A range of methods is available for the enumeration of microorganisms in food. The choice of method will depend on a number of factors. • Type of sample. • Characteristics, including the physiological state, of specific organisms sought. • Characteristics of specific media. • Lower limit of enumeration required. • Purpose of the examination. | 5 Enumeration of microorganisms Dip slide culture Membrane filtration Pour plate Spiral plate Surface drop Surface spread plate Multiple tube most probable number methods Surface contact methods Surface swabs Membrane slide cultures Rinse method for watercress other leaf vegetables and acidic berry fruits Bottle rinse and plate count Choice of method A range of methods is available for the enumeration of microorganisms in food. The choice of method will depend on a number of factors. Type of sample. Characteristics including the physiological state of specific organisms sought. Characteristics of specific media. Lower limit of enumeration required. Purpose of the examination. Time available. Legislation sometimes prescribes a specific counting method for the enumeration of microorganisms in a particular product for example the pour plate method is specified in European Union EU milk legislation. For environmental samples such as surfaces utensils and equipment a surface contact technique may be the most useful method to choose. Any of a number of methods given in this section may be selected for enumeration of microorganisms in food. Whilst the pour plate method using plate count agar is regarded as the standard international method of enumeration for a total aerobic colony count it is common for laboratories to use surface methods such as the surface drop and spiral plate. Apart from the obvious convenience of using pre-poured plates these surface methods have the advantages that they eliminate possible heat stress to the organisms from the molten agar provide fully aerobic conditions of growth and facilitate identification of the organism types present. Pour plate methods require the use of a clear growth medium to allow counting of colonies that have grown below the surface of the medium. This also applies to counts performed by automated colony counters using transmitted light. Enumeration of microorganisms 105
