Preparation and validation of skim milk-based method for preservation of bacterial strain at resource-limited setup.

Introduction and Objectives: To ensure quality control, microbiology laboratories should maintain a variety of bacterial and fungal strains. Once cultivated, these organisms can survive on the media as long as nutrients are available. Skim milk, a common freeze-drying protectant, can be used as a freezing solution for bacteria at low temperatures. The aim of this study was to prepare and validate a skim milk solution for bacterial strain storage. Methods: A 10% skim milk solution was prepared and stored into 2-3 ml aliquots in sterile plastic vials. Subcultured organisms were inoculated into vials and stored at -20°C in a deep freezer. We checked the quality of the bacterial strains at regular intervals: 7 days, 15 days, 1 month, 2 months, and 3 months. Each time, a subculture was performed from the same vial to check the characteristics of the bacterial strain. All characteristics were recorded in the study Performa until the study's completion. Results: A total of 15 strains, including Gram-positive cocci, Gram-negative bacilli, and yeast strains, were included in the study. At all subculture times, organisms grew properly and exhibited similar colony characteristics. No changes were observed in the subculture from the first to the last inoculation of the bacterial organisms. Biochemical tests were conducted every time a subculture was performed to check for variations in properties, which were found to be consistent over time. Conclusion: The skim milk solution offers an effective method for storing, maintaining, and reviving bacterial strains, for up to 3 months. This approach significantly reduces the labor-intensive work associated with regular subculture methods used for storing or reviving bacterial strains.