Antimicrobial activity and stability of weakly acidified chlorous acid water

Antiviral agents and disinfectants for foot‑and‑mouth disease (Review)

Fluorouracil, 5-azacytidine, 6-azauridine, ribavirin, favipiravir (T-705) and its derivative (T-1105) exhibit anti-foot-and-mouth disease virus (FMDV) effects. In particular, T-1105 exhibits promising results when administered to guinea pigs orally, and pigs in their feed. FMDV is excreted in the early stages of infection in aerosols and oral or nasal droplets from animals. T-1105 along with the FMDV vaccine can be used to combat foot-and-mouth disease (FMD) epidemics. Several studies have shown that sodium hypochlorous solutions are widely used to inactivate viruses, including FMDV. However, these solutions must be stored under cool and dark conditions to maintain their virucidal effects. Interestingly, a study indicated that the virucidal activity of a calcium bicarbonate solution with a mesoscopic structure (CAC-717) did not decrease after storage at room temperature for at least four years outside direct sunlight. Numerous lessons acquired from the 2010 FMD outbreak in Japan are relevant for the control of COVID-19. However, the widespread use of chlorite can cause environmental issues. Chlorite can be combined with nitrogen to produce chloramine or N-nitrosodimethylamine, which plays a role in carcinogenesis. Therefore, risk assessments should be conducted in aquatic environments. Moreover, there is a need to develop nonchlorine disinfectants that can be used during epidemics, including FMD. The approach of 'One Health' should be shared between the public health and veterinary fields to improve the management of viral outbreaks, including those due to FMD.

Atypical Melissococcus plutonius strains: their characteristics, virulence, epidemiology, and mysteries

Melissococcus plutonius is a Gram-positive lanceolate coccus that is the causative agent of European foulbrood, an important bacterial disease of honey bee brood. Although this bacterium was originally described in the early 20th century, a culture method for this bacterium was not established until more than 40 years after its discovery due to its fastidious characteristics, including the requirement for high potassium and anaerobic/microaerophilic conditions. These characteristics were considered to be common to the majority of M. plutonius strains isolated worldwide, and M. plutonius was also thought to be genetically homologous or clonal for years. However, non-fastidious variants of this species (designated as atypical M. plutonius) were very recently identified in Japan. Although the morphology of these unusual strains was similar to that of traditionally well-known M. plutonius strains, atypical strains were genetically very different from most of the M. plutonius strains previously isolated and were highly virulent to individual bee larva. These atypical variants were initially considered to be unique to Japan, but were subsequently found worldwide; however, the frequency of isolation varied from country to country. The background of the discovery of atypical M. plutonius in Japan and current knowledge on atypical strains, including their biochemical and culture characteristics, virulence, detection methods, and global distribution, are described in this review. Remaining mysteries related to atypical M. plutonius and directions for future research are also discussed.

Elucidation of composition of chlorine compounds in acidic sodium chlorite solution using ion chromatography

With the spread of coronavirus infections, the demand for disinfectants, such as a sodium chlorite solution, has increased worldwide. Sodium chlorite solution is a food additive and is used in a wide range of applications. There is evidence that chlorous acid or sodium chlorite is effective against various bacteria, but the actual mechanism is not well understood. One reason for this is that the composition of chlorine-based compounds contained in sodium chlorite solutions has not been clearly elucidated. The composition can vary greatly with pH. In addition, the conventional iodometric titration method, the N,N-diethyl-p-phenylenediamine sulfate (DPD) method and the absorption photometric method cannot clarify the composition. In this study, we attempted to elucidate the composition of a sodium chlorite solution using absorption spectrophotometry and ion chromatography (IC). IC is excellent for qualitative and quantitative analysis of trace ions. Through this, we aimed to develop an evaluation method that allows anyone to easily determine the bactericidal power of sodium chlorite. We found that commercially available sodium chlorite solution is 80% pure, with the remaining 20% potentially containing sodium hypochlorite solution. In addition, when sodium chlorite solution became acidified, its absorption spectrum exhibited a peak at 365 nm. Sodium chlorite solution is normally alkaline, and it cannot be measured by the DPD method, which is only applicable under acidic conditions. The presence of a peak at 365 nm indicates that the acidic sodium chlorite solution contains species with oxidizing power. On the other hand, the IC analysis showed a gradual decrease in chlorite ions in the acidic sodium chlorite solution. These results indicate that chlorite ions may not react with this DPD reagent, and other oxidizing species may be present in the acidic sodium chlorite solution. In summary, when a sodium chlorite solution becomes acidic, chlorine-based oxidizing species produce an absorption peak at 365 nm. Sodium hypochlorite and sodium chlorite solutions have completely different IC peak profiles. Although there are still many problems to be solved, we believe that the use of IC will facilitate the elucidation of the composition of sodium chlorite solution and its sterilization mechanism.

