gelatin liquefaction) is indicated by the presence of a clear zone round the colony after 10 minutes contact with either reagent. Particularly with plate cultures, gelatin liquefaction may be detected sooner by the `Stone reaction’ ( Stone 6 ): add a drop of saturated aqueous ammonium sulphate solution, or of fresh 20% aqueous sulpho-salicylic acid solution, to an individual colony growing on Nutrient Gelatin. Besides its presence or absence, the shape and nature of the liquefied portion of the stab culture are often useful identifying characteristics. Especially where prolonged incubation is necessary, it is important to ensure adequate closure of the containers in order to prevent dehydration of the medium. For practical purposes, a maximum incubation period of 14 days is suggested 3,4.Ĭonsiderably longer incubation may be necessary, some strains of Enterobacter cloacae liquefied gelatin only after 3 months at 20-22☌ 5. Rates of liquefaction vary considerably, so that some organisms produce liquefaction within a few days whereas others require several weeks. If the medium is incubated at a higher temperature it is necessary to employ uninoculated controls to allow for the hydrolytic effect of heat and other factors. The latter method not only allows determinations to be carried out on organisms which grow slowly or not at all at 20-22☌ but also usually avoids false positive results produced by the release of enzymes after the death of the organisms 2. Alternatively, incubate at a higher temperature (usually optimum for the organism under investigation) and then transfer the tube to a refrigerator or into cold water before observation. Test for gelatin liquefaction by incubating a Nutrient Gelatin stab or plate culture at 20-22☌. Gelatin is liquefied in a characteristic manner by certain proteolytic organisms whereas gelatin media have been largely superseded by agar media for most purposes, nutrient gelatin is still employed for differentiation of micro-organisms by their proteolytic effects. Nutrient Gelatin, a solid medium at 22☌ and below, is employed for the determination of gelatin liquefaction and for the 20☌ plate count 1. Mix well before pouring and cool below 20☌ or leave to set in a refrigerator. Sterilise by autoclaving at 121☌ for 15 minutes.
Bring to the boil to dissolve completely. Suspend 128g in 1 litre of distilled water. * Adjusted as required to meet performance standards
For determination of gelatin liquefaction, and for the 20☌ plate count.
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