Thirst (1957) describes a novel slide microtest for detecting gelatin liquefaction in bacteria, adapted from a method previously used for helminth larvae. The technique involves applying small bacterial samples to gelatin-coated microscope slides, incubating at 22°C, then fixing and staining to visualize clear areas where gelatin has been hydrolyzed by bacterial gelatinase enzymes. The method was tested on strains from the genera Cloaca, Bacillus, and Clostridium obtained from the National Collection of Type Cultures. Results showed general concordance between slide reactions and traditional gelatin stab culture tests, with notable differences among bacterial groups. Cloaca cloacae strains showing rapid stab liquefaction consistently gave positive slide reactions, while slower-acting strains were less likely to be positive. Bacillus species showed more variable results, with serum-digesting strains demonstrating higher positive rates regardless of stab-test timing. Clostridium welchii exhibited particularly strong gelatinase activity. The main limitation is that the method detects only pre-formed enzymes, unable to detect adaptive enzyme production. Key advantages include minimal culture requirements and the ability to test organisms under any selected growth conditions.

Key findings

• A rapid gelatin liquefaction microtest using thin gelatin films on glass slides can detect bacterial gelatinase activity with improved sensitivity over traditional methods, requiring only small sample volumes.
• Among Cloaca cloacae strains, those liquefying gelatin stabs rapidly showed consistent positive slide reactions, while slow-acting strains were less likely to test positive on slides.
• Clostridium welchii demonstrated notably high gelatinase activity, with 5 of 6 strains tested giving positive reactions within 135 minutes.
• The slide test detects only readily-formed gelatinase enzymes and cannot detect adaptive enzyme synthesis, limiting its sensitivity for some bacterial strains.
• Bacillus serum-digesting strains showed elevated gelatinase activity on slides, suggesting serum-digesting enzymes may reinforce but are not entirely responsible for gelatin hydrolysis.