Virus purification highlights the high susceptibility of SARS-CoV-2 to a chlorine-based disinfectant, chlorous acid

Chlorous acid water (HClO2) is known for its antimicrobial activity. In this study, we attempted to accurately assess the ability of chlorous acid water to inactivate SARS-CoV-2. When using cell culture supernatants of infected cells as the test virus, the 99% inactivation concentration (IC99) for the SARS-CoV-2 D614G variant, as well as the Delta and Omicron variants, was approximately 10ppm of free chlorine concentration with a reaction time of 10 minutes. On the other hand, in experiments using a more purified virus, the IC99 of chlorous acid water was 0.41-0.74ppm with a reaction time of 1 minute, showing a strong inactivation capacity over 200 times. With sodium hypochlorite water, the IC99 was 0.54ppm, confirming that these chlorine compounds have a potent inactivation effect against SARS-CoV-2. However, it became clear that when using cell culture supernatants of infected cells as the test virus, the effect is masked by impurities such as amino acids contained therein. Also, when proteins (0.5% polypeptone, or 0.3% BSA + 0.3% sheep red blood cells, or 5% FBS) were added to the purified virus, the IC99 values became high, ranging from 5.3 to 76ppm with a reaction time of 10 minutes, significantly reducing the effect. However, considering that the usual usage concentration is 200ppm, it was shown that chlorous acid water can still exert sufficient disinfection effects even in the presence of proteins. Further research is needed to confirm the practical applications and effects of chlorous acid water, but it has the potential to be an important tool for preventing the spread of SARS-CoV-2.

Efficacy of sodium chlorite in inactivating Vibrio parahaemolyticus attached to polyethylene terephthalate surfaces

The inactivation of Vibrio parahaemolyticus cells attached to a polyethylene terephthalate (PET) disc in a sodium chlorite (NaClO2) solution was kinetically studied in a weakly acidic pH range of 4.0 - 6.5. The logarithmic reduction in the survival ratio depended on the concentration-time product. All inactivation curves showed a linear reduction phase, and the reduction in viable cells was greater than 4-log. No significant desorption of attached cells was observed during the inactivation treatment. The first-order inactivation rate constant (k) increased by approximately 4.5-fold for every 1.0 unit fall in pH. At all pH values, the k values calculated for the attached cells were approximately half of those for the unattached cells. These findings indicate that a weakly acidic NaClO2 solution is effective in inactivating bacteria attached to hard surfaces.

Comparative evaluation of chlorous acid and sodium hypochlorite activity against SARS-CoV-2

A novel coronavirus, named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), suddenly emerged in China in 2019, spread globally and caused the present COVID-19 pandemic. Therefore, to mitigate SARS-CoV-2 infection effective measures are essential. Chlorous acid (HClO₂) has been shown to be an effective antimicrobial agent. However, at present there is no experimental evidence showing that HClO₂ can inactivate SARS-CoV-2. Therefore, in this study, we examined the potential of HClO₂ to inactivate SARS-CoV-2 in presence or absence of organic matter and the results were compared with that of sodium hypochlorite (NaClO), another potent antimicrobial agent. When concentrated SARS-CoV-2 was incubated with 10 ppm HClO₂ for 10 s, viral titre was decreased by 5 log of 50% tissue culture infective dose per mL (TCID₅₀ ml⁻¹). However, the same concentration of NaClO could not inactivate SARS-CoV-2 as effectively as HClO₂ did even after incubation for 3 min. Furthermore, 10 ppm HClO₂ also inactivated more than 4.0 log of TCID₅₀ within 10 s in the presence of 5 % fetal bovine serum used as mixed organic matters. Our results obtained with HClO₂ are more effective against SARS-CoV-2 as compared to NaClO that can be used for disinfectant against SARS-CoV-2 .

Universal Virucidal Activity of Calcium Bicarbonate Mesoscopic Crystals That Provides an Effective and Biosafe Disinfectant

We investigated the virucidal effects in solution of a new type of disinfectant, calcium bicarbonate mesoscopic crystals, designated CAC-717, against various types of virus. CAC-717 in solution is alkaline (pH 12.4) and has a self-electromotive force that generates pulsed electrical fields. Upon application to human skin, the pH of the solution becomes 8.4. CAC-717 contains no harmful chemicals and is thus non-irritating and harmless to humans and animals. Its virucidal effects were tested against six types of animal virus: enveloped double-strand (ds)-DNA viruses, non-enveloped ds-DNA viruses, non-enveloped single strand (ss)-DNA viruses, enveloped ss-RNA viruses, non-enveloped ss-RNA viruses, and non-enveloped ds-RNA viruses. The treatment resulted in a reduction in viral titer of at least 3.00 log₁₀ to 6.38 log₁₀. Fetal bovine serum was added as a representative organic substance. When its concentration was ≥20%, the virucidal effect of CAC-717 was reduced. Real-time PCR revealed that CAC-717 did not reduce the quantity of genomic DNA of most of the DNA viruses, but it greatly reduced that of the genomic RNA of most of the RNA viruses. CAC-717 may therefore be a useful biosafe disinfectant for use against a broad range of viruses.

Inactivation of human norovirus by chlorous acid water, a novel chlorine-based disinfectant

INTRODUCTION

Human norovirus (HuNoV) is a leading cause of infectious gastroenteritis. Since HuNoV shows resistance to alcohol, chlorine-based sanitizers are applied to decontaminate the virus on environmental surfaces. Chlorous acid water (CA) has been recently approved as a novel chlorine-based disinfectant categorized as a Type 2 OTC medicine in Japan. In this study, we aimed to evaluate the capability of CA to inactivate HuNoV.

METHODS

HuNoV (genogroups GII.2 and GII.4) was exposed to the test disinfectants including CA and sodium hypochlorite (NaClO), and the residual RNA copy was measured by reverse transcription quantitative PCR (RT-qPCR) after pretreatment with RNase. In addition, the log₁₀ reduction of HuNoV RNA copy number by CA and NaClO was compared in the presence of bovine serum albumin (BSA), sheep red blood cells (SRBC), polypeptone, meat extract or amino acids to evaluate the stability of these disinfectants under organic-matter-rich conditions.

RESULTS

In the absence of organic substances, CA with 200 ppm free available chlorine provided >3.0 log₁₀ reduction in the HuNoV RNA copy number within 5 min. Even under high organic matter load (0.3% each of BSA and SRBC or 0.5% polypeptone), 200 ppm CA achieved >3.0 log₁₀ reduction in HuNoV RNA copy number while less than 1.0 log₁₀ reduction was observed with 1,000 ppm sodium hypochlorite (NaClO) in the presence of 0.5% polypeptone. CA reacted with only cysteine, histidine and glutathione while NaClO reacted with all of the amino acids tested.

CONCLUSIONS

CA is an effective disinfectant to inactivate HuNoV under organic-matter-rich conditions.

Efficacy of Bioshell Calcium Oxide Water as Disinfectants to Enable Face Mask Reuse

Bioshell calcium oxide (BiSCaO) is derived from scallop shells and after heat treatment exhibits broad microbicidal activity. BiSCaO Water is a disinfectant prepared by collecting the aqueous layer after adding BiSCaO powder to water, is colorless and transparent, and has a pH of 12.8. We compared the utility of commercially available BiSCaO Water, ethanol, sodium hypochlorite, hypochlorous acid and hydrogen peroxide solutions as sterilization agents to enable the reuse of surgical and N95 face masks. The microbicidal efficacy of each disinfectant was evaluated using pieces of surgical and N95 face masks contaminated with normal bacterial flora. The results suggest that BiSCaO Water has excellent disinfection activity toward contaminated polypropylene masks and has minimal adverse effect on the structure of non-woven masks.

Microbicidal effects of slightly acidic hypochlorous acid water and weakly acidified chlorous acid water on foulbrood pathogens

Paenibacillus larvae and Melissococcus plutonius are bacterial pathogens of honey bee brood. As decontamination of beekeeping equipment, including combs, is essential to control these pathogens, we evaluated the disinfecting effects of slightly acidic hypochlorous acid water (SAHAW) and weakly acidified chlorous acid water (WACAW) on the pathogens. Both disinfectants exhibited strong disinfecting effects in suspension tests under no organic matter conditions and reduced both pathogens by >5 log₁₀ CFU/ml. Although the microbicidal activity of SAHAW with an available chlorine concentration (ACC) of 10–30 ppm was decreased by organic matter, it reduced viable P. larvae spores in combs more efficiently than H₂O when the comb was not as dirty. However, its efficacy on combs decreased at 4°C and when overused or highly contaminated combs were tested. WACAW with an ACC of ≥600 ppm had a higher disinfecting capacity than SAHAW, and efficiently removed P. larvae spores from combs even under organic matter-rich and low-temperature conditions. However, even by WACAW, the amount of viable spores in combs was not markedly reduced depending on contamination levels and P. larvae genotypes. These results suggest the usefulness of both disinfectants for decontaminating beekeeping equipment depending on the situations expected.

Comparison of Various Disinfectants on Bactericidal Activity Under Organic Matter Contaminated Environments

The bactericidal activity of heated bio-shell calcium oxide (BiSCaO) powder suspension (pH 12.4) , hypochlorous acid (HClO; pH 6), sodium hypochlorite (NaClO; pH 8) , povidone-iodine (Isodine solution^®) , and chlorhexidine gluconate (Hibiscrub^®) under organic matter contaminated environments were compared for tests conducted on wood scraps and pig skin pieces that were incubated with normal bacterial flora (total viable counts and coliform bacteria) . The test results showed that BiSCaO suspension had higher bactericidal activity than HClO and NaClO. Furthermore, more than 10-fold higher concentrations of antiseptics such as povidone-iodine and chlorhexidine gluconate were required to achieve bactericidal activity comparable to that of BiSCaO suspension. Our results demonstrate the possibility of using BiSCaO suspension under organic matter contaminated environments as a disinfectant for environmental and food hygiene applications.

Stability of Weakly Acidic Hypochlorous Acid Solution with Microbicidal Activity

　Hypochlorous acid (HOCl) solution (200 ppm, pH 6) was prepared and evaluated for their stabilities and microbicidal activities. We demonstrated that HOCl is unstable against ultraviolet (UV) light, sunshine, contact with air, and elevated temperature (≧25℃). Furthermore, in the HOCl solution, the presence of excess NH₂- or CHO-containing organic compounds such as proteins and carbohydrates, or of inorganic ions such as NO₂^-, SO₃^-, PO₃^-, Fe²⁺, Cu²⁺, and CuS, resulted in the rapid consumption of HOCl by oxidation reactions, and significantly decreased the microbicidal activity of the HOCl solution against coliform bacteria and total viable cell count. Thus, production of stable HOCl solution requires formulation in pure water harboring concentrations as low as possible of various compounds and ions, as well as storage in dark and cool conditions (<10℃) to maintain the concentration of HOCl molecules and microbicidal activity.

Antimicrobial effect of a combination of herb extract and organic acid against Bacillus subtilis spores

The sporicidal activities of the herbs were investigated to screen for novel antimicrobial substances against Bacillus subtilis spores. The bacterial inactivation effects of ethanol extracts of coriander, caraway, mace at concentrations of 0.5% (w/v) and 1.0-2.5% were about 10- and 100-fold respectively against spores. At pH 5, the antimicrobial activity was about 92%, but at pH 4 the sporicidal activity was particularly high, reducing the spore count by 99.99%. The 0.1-2.5% ethanol extract of herbs adjusted to pH 4-5 exhibited significantly marked deactivation effects, with 3-4 log CFU/mL reductions. The herb-acid combination exerted a further increase in sporicidal activity, with an additional 1-3 log CFU/mL reduction. The sporicidal mechanism was assumed to involve a two-step: (1) the hydrophobic binding of surfactants in the herbs onto the spore coat destroys its protein, and (2) the acid then penetrates into the interior, generating unstable growth conditions.

Microbicidal effects of weakly acidified chlorous acid water against feline calicivirus and Clostridium difficile spores under protein-rich conditions

Sanitation of environmental surfaces with chlorine based-disinfectants is a principal measure to control outbreaks of norovirus or Clostridium difficile. The microbicidal activity of chlorine-based disinfectants depends on the free available chlorine (FAC), but their oxidative potential is rapidly eliminated by organic matter. In this study, the microbicidal activities of weakly acidified chlorous acid water (WACAW) and sodium hypochlorite solution (NaClO) against feline calcivirus (FCV) and C. difficile spores were compared in protein-rich conditions. WACAW inactivated FCV and C. difficile spores better than NaClO under all experimental conditions used in this study. WACAW above 100 ppm FAC decreased FCV >4 log10 within 30 sec in the presence of 0.5% each of bovine serum albumin (BSA), polypeptone or meat extract. Even in the presence of 5% BSA, WACAW at 600 ppm FAC reduced FCV >4 log10 within 30 sec. Polypeptone inhibited the virucidal activity of WACAW against FCV more so than BSA or meat extract. WACAW at 200 ppm FAC decreased C. difficile spores >3 log10 within 1 min in the presence of 0.5% polypeptone. The microbicidal activity of NaClO was extensively diminished in the presence of organic matter. WACAW recovered its FAC to the initial level after partial neutralization by sodium thiosulfate, while no restoration of the FAC was observed in NaClO. These results indicate that WACAW is relatively stable under organic matter-rich conditions and therefore may be useful for treating environmental surfaces contaminated by human excretions.

Kinetics of the Inactivation of Vibrio parahaemolyticus in Weakly Acidic Sodium Chlorite Solution

　The kinetics of the inactivation of Vibrio parahaemolyticus in sodium chlorite (NaClO₂) solution was studied in the weakly acidic pH range of 4.0 to 6.5 and at various temperatures. The logarithmic reduction of the survival ratio depended on the concentration-time product, and all the inactivation curves showed a linear reduction phase. The first-order inactivation rate constant (k) increased by approximately twice for every 0.44 unit fall in pH. During the inactivation experiments, no formation of chlorine dioxide occurred. These data indicated that undissociated HClO₂ was the active species governing the inactivation of V. parahaemolyticus. It was also shown that the use of weakly acidic NaClO₂ solutions containing high concentrations of ionized ClO₂^- gave slower kinetics of the inactivation, whereas it could achieve the significant reduction of viable cells of more than 4-log. The k value showed an Arrhenius-type temperature dependence in the temperature range of 5 to 40℃. The apparent activation energy for the inactivation of V. parahaemolyticus was estimated to be 43.5 kJ/mol. The k value increased by approximately 1.8 times for every 10℃ rise in temperature.