A mobile genetic element profoundly increases heat resistance of bacterial spores

Abstract

Bacterial endospores are among the most resilient forms of life on earth and are intrinsically resistant to extreme environments and antimicrobial treatments. Their resilience is explained by unique cellular structures formed by a complex developmental process often initiated in response to nutrient deprivation. Although the macromolecular structures of spores from different bacterial species are similar, their resistance to environmental insults differs widely. It is not known which of the factors attributed to spore resistance confer very high-level heat resistance. Here, we provide conclusive evidence that in Bacillus subtilis, this is due to the presence of a mobile genetic element (Tn1546-like) carrying five predicted operons, one of which contains genes that encode homologs of SpoVAC, SpoVAD and SpoVAEb and four other genes encoding proteins with unknown functions. This operon, named spoVA, confers high-level heat resistance to spores. Deletion of spoVA in a B. subtilis strain carrying Tn1546 renders heat-sensitive spores while transfer of spoVA into B. subtilis 168 yields highly heat-resistant spores. On the basis of the genetic conservation of different spoVA operons among spore-forming species of Bacillaceae, we propose an evolutionary scenario for the emergence of extremely heat-resistant spores in B. subtilis, B. licheniformis and B. amyloliquefaciens. This discovery opens up avenues for improved detection and control of spore-forming bacteria able to produce highly heat-resistant spores.

Spore Heat Activation Requirements and Germination Responses Correlate with Sequences of Germinant Receptors and with the Presence of a Specific spoVA 2mob Operon in Foodborne Strains of Bacillus subtilis

“…Recently, variation in spore germination and spore heat resistance has been partly attributed to interstrain differences in the absence or presence (and copy number) of the heptacistronic spoVA 2mob operon that is usually carried on the Tn1546-like transposon element (which lost the ability of transposition) (27,28). Tn1546 additionally carries four other operons that alone do not have apparent effects on spore heat resistance or germination (27,28).…”

“…Additionally, B. subtilis 168 contains two GR operons, yndDEF and yfkQRTS, that seem to be inactive and poorly transcribed (25). Moreover, certain foodborne B. subtilis strains comprise the incomplete and likely nonfunctional gerX germination operon that encodes putative GR subunits A and C but lacks a gene for the B subunit (27,28).…”

Bacillus cereus growth and biofilm formation: the impact of substratum, iron sources, and transcriptional regulator Sigma 54

“…The functions of genes carried on this transposon (SpoVAC, SpoVAD, SpoVAEb and four other unknown genes) are not completely known but the presence of the transposon results in about 50 % higher dipicolinic acid (DPA) content (Berendsen et al 2016). B. cereus strains belonging to various phylogenetic groups displayed lower variability in their heat resistance, having D 95°C values between 2 and 30 min, excluding psychrotolerant strains with low heat resistance (Luu-Thi et al 2014).…”

“…Intrinsic factors encoded by genotypic differences can have a large impact on spore heat resistance, such as a recently discovered mobile genetic element (Tn1546-like), causing 130 fold higher D 120°C values for B. subtilis strains (Berendsen et al 2015, Berendsen et al 2016. The functions of genes carried on this transposon (SpoVAC, SpoVAD, SpoVAEb and four other unknown genes) are not completely known but the presence of the transposon results in about 50 % higher dipicolinic acid (DPA) content (Berendsen et al 2016).…”

“…It can be used to correlate phenotypes with gene presence/absence data, and to predict possible gene functions, as demonstrated for L. plantarum and Lactococcus lactis strains in relation to phenotypes such as sugar utilisation, metal resistance and arginine metabolism (Bayjanov et al 2012, Bayjanov et al 2013. This approach was shown preciously successful in identification of a mobile genetic element responsible for high heat resistance of B. subtilis spores (Berendsen et al 2016). We applied gene-trait matching to predict genes involved in biofilm formation of B. cereus.…”