Bacillus spore wet heat resistance and evidence for the role of an expanded osmoregulatory spore cortex

Abstract

Bacillus spores' low core water content is a major factor in their wet heat resistance. One suggested mechanism for achieving low spore core water content is osmoregulated expansion of spores' peptidoglycan cortex. Evidence for this mechanism includes a report that decoated Bacillus cereus spores incubated in 4 mol l(-1) CaCl2 exhibit drastically reduced heat resistance. The current work shows that this heat sensitization of decoated spores of three Bacillus species is most likely due to inactivation of some crucial spore germination protein(s), since while treated spores appear dead, their apparent low viability is rescued by triggering spore germination with lysozyme.

What’s new and notable in bacterial spore killing!

“…These include DNA damaging agents and some chemicals that can damage proteins (Cortezzo et al 2004 ; Mokashi et al 2020 ). The low water content in the core protects core enzymes against wet heat damage, and both CaDPA accumulation and cortex action contribute to the core’s low water content (Coleman et al 2007 , 2010 ; Coleman and Setlow 2009 ; Rao et al 2016 ). The α/β-type SASP also protect DNA against wet heat damage, and CaDPA may assist in protecting DNA from some damage by immobilizing it (Setlow et al 2006 ), although generally not against radiation induced damage.…”

“…This can also be true for some percentage of spores in populations treated with wet heat or oxidizing agents. While some spores appear truly dead, others are only damaged and if recovered, generally on a very rich medium that often includes glucose, can grow and the damage is then not permanent (Coleman and Setlow 2009 ; Rao et al 2016 ; Russell and Loosemore 1964 ). Damaged spores can also be more sensitive to elevated temperatures or salt concentrations in recovery medium, so it is generally essential to test such variables to be sure treated spores are truly dead (Camilleri et al 2020 ).…”

“…Notably, spores killed by many of the agents listed above can often still germinate, albeit sometimes slower than untreated spores. This is true for spores killed by radiation, dry and wet heat and many chemicals (Mtimet et al 2017 ; Rao et al 2016 ; Russell and Loosemore 1964 ; Setlow et al 2013 ). Agents which target proteins involved in germination include wet heat and some chemicals.…”

“…The latter observation, as well as that NaOH-treated spores release CaDPA following germinant addition but never complete germination, suggests that strong alkali inactivates all spore CLEs which are outside the spore core, but the core components can be undamaged. Lysozyme at low levels then takes the place of CLEs in germinating the dormant spores which presumably have had sufficient coat proteins removed by the NaOH to allow lysozyme to access the cortex PG (Atrih and Foster 2001 ; Rao et al 2016 ; Setlow et al 2002 ).…”

“…Next is a large peptidoglycan (PG) layer, the cortex, whose structure is different to that of growing cell PG. The cortex appears to play an important role in reducing the water content in the spore core, which is important in spore dormancy and resistance to wet heat (Rao et al 2016 ). Under the cortex is the germ cell wall with a PG structure identical to that in the growing cell wall.…”