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American Society for Microbiology Presentation
2021 World Microbe Forum
June