• Membrane filtration
• Surface spread plates
• The "most probable number" approach
Pour plates and membrane filtration are widely used; surface spread plates are more commonly used for counting microorganisms in pure culture work. The most probable number approach is a method of last resort and its supporting statistical tables are limited, as they do not properly address mold recovery. Each of these four approaches splits into two methods.
• Counting numbers of aerobic bacteria per unit weight or volume of product
• Counting numbers of yeasts and moulds per unit weight or volume of product
Microbiologists debate endlessly whether the two methods are mutually exclusive, and how to derive a total count if they are not. Most of this debate is only of academic interest. For quality control (QC) purposes safe conservative assumptions are that:
• Bacteria are never recovered in the test for yeasts and molds
• Yeasts and molds are never recovered in the test for aerobic bacteria
• The total count is the sum of the two
With properly formulated preparations manufactured under controlled GMP conditions, actual recoverable numbers are rarely within a decimal order of magnitude of the specified limits. Any manufacturer producing a nonsterile aqueous-based preparation close to (or frequently failing) the pharmacopoeial limits for numbers of microorganisms, is already in trouble!
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