Often in the microbial examination of products, physical issues occur that force adaptations to the sample preparation. Some of these challenges include viscosity, the need for mixing or blending to achieve homogeneity, opacity, and insolubility. Viscosity makes mixing the product difficult, and also decreases ease and accuracy of pipetting and transferring material. Sometimes it is necessary to perform a density study and/or transfer by weight to ensure the appropriate quantity of sample is tested. When testing semi-solid substances such as gum, a sufficient blending or stomaching method must be employed to ensure adequate extraction and dispersion of any microbes into the suspension. Opacity of products, such as dyes, mascara, and certain tablets can lead to the need for membrane filtration, increased dilution, and/or addition of biological growth indicators to make plate counts readable. In the case of mascara, insolubility is also an issue, which is typically resolved with an additive such as polysorbate or isopropyl myristate.
When multiple difficulties coalesce, initially challenging samples can seem impossible to process and validate. For instance, the confluence of physical challenges and inhibitory properties: A drug tablet is typically easy enough to suspend in diluent and test by direct plating, but fill it with an antimicrobial and an excipient like guar gum, and it’s not so simple. Guar gum will turn most solutions into a gel. An unknown antimicrobial or antibiotic can take several attempts to neutralize sufficiently.
Add to this the fact that formulations are rarely shared prior to testing with the contract lab, and things get very exciting. However, using the tools described and some ingenuity, even the most difficult challenges can be overcome. In the example above, processing guar gum with cellulase will reduce the viscosity and the gel will become a liquid again. Adding in neutralizers may help take care of the antimicrobial. If not, dilution and/or complex centrifugation, followed by membrane filtration could yield a suitable method.