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Abstract Viable count of six pure cultures of bacteria as well as those of water and raw milk samples were worked out by the standard spread plate method and economically designed miniagar discs using lesser amounts of the aliquots. The transfer of the material was also attempted with the help of micropipette in addition to a metal spreader. Micro-colonies appeared within 6 to 10 hours. The results obtained indicated that colony forming units (C.F.U.) both of natural samples as well as experimental cultures can be established economically by employing the modified miniagar discs method, a modified protocol.
Keywords: Colony forming units; microcolonies; miniagar discs for C.F.U.
INTRODUCTION Estimating bacterial population densities is important in environmental assessments of air, water and soil as well as in the experimental cultures. Various methods are also employed by public health authorities to keep check on microbial quality of milk, water and food. In this regard a number of techniques are used in food industries. Physical methods are used to estimate total population i.e. dead as well as live microorganisms. They include direct counting, measurement of turbidity, determining biomass and estimating growth dependant metabolites. The biological methods are used for estimating the number of viable units (Pelczar et al., 1986; Morgan and Carter, 1996; Banwart, 1998).
The number of viable cells in a sample is assessed from the number of colonies, which develop on incubation of known amount of a sample inoculated on a solid medium. Plate count, roll tube count, drop count, surface count, dip slide count, contact plate, membrane filter count and estimations of most probable number are the familiar methods of measuring colony forming units (C.F.U.) in a sample. A viable count method assumes that a visible colony will develop from each microorganism. However, a single colony may develop from one or from hundreds or even thousands of microorganisms physically associated with each other or distributed at one location during the process of inoculation. Thus each colony develops form one viable unit; therefore viable counts are usually given as number of C.F.U. per unit volume rather than number of microbes (Collins et al., 1995).
The selectivity of medium and the conditions of incubation may significantly affect the number of viable cells, which give rise to colonies. Viable...