The elastase test is conducted by inoculating a pure culture of D. nodosus onto an agar culture medium which contains large particles of elastin, which is a protein.
If the bacteria produce an active form of the proteolytic enzyme called elastase, it diffuses into the agar and gradually digests the elastin particles, resulting in a clear zone.
Strains of D. nodosus that cause virulent footrot tend to have greater elastase activity than strains associated with benign footrot (Stewart, 1979; Claxton, 1986). In general terms, the time taken for digestion of elastin is inversely proportion to the virulence of the isolate of D. nodosus. However, there are exceptions.
The results of the elastase test can be reported as days to positive, and in that case cut-points can be allocated, for example:
So-called virulent strains of D. nodosus can digest elastin within 10 days.
So-called intermediate strains of D. nodosus can digest elastin between 10 and 21 days.
So-called benign strains of D. nodosus fail to digestion election within 28 days.
Clinically virulent footrot is associated with strains that digest elastase up to 21 days (Claxton, 1986).
Figure 1. Lack of digestion of elastin particles by a so-called benign isolate of D. nodosus after 28 days of incubation. Photo: Ian Links
Figure 2. Digestion of elastin particles by a so-called virulent isolate of D. nodosus after 7 days of incubation. Photo: Ian Links
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