Bacillus thermoamylovorans Spores with Very-High-Level Heat Resistance Germinate Poorly in Rich Medium despite the Presence of ger Clusters but Efficiently upon Exposure to Calcium-Dipicolinic Acid.
High-level heat resistance of spores of Bacillus thermoamylovorans poses challenges to the food industry, as industrial sterilization processes may not inactivate such spores, resulting in food spoilage upon germination and outgrowth. In this study, the germination and heat resistance properties of spores of four food-spoiling isolates were determined. Flow cytometry counts of spores were much higher than their counts on rich medium (maximum, 5%). Microscopic analysis revealed inefficient nutrient-induced germination of spores of all four isolates despite the presence of most known germination-related genes, including two operons encoding nutrient germinant receptors (GRs), in their genomes. In contrast, exposure to nonnutrient germinant calcium-dipicolinic acid (Ca-DPA) resulted in efficient (50 to 98%) spore germination. All four strains harbored cwlJ and gerQ genes, which are known to be essential for Ca-DPA-induced germination in Bacillus subtilis. When determining spore survival upon heating, low viable counts can be due to spore inactivation and an inability to germinate. To dissect these two phenomena, the recoveries of spores upon heat treatment were determined on plates with and without preexposure to Ca-DPA. The high-level heat resistance of spores as observed in this study (D120°C, 1.9 ± 0.2 and 1.3 ± 0.1 min; z value, 12.2 ± 1.8°C) is in line with survival of sterilization processes in the food industry. The recovery of B. thermoamylovorans spores can be improved via nonnutrient germination, thereby avoiding gross underestimation of their levels in food ingredients.
Top Institute Food and Nutrition, Wageningen, The Netherlands Laboratory of Molecular Genetics, University of Groningen, Groningen, The Netherlands NIZO Food Research B.V., The Netherlands.
Erwin M
EM
Berendsen
Top Institute Food and Nutrition, Wageningen, The Netherlands Laboratory of Molecular Genetics, University of Groningen, Groningen, The Netherlands.
Antonina O
AO
Krawczyk
Top Institute Food and Nutrition, Wageningen, The Netherlands NIZO Food Research B.V., The Netherlands.
Verena
V
Klaus
Top Institute Food and Nutrition, Wageningen, The Netherlands Laboratory of Molecular Genetics, University of Groningen, Groningen, The Netherlands.
Anne
A
de Jong
Top Institute Food and Nutrition, Wageningen, The Netherlands NIZO Food Research B.V., The Netherlands.
Jos
J
Boekhorst
Top Institute Food and Nutrition, Wageningen, The Netherlands Laboratory of Molecular Genetics, University of Groningen, Groningen, The Netherlands NIZO Food Research B.V., The Netherlands.
Robyn T
RT
Eijlander
Top Institute Food and Nutrition, Wageningen, The Netherlands Laboratory of Molecular Genetics, University of Groningen, Groningen, The Netherlands o.p.kuipers@rug.nl Marjon.Wells-Bennik@nizo.com.
Oscar P
OP
Kuipers
Top Institute Food and Nutrition, Wageningen, The Netherlands NIZO Food Research B.V., The Netherlands o.p.kuipers@rug.nl Marjon.Wells-Bennik@nizo.com.
Marjon H J
MH
Wells-Bennik
D000818:Animals; D001407:Bacillus / Q000235:genetics / Q000502:physiology*; D005516:Food Microbiology; D006170:Gum Arabic; D006358:Hot Temperature; D008892:Milk / Q000382:microbiology; D010848:Picolinic Acids / Q000378:metabolism*; D013171:Spores, Bacterial / Q000254:growth & development
D010848:Picolinic Acids; D006170:Gum Arabic; C004860:dipicolinic acid