Detection of bacterial gelatinases by gelatin-agar plate methods.

Many methods have been devised to detect the action of bacterial cultures in or on gelatin media. The traditional gelatin stab technique involves incubation of the culture below the thermal liquefaction point of the gelatin in order to inaintain the gel structure so that the shape and size of the liquefied portion can be observed. A long incubation time is usually required. Bertiau (1914) suggested the use of higher incubation temperatures, above the thermal liquefaction point of gelatin, with subsequent chilling to determine if a gel would form again. This method would give all-or-none results. If a sufficient number of intact gelatin molecules remained, a gel would form. Small amounts of gelatin degradation would not be detected. Plate methods based upon zonal changes in agar media containing gelatin after the addition of protein precipitants have been used. Frazier (1926) used tannic acid or acidic mercuric chloride; Chapman (1946, 1953) employed either a saturated solution of ammonium sulfate or a 20 per cent solution of sulfosalicylic acid. One (letermination per plate could be made since the reagents used for the visualization of the zone(s) sterilize the culture. Chapman (1948) incorporated high concentrations of ammonium sulfate and sodium chloride into a medium so that the zones of clearing could be observed directly in the relatively opaque gelatin-agar medium. However, the high salt content inhibited the growth of organisms other than staphylococei and streptobacilli. The present paper describes various media containing gelatin-agar mixtures. These media permit direct detection of gelatin degradation, brought about by growing cultures of bacteria. These media may be used at conventional incubation temperatures without subsequent chilling and the zonal changes can be detected without the addition of a developer. MATERIALS AND METHODS