test, there needs to be a reference capture kit tests (see Chapter 6).
standard against which the test sample is mea-
sured and/or a standard curve prepared to allow
an accurate estimate of the number of bacteria 4.5 Antimicrobial sensitivity testing
represented by a specific reading. This method is and antimicrobial use
simple to perform but has the disadvantage that Karen Liljebjelke and Susan C. Cork
it measures both live and dead bacteria whereas
the colony counting techniques measure only The principle of the antibiotic sensitivity test is
the viable bacteria (colony forming units). Other to determine whether or not bacterial growth

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