Screening mutant library of Lactic Acid Bacteria for development of strains with improved physiological traits desired in the dairy fermentation industry

Abstract

Conventional mutagenesis and mutant library screening has long been used to generate mutant strains of Lactic Acid Bacteria with improved physiological and genetic characteristics. This approach was proved to be successful in the fermentation industry by supplying isolates capable of tolerating operational conditions. Acid development and persistence during dairy fermentation and poor growth are the main hindrances the industry is facing. There in order to alleviate this problem observed in Ethiopian cottage and industrial dairy fermentation, this study was designed with objectives of screening and selecting elite mutant and wild types isolates with improved acid tolerance and elevated growth performance. Hence, mutant library of Lactic Acid Bacteria and their respective wild types were screened for acid tolerance at various temperatures. The Survivors were evaluated for growth performance at 25, 35, 45 and 55 °C and pH 7, 6 and 5. Two mutant isolates (MAS03 and MAB18) and their respective wild type isolates (B6S01 and A6B08) were found be better performing and were selected for growth monitoring test. Acid stress tolerance (pH 3 and 4) was found to be higher at 35 °C and 12 hours of incubation. The growth performance of mutant isolates was found to excel the wild types at pH 7, 6 and 5. Mean Optical Density as high as 8±0.00 were measured at 45 °C and 8th hours by the mutants type MAB18. Similarly at pH 6 Optical density 6.712±0.025 was observed at 35 °C and 12th hour of incubation by the other mutant type MAS03. However, the respective wild types performance at the respective conditions were 2.417±0.021 (A6B08) and 2.999±0.001 (B6S01). The mutant types’ lag time was as short as 2 and 4 hours at pH 7 and 6, and 35 and 45°C while it took the wild types 6 to 8 hours. The mutants were found be robust in faster exponential growth, stress tolerance and highly reduced lag time. These traits are desirable industrially; therefore, considering them for cottage and industrial dairy processes could reduce production cost and boost productivity.