Short communication: Dynamic changes in bacterial diversity during the production of powdered infant formula by PCR-DGGE and high-throughput sequencing

Abstract

Microorganisms such as thermophilic and psychrotrophic bacteria cause spoilage of milk and milk products [e.g., powdered infant formula (PIF)], mainly because they produce heat-stable extracellular enzymes. However, the dynamic changes in microbial diversity during PIF production are still not well understood. We used denaturing gradient gel electrophoresis (DGGE) and high-throughput sequencing (HTS) to investigate bacterial community structure and distribution during the major stages of PIF production: raw milk, pasteurization, mixing, evaporation, and spray-drying. Our PCR-DGGE analysis indicated that Lactobacillus and Pseudomonas spp. were the dominant bacteria at the raw milk and pasteurization stages; Lactococcus, Streptococcus, Enterococcus, and Lactobacillus spp. were abundant during mixing, evaporation, and spray-drying. Our HTS analysis showed that Pseudomonas had an abundance of 96.79% at the raw milk stage. Lactobacillus, Streptococcus, Thermus, Acinetobacter, and Bacteroides spp. were most common after pasteurization. The index of bacterial diversity was highest at the evaporation stage, suggesting a high potential risk of microbial contamination. The results from DGGE and HTS were consistent in reflecting changes in dominant flora, but different in reflecting the richness of bacterial communities present during PIF production: HTS revealed a much higher richness of bacterial species than DGGE. Our findings from DGGE and HTS showed that psychrophilic and thermophilic bacteria were the main flora present during PIF production: psychrophilic bacteria were mainly Pseudomonas spp. and thermophilic bacteria were mainly Lactobacillus, Streptococcus, and Bacillus spp. To our knowledge, this is the first study to report dynamic changes in microbial communities during PIF production. Our results provide insight into bacterial communities and identify potential contamination sources that could serve as a guide for reducing microbial risk.