The principle of this test is based on an observation that so-called virulent strains of D. nodosus produced heat stable protease whereas so-called benign strains produced heat labile protease (Kortt et al., 1982; Depiazzi et al., 1991).
This evolved into the gelatin gel test that was adopted as the preferred test across Australia in the 1990s (Palmer, 1993; Richards and Mitchell, 1993; Eggleston, 1992). However, variations in methodology and interpretation existed between laboratories (Palmer, 1993; Stewart and Claxton, 1993).
The fluid from a broth culture of D. nodosus is placed onto a layer of solid gelatin agar, after heating. The result is compared to a sample of fluid that has not been heated. If gelatin is digested, a clear spot appears.
Figure 1. So-called virulent isolate of D. nodosus. There is digestion of gelatin (cleared spots) even after heating the culture. Image (adapted) from Ian Links
Figure 2. So-called benign isolate of D. nodosus. There is partial failure to digest gelatin after heating the culture. Image (adapted) from Ian Links
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