Short communication: Growth of dairy isolates of Geobacillus thermoglucosidans in skim milk depends on lactose degradation products supplied by Anoxybacillus flavithermus as secondary species

摘要

ABSTRACTThermophilic bacilli such asAnoxybacillusandGeobacillusare important contaminants in dairy powder products. Remarkably, one of the common contaminants,Geobacillus thermoglucosidans, showed poor growth in skim milk, whereas significant growth ofG. thermoglucosidanswas observed in the presence of anAnoxybacillus flavithermusdairy isolate. In the present study, we investigated the underlying reason for this growth dependence ofG. thermoglucosidans. Whole-genome sequences of 4A. flavithermusstrains and 4G. thermoglucosidansstrains were acquired, with special attention given to carbohydrate utilization clusters and proteolytic enzymes. Focusing on traits relevant for dairy environments, comparative genomic analysis revealed that allG. thermoglucosidansstrains lacked the genes necessary for lactose transport and metabolism, showed poor growth in skim milk, and produced white colonies on X-gal plates, indicating the lack of β-galactosidase activity. TheA. flavithermusisolates scored positive in these tests, consistent with the presence of a putative lactose utilization gene cluster. All tested isolates from both species showed proteolytic activity on milk plate count agar plates. Adding glucose or galactose to liquid skim milk supported growth ofG. thermoglucosidansisolates, in line with the presence of the respective monosaccharide utilization gene clusters in the genomes. Analysis by HPLC ofA. flavithermusTNO-09.006 culture filtrate indicated that the previously described growth dependence ofG. thermoglucosidansin skim milk was based on the supply of glucose and galactose byA. flavithermusTNO-09.006.