Revival of biocide-treated spores of Bacillus subtilis

The differential effects of heat-shocking on the viability of spores from Bacillus anthracis, Bacillus subtilis, and Clostridium sporogenes after treatment with peracetic acid- and glutaraldehyde-based disinfectants

This study investigated (1) the susceptibility of Bacillus anthracis (Ames strain), Bacillus subtilis (ATCC 19659), and Clostridium sporogenes (ATCC 3584) spores to commercially available peracetic acid (PAA)- and glutaraldehyde (GA)-based disinfectants, (2) the effects that heat-shocking spores after treatment with these disinfectants has on spore recovery, and (3) the timing of heat-shocking after disinfectant treatment that promotes the optimal recovery of spores deposited on carriers. Suspension tests were used to obtain inactivation kinetics for the disinfectants against three spore types. The effects of heat-shocking spores after disinfectant treatment were also determined. Generalized linear mixed models were used to estimate 6-log reduction times for each spore type, disinfectant, and heat treatment combination. Reduction times were compared statistically using the delta method. Carrier tests were performed according to AOAC Official Method 966.04 and a modified version that employed immediate heat-shocking after disinfectant treatment. Carrier test results were analyzed using Fisher's exact test. PAA-based disinfectants had significantly shorter 6-log reduction times than the GA-based disinfectant. Heat-shocking B. anthracis spores after PAA treatment resulted in significantly shorter 6-log reduction times. Conversely, heat-shocking B. subtilis spores after PAA treatment resulted in significantly longer 6-log reduction times. Significant interactions were also observed between spore type, disinfectant, and heat treatment combinations. Immediately heat-shocking spore carriers after disinfectant treatment produced greater spore recovery. Sporicidal activities of disinfectants were not consistent across spore species. The effects of heat-shocking spores after disinfectant treatment were dependent on both disinfectant and spore species. Caution must be used when extrapolating sporicidal data of disinfectants from one spore species to another. Heat-shocking provides a more accurate picture of spore survival for only some disinfectant/spore combinations. Collaborative studies should be conducted to further examine a revision of AOAC Official Method 966.04 relative to heat-shocking.

Mechanisms of Bacillus subtilis spore resistance to and killing by aqueous ozone

AIMS

To determine the mechanisms of Bacillus subtilis spore killing by and resistance to aqueous ozone.

METHODS AND RESULTS

Killing of B. subtilis spores by aqueous ozone was not due to damage to the spore's DNA, as wild-type spores were not mutagenized by ozone and wild-type and recA spores exhibited very similar ozone sensitivity. Spores (termed alpha-beta-) lacking the two major DNA protective alpha/beta-type small, acid-soluble spore proteins exhibited decreased ozone resistance but were also not mutagenized by ozone, and alpha-beta- and alpha-beta-recA spores exhibited identical ozone sensitivity. Killing of spores by ozone was greatly increased if spores were chemically decoated or carried a mutation in a gene encoding a protein essential for assembly of the spore coat. Ozone killing did not cause release of the spore core's large depot of dipicolinic acid (DPA), but these killed spores released all of their DPA after a subsequent normally sublethal heat treatment and also released DPA much more readily when germinated in dodecylamine than did untreated spores. However, ozone-killed spores did not germinate with either nutrients or Ca(2+)-DPA and could not be recovered by lysozyme treatment.

CONCLUSIONS

Ozone does not kill spores by DNA damage, and the major factor in spore resistance to this agent appears to be the spore coat. Spore killing by ozone seems to render the spores defective in germination, perhaps because of damage to the spore's inner membrane.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results provide information on the mechanisms of spore killing by and resistance to ozone.

Mechanisms of killing of Bacillus subtilis spores by Decon and OxoneTM, two general decontaminants for biological agents

AIMS

To determine the mechanisms of Bacillus subtilis spore killing by and resistance to the general biological decontamination agents, Decon and Oxone.

METHODS AND RESULTS

Spores of B. subtilis treated with Decon or Oxone did not accumulate DNA damage and were not mutagenized. Spore killing by these agents was increased if spores were decoated. Spores prepared at higher temperatures were more resistant to these agents, consistent with a major role for spore coats in this resistance. Neither Decon nor Oxone released the spore core's depot of dipicolinic acid (DPA), but Decon- and Oxone-treated spores more readily released DPA upon a subsequent normally sublethal heat treatment. Decon- and Oxone-killed spores initiated germination with dodecylamine more rapidly than untreated spores, but could not complete germination triggered by nutrients or Ca(2+)-DPA and did not degrade their peptidoglycan cortex. However, lysozyme treatment did not recover these spores.

CONCLUSIONS

Decon and Oxone do not kill B. subtilis spores by DNA damage, and a major factor in spore resistance to these agents is the spore coat. Spore killing by both agents renders spores defective in germination, possibly because of damage to the inner membrane of spore.

SIGNIFICANCE AND IMPACT OF STUDY

These results provide information on the mechanisms of the killing of bacterial spores by Decon and Oxone.

Studies on the mechanism of killing of Bacillus subtilis spores by hydrogen peroxide

AIMS

To determine the mechanism of killing of Bacillus subtilis spores by hydrogen peroxide.

METHODS AND RESULTS

Killing of spores of B. subtilis with hydrogen peroxide caused no release of dipicolinic acid (DPA) and hydrogen peroxide-killed spores were not appreciably sensitized for DPA release upon a subsequent heat treatment. Hydrogen peroxide-killed spores appeared to initiate germination normally, released DPA and hydrolysed significant amounts of their cortex. However, the germinated killed spores did not swell, did not accumulate ATP or reduced flavin mononucleotide and the cores of these germinated spores were not accessible to nucleic acid stains.

CONCLUSIONS

These data indicate that treatment with hydrogen peroxide results in spores in which the core cannot swell properly during spore germination.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results provide further information on the mechanism of killing of spores of Bacillus species by hydrogen peroxide.

Studies on the mechanisms of the sporicidal action of ortho-phthalaldehyde

AIMS

To determine the mechanism of killing of spores of Bacillus subtilis by ortho-phthalaldehyde (OPA), an aromatic dialdehyde currently in use as an antimicrobial agent.

METHODS AND RESULTS

OPA is sporicidal, although spores are much more OPA resistant than are vegetative cells. Bacillus subtilis mutants deficient in DNA repair, spore DNA protection and spore coat assembly have been used to show that (i) the coat appears to be a major component of spore OPA resistance, which is acquired late in sporulation of B. subtilis at the time of spore coat maturation, and (ii) B. subtilis spores are not killed by OPA through DNA damage but by elimination of spore germination. Furthermore, OPA-treated spores that cannot germinate are not recovered by artificial germinants or by treatment with NaOH or lysozyme.

CONCLUSIONS

OPA appears to kill spores by blocking the spore germination process.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work provides information on the mechanism of spore resistance to, and spore killing by, the disinfectant, OPA.

Mechanisms of killing spores of Bacillus subtilis by acid, alkali and ethanol

AIMS

To determine the mechanisms of killing of Bacillus subtilis spores by ethanol or strong acid or alkali.

METHODS AND RESULTS

Killing of B. subtilis spores by ethanol or strong acid or alkali was not through DNA damage and the spore coats did not protect spores against these agents. Spores treated with ethanol or acid released their dipicolinic acid (DPA) in parallel with spore killing and the core wet density of ethanol- or acid-killed spores fell to a value close to that for untreated spores lacking DPA. The core regions of spores killed by these two agents were stained by nucleic acid stains that do not penetrate into the core of untreated spores and acid-killed spores appeared to have ruptured. Spores killed by these two agents also did not germinate in nutrient and non-nutrient germinants and were not recovered by lysozyme treatment. Spores killed by alkali did not lose their DPA, did not exhibit a decrease in their core wet density and their cores were not stained by nucleic acid stains. Alkali-killed spores released their DPA upon initiation of spore germination, but did not initiate metabolism and degraded their cortex very poorly. However, spores apparently killed by alkali were recovered by lysozyme treatment.

CONCLUSIONS

The data suggest that spore killing by ethanol and strong acid involves the disruption of a spore permeability barrier, while spore killing by strong alkali is due to the inactivation of spore cortex lytic enzymes.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results provide further information on the mechanisms of spore killing by various chemicals.

Killing of spores of Bacillus subtilis by peroxynitrite appears to be caused by membrane damage

During an infection of a higher eukaryote, dormant spores of a Bacillus species have been previously shown to be present in cells that can generate the toxic agent peroxynitrite (PON). Dormant spores of Bacillus subtilis were much more resistant to killing by PON than were growing cells, and spore-coat alteration or removal greatly decreased PON resistance. Spores were not killed by PON through DNA damage and lost no dipicolinic acid (DPA) during PON treatment. However, PON-killed spores lost DPA during subsequent heat treatments that caused much less DPA release from untreated spores. Although dead, the PON-killed spores germinated and initiated metabolism but never went through outgrowth; the great majority of germinated PON-killed spores also took up propidium iodide, indicating that they had suffered significant membrane damage and were dead. Together these data suggest that spore killing by PON is through some type of damage to the spore's inner membrane.

Analysis of the killing of spores of Bacillus subtilis by a new disinfectant, Sterilox

AIMS

To determine the mechanism whereby the new disinfectant Sterilox kills spores of Bacillus subtilis.

METHODS AND RESULTS

Bacillus subtilis spores were readily killed by Sterilox and spore resistance to this agent was due in large part to the spore coats. Spore killing by Sterilox was not through DNA damage, released essentially no spore dipicolinic acid and Sterilox-killed spores underwent the early steps in spore germination, including dipicolinic acid release, cortex degradation and initiation of metabolism. However, these germinated spores never swelled and many had altered permeability properties.

CONCLUSIONS

We suggest that Sterilox treatment kills dormant spores by oxidatively modifying the inner membrane of the spores such that this membrane becomes non-functional in the germinated spore leading to spore death.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work provides information on the mechanism of spore resistance to and spore killing by a new disinfectant.

Mechanisms of killing of spores of Bacillus subtilis by iodine, glutaraldehyde and nitrous acid

Treatment of wild-type spores of Bacillus subtilis with glutaraldehyde or an iodine-based disinfectant (Betadine) did not cause detectable mutagenesis, and spores (termed alpha-beta-) lacking the major DNA-protective alpha/beta-type, small, acid-soluble proteins (SASP) exhibited similar sensitivity to these agents. A recA mutation did not sensitize wild-type or alpha-beta- spores to Betadine or glutaraldehyde, nor did spore treatment with these agents result in significant expression of a recA-lacZ fusion when the treated spores germinated. Spore glutaraldehyde sensitivity was increased dramatically by removal of much spore coat protein, but this treatment had no effect on Betadine sensitivity. In contrast, nitrous acid treatment of wild-type and alpha-beta- spores caused significant mutagenesis, with alpha-beta- spores being much more sensitive to this agent. A recA mutation further sensitized both wild-type and alpha-beta- spores to nitrous acid, and there was significant expression of a recA-lacZ fusion when nitrous acid-treated spores germinated. These results indicate that: (a) nitrous acid kills B. subtilis spores at least in part by DNA damage, and alpha/beta-type SASP protect against this DNA damage; (b) killing of spores by glutaraldehyde or Betadine is not due to DNA damage; and (c) the spore coat protects spores against killing by glutaraldehyde but not Betadine. Further analysis also demonstrated that spores treated with nitrous acid still germinated normally, while those treated with glutaraldehyde or Betadine did not.

Antiseptics and disinfectants: activity, action, and resistance

Antiseptics and disinfectants are extensively used in hospitals and other health care settings for a variety of topical and hard-surface applications. A wide variety of active chemical agents (biocides) are found in these products, many of which have been used for hundreds of years, including alcohols, phenols, iodine, and chlorine. Most of these active agents demonstrate broad-spectrum antimicrobial activity; however, little is known about the mode of action of these agents in comparison to antibiotics. This review considers what is known about the mode of action and spectrum of activity of antiseptics and disinfectants. The widespread use of these products has prompted some speculation on the development of microbial resistance, in particular whether antibiotic resistance is induced by antiseptics or disinfectants. Known mechanisms of microbial resistance (both intrinsic and acquired) to biocides are reviewed, with emphasis on the clinical implications of these reports.

Revival of Bacillus subtilis spores from biocide-induced injury in the germination process

Spores of Bacillus subtilis NCTC 8236 were treated with glutaraldehyde, Lugol's iodine, polyvinylpyrrolidone-iodine (PVP-I), sodium hypochlorite or sodium dichloroisocyanurate (NaDCC). After exposure survivors were enumerated on nutrient agar containing potential revival agents (subtilisin, lysozyme, calcium dipicolinate, calcium lactate). Of these, only calcium lactate had any significant enhancing effect and then only with iodine-treated spores. Calcium lactate (9 mmol l-1) in nutrient broth enhanced the rate and extent of germination of iodine-treated spores but not of spores previously subjected to glutaraldehyde, hypochlorite or NaDCC.