Emergence and development of resistance to antimicrobial chemicals and heat in spores of Bacillus subtilis

Akwaton, polyhexamethylene-guanidine hydrochloride-based sporicidal disinfectant: a novel tool to fight bacterial spores and nosocomial infections

Bacterial spores are of continuing interest to the food and medical industries. In efforts to eliminate bacterial spore contamination, a number of sporicidal agents have been developed. Most of these compounds must be used carefully in very specific circumstances as they are toxic to humans. The sporicidal activity of Akwaton, a polyhexamethylene-guanidine hydrochloride (PHMGH)-based disinfectant, was tested against Bacillus subtilis spores. PHMGH is a colourless, odourless, non-corrosive and non-irritating antimicrobial biocide of the guanidine family. Spores suspended in distilled water and spores placed on solid surfaces (stainless steel and glass) were used to determine the log(10) reduction after exposure to varying concentrations of Akwaton. The minimum sporostatic concentration, the minimum sporicidal concentration and the time required for sporicidal activity corresponded to 0.06% (w/v), 0.08 % (w/v) and 8.5 min, respectively. Disinfectant concentrations of 0.24 % (w/v) and 0.44 % (w/v) killed all spores suspended in distilled water within 3 min and 90 s, respectively. The sporicidal activity against suspended spores was linearly dependent with respect to the concentration of PHMGH and contact time (y(3 min) = 40x-1.6 and y(90 s) = 20x-0.8 thus y(3 min) = 2y(90 s)). Spores placed on surfaces were more resistant to the effect of the disinfectant and the positive linear correlation between the sporicidal activity and concentration was not observed. The concentration required to kill all spores placed on a surface (stainless steel or glass) corresponded to 0.52 % (w/v) for 90 s of contact and 0.36 % (w/v) for 3 min. This study demonstrated that PHMGH is an effective sporicidal disinfectant with great potential for use in hospitals, laboratories, food industries and households.

Canine coronavirus inactivation with physical and chemical agents

Canine coronavirus (CCoV) is responsible for mild or moderate enteritis in puppies. The virus is highly contagious and avoiding contact with infected dogs and their excretions is the only way to ensure disease prevention. Since no studies have yet focused on the sensitivity of CCoV to chemical biocides the present investigation examined the efficiency of physical and chemical methods of viral inactivation. CCoV infectivity was stable at +56 degrees C for up to 30 min, but tended to decrease rapidly at +65 degrees C and +75 degrees C. Germicidal ultra-violet (UV-C) light exposure demonstrated no significant effects on virus inactivation for up to 3 days. CCoV was observed to be more stable at pH 6.0-6.5 while extreme acidic conditions inactivated the virus. Two tested aldehydes inactivated the virus but their action was temperature- and time-dependent. The methods for CCoV inactivation could be applied as animal models to study human coronavirus infection, reducing the risk of accidental exposure of researchers to pathogens during routine laboratory procedures.

Mechanisms of killing of Bacillus subtilis spores by hypochlorite and chlorine dioxide

AIMS

To determine the mechanisms of Bacillus subtilis spore killing by hypochlorite and chlorine dioxide, and its resistance against them.

METHODS AND RESULTS

Spores of B. subtilis treated with hypochlorite or chlorine dioxide did not accumulate damage to their DNA, as spores with or without the two major DNA protective alpha/beta-type small, acid soluble spore proteins exhibited similar sensitivity to these chemicals; these agents also did not cause spore mutagenesis and their efficacy in spore killing was not increased by the absence of a major DNA repair pathway. Spore killing by these two chemicals was greatly increased if spores were first chemically decoated or if spores carried a mutation in a gene encoding a protein essential for assembly of many spore coat proteins. Spores prepared at a higher temperature were also much more resistant to these agents. Neither hypochlorite nor chlorine dioxide treatment caused release of the spore core's large depot of dipicolinic acid (DPA), but hypochlorite- and chlorine dioxide-treated spores much more readily released DPA upon a subsequent normally sub-lethal heat treatment than did untreated spores. Hypochlorite-killed spores could not initiate the germination process with either nutrients or a 1 : 1 chelate of Ca2+-DPA, and these spores could not be recovered by lysozyme treatment. Chlorine dioxide-treated spores also did not germinate with Ca2+-DPA and could not be recovered by lysozyme treatment, but did germinate with nutrients. However, while germinated chlorine dioxide-killed spores released DPA and degraded their peptidoglycan cortex, they did not initiate metabolism and many of these germinated spores were dead as determined by a viability stain that discriminates live cells from dead ones on the basis of their permeability properties.

CONCLUSIONS

Hypochlorite and chlorine dioxide do not kill B. subtilis spores by DNA damage, and a major factor in spore resistance to these agents appears to be the spore coat. Spore killing by hypochlorite appears to render spores defective in germination, possibly because of severe damage to the spore's inner membrane. While chlorine dioxide-killed spores can undergo the initial steps in spore germination, these germinated spores can go no further in this process probably because of some type of membrane damage.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results provide information on the mechanisms of the killing of bacterial spores by hypochlorite and chlorine dioxide.

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 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.

Molecular mechanisms of resistance to heat and oxidative damage

Spore heat resistance can be predicted within reasonable limits from knowledge of optimal growth temperature of the sporeformer, the temperature of sporulation, water content of the spore protoplast, cortex size, total mineralization and specific mineralization. The molecular mechanisms by which dehydration and mineralization act to stabilize spores against heat damage are unknown. A major need for further progress is to identify the principal targets for lethal damage. In this review the hypothesis was explored that heat killing may be related to oxidative killing. The proposed common denominator for the two is the formation of radicals able to react with, and irreversibly damage, spore polymers such as proteins or DNA.

Revival of biocide-treated spores of Bacillus subtilis

Spores of Bacillus subtilis NCTC 8236 were treated with biocides and then subjected to various revival procedures. Sodium hydroxide (optimum concentration 25 mmol l-1) revived a small portion of glutaraldehyde-treated spores but not of spores exposed to formaldehyde, polyvinylpyrrolidone-iodine (PVP-I), Lugol's iodine, sodium hypochlorite or sodium dichloroisocyanurate (NaDCC). Post-treatment heat shock (at 70 degrees or 80 degrees C) increased the numbers of colony-forming units (cfu) of formaldehyde-injured spores. Coat-extraction procedures had the greatest effect on iodine-pretreated spores. The uptake of iodine and chlorine was more rapid and occurred to a greater extent with outgrowing, germinating and especially coat-deficient spores than with mature, resting spores.

Bacterial spores and chemical sporicidal agents

Bacterial spores are among the most resistant of all living cells to biocides, although the response depends on the stage of sporulation. The development of resistance to some agents such as chlorhexidine occurs much earlier in sporulation than does resistance to glutaraldehyde, which is a very late event. During germination or outgrowth or both, resistance is lost and the cells become as susceptible to biocides as nonsporulating bacteria. Mechanisms of spore resistance to, and the action of, biocides are discussed, and possible means of enhancing antispore activity are considered. The clinical and other uses of sporicidal and sporostatic chemical agents are described.

Possible mechanisms for the revival of glutaraldehyde-treated spores of Bacillus subtilis NCTC 8236

Spores of Bacillus subtilis NCTC 8236 were exposed to 2% alkaline glutaraldehyde and subsequently subjected to various treatments in an attempt to revive injured spores. Treatment with alkali (sodium or potassium hydroxide or, to a lesser extent, sodium bicarbonate) proved to be most successful. Some revival was achieved after thermal treatment. No revival was obtained with lysozyme or with various types of coat-removing agents. Experiments designed to distinguish between germination and outgrowth in the revival process established that sodium hydroxide (optimum concentration, 20 mmol/l) added to glutaraldehyde-treated spores increased the potential for germination. In contrast, spores which had been allowed to germinate before exposure to low concentrations of glutaraldehyde and then to sodium hydroxide were inhibited at the outgrowth phase to a much greater extent than germinated spores treated with the dialdehyde without subsequent alkali exposure. The results overall are discussed in terms of the possible mechanism and site of action of glutaraldehyde and the practical implications and significance of its use as a sporicide.

Effects of aqueous and alcoholic povidone-iodine on spores of Bacillus subtilis

Spores of Bacillus subtilis NCTC 10073 were examined for susceptibility to two proprietary brands of povidone-iodine: an aqueous solution, Betadine and an alcoholic solution, Videne. Spores were converted to ion-exchange (Ca, H) and coat-defective (SLS-, UME-, UMS-, UDT- and UDS-treated) forms. The resistance of these to povidone-iodine was compared and related to uptake. Effects on spore protoplasts and cortex in relation to hexosamine release were also examined. The degree of spore penetration and site of action of povidone-iodine is discussed.