As previously discussed, the range of organisms must contain bacteria and fungi. Within these we must find Gram-positive and Gram-negative bacteria because the structure of the bacterial wall influences the penetration and thus the efficacy of the preservating agent. For the fungi, representatives of the two fungal forms must be used, namely the vegetative yeast cell and the mould spore. The choice of species is directed by their skin and mucosal pathogenicity for cosmetics. Product degradation capabilities are also taken into account to choose the species. So among the Gram-positive species, Staphylococcus aureus is an important skin pathogen, as is Pseudomonas aeruginosa for the Gram-negative bacteria. This latter organism is also able to use many compounds, such as preservatives or even disinfectants, as a carbon source and is very adaptative in adverse environmental conditions even in pure water [60,61]. For the yeast, Candida albicans is a skin pathogen and Aspergillus niger is a representative of the degradation flora. The choice of strains for a standardized assay must be guided by the need to compare results obtained in different laboratories, and in this way culture-collection strains are chosen coming from the American Type Culture Collection (ATCC). The strains normally used are as follows: (1) Staphylococcus aureus ATCC 6538, (2) Pseudomonas aeruginosa ATCC 9027, (3) Candida albicans ATCC 10231, and (4) Aspergillus niger ATCC 16404. These strains are to some extent resistant to the antimicrobials, and some are also used for testing disinfectants or antibiotics. For a representative preservation-efficacy test, it is also recommended to add strains isolated from the environment, water, or contaminated products. These strains live in the vicinity of or even inside the product, are well adapted to adverse conditions, and are often resistant to preservatives or even disinfectants [62-64]. Nevertheless, after a -c few passages in culture media, this particular resistance can disappear. Precautions must be taken to avoid this, such as immediate storage in appropriate medium by deep freezing or in liquid nitrogen. jjj
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