Improvement of the Pour Plate Method by Separate Sterilization of Agar and Other Medium Components and Reduction of the Agar Concentration

Abstract

Although the pour plate method is widely employed in microbiological quality control, it has certain drawbacks, including having to melt the culture medium before seeding. The preparation of the culture media was modified by using a lower concentration of agar (10g/L), which was separated from the nutrients during sterilization. The new protocol was assessed in frequently media used in microbiological quality control of food, cosmetics and pharmaceutical products, with Tryptic Soy Agar (TSA), Sabouraud 4% Dextrose Agar (SDS) and Violet Red Bile Glucose Agar (VRBG). In comparison with the conventionally produced media, the modifications significantly improved the growth of Saccharomyces cerevisiae in SDS, Staphylococcus aureus, Salmonella Salmonella enterica subsp. enterica ser. Typhimurium Typhimurium and Candida albicans in TSA, and Escherichia coli ATCC 8739 and ATCC 25922 and S. Typhimurium Salmonella enterica subsp. enterica ser. Typhimurium in VRBG. The modified VRBG was also more selective for Pseudomonas aeruginosa. Regarding physicochemical properties, a significantly lower pH was observed in TSA and VRBG, and lower strength values in TSA. Sterilizing agar separately from the other media components and reducing the agar concentration to 10g/L can improve microorganism growth and enhance the selectivity of differential media in the pour plate method. These modifications could facilitate the automation of this culture technique.