Tests for Absence of Specific Indicator Microorganisms

Both pharmacopoeias describe methods for determining if particular microorganisms are present or absent from specified weights or volumes (10g or 10 ml in USP, 1 g or 1 ml in PhEur) of pharmaceutical preparations. These methods involve incubation of the pharmaceutical preparation in enrichment media, which encourages growth of one particular microorganism at the expense of others. This is followed by surface spread plating on media (selective and differential) upon which the particular microorganism takes a distinctive and easily recognizable colonial appearance.

The pharmacopoeias specify methods applying to Staphylococcus aureus, Pseudomonas aeruginosa, E. coli and Salmonella spp., with a method for each microorganism. Within each method there are options around choice of media and, in some cases, incubation temperatures. PhEur includes a broader category for "enterobacteria and certain other Gram-negative microorganisms" and recommends its application to Category 2 products (topical and respiratory preparations and transdermal patches).

Microorganisms for which methods are described in the pharmacopoeias are pathogenic. However, they have not been included solely for their pathogenicity: they are "indicator" or "index" microorganisms, chosen because they are easily recoverable and recognizable using robust and readily available methodology.

They are indicators of excessively contaminated raw materials and unhygienic manufacturing practices, providing a risk index for the presence of low numbers of other pathogens, for which feasible methods of detection are not available.

It is out of the question that batches of routinely produced pharmaceutical preparations should be critically examined and certified free from all pathogens and potential pathogens; quite simply, nothing would ever be released within its shelf life. However it would be extremely naive to believe that absence of all four selected indicator microorganisms is synonymous with absence of all pathogens and potential pathogens.

A technical curiosity of the methods, media and conditions recommended in the pharmacopoeias for recovery of these selected indicator microorganisms is that they derive from medical microbiology. As such, the media were originally designed to encourage the growth of a particular type of microorganism that would be most likely present as a minor component of a much larger microbial population in pathology samples, faeces, etc.

In pharmaceutical preparations, the selected indicator microorganisms would most likely be accompanied by very few other types, if present at all: Salmonella, for instance, has no recent history of ever being isolated from properly manufactured and formulated pharmaceutical preparations. There are substantial data to show that under these circumstances, standard general-purpose media are just as effective for the isolation of these selected indicator microorganisms as the complicated enrichment and selective media recommended.3

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