Determining the Benzo[a]pyrene Degradation, Tolerance, and Adsorption Mechanisms of Kefir-Derived Bacterium Bacillus mojavensis TC-5

Abstract

Microbial detoxification, as an environmentally friendly strategy, has been widely applied for benzo[a]pyrene (BaP) degradation. Within this approach, food-derived microbial strains offer unique advantages in safety, specificity, and sustainability for detoxifying food-borne BaP. In this study, we aimed to explore the potential of such strains in BaP degradation. Bacillus mojavensis TC-5, a strain that degrades BaP, was isolated from kefir grains. Surprisingly, 12 genes encoding dehydrogenases, synthases, and oxygenases, including betB, fabHB, qdoI, cdoA, and bioI, which are related to BaP degradation, were up-regulated by 2.01-fold to 4.52-fold in TC-5. Two potential degradation pathways were deduced. In pathway I, dioxygenase, betaine aldehyde dehydrogenase, and beta-ketoacyl-ACP synthase III FabHB act sequentially on BaP to form 4H-pyran-4-one,2,3-dihydro-3,5-dihydroxy-6-methyl via the phthalic acid pathway. In the presence of the cytochrome P450 enzyme, BaP progressively mediates ring cleavage via the anthracene pathway, eventually forming 3-methyl-5-propylnonane in pathway II. Notably, TC-5 achieved an impressive BaP removal efficiency of up to 63.94%, with a degradation efficiency of 32.89%. These results suggest that TC-5 has significant potential for application in addressing food-borne BaP contamination. Moreover, our findings expand the application possibilities of Xinjiang fermented milk products and add to the available green strategies for BaP degradation in food systems.

Keywords: Bacillus mojavensis; benzo[a]pyrene; degradation pathway; kefir.

Figure 1

Morphological observations of strain TC-5. (a) Colony morphology on plate; (b) Gram stain under the microscope; (c) external appearance of the bacterium under SEM; (d) growth curves of TC-5 at different concentrations of BaP; (e) degradation chromatograms of the control and treatment groups; (f) phylogenetic tree of genes in TC-5 that may have BaP degradation functions.

Figure 2

(a) Heatmap of correlation analysis between BaP and CK groups; (b) volcano map of expressed genes; (c) Venn analysis of expressed genes.

Figure 3

Heatmap of clustering of different functional genes in TC-5.

Figure 4

Schematic illustration about mechanism of action of BaP.

Figure 5

The speculated pathway of BaP degradation by TC-5. A–D represent different enzymes: A: quercetin 2,3-dioxygenase/cysteine dioxygenase family protein; B: cytochrome P450 enzyme; C: betaine aldehyde dehydrogenase; D: beta-ketoacyl-ACP synthase III FabHB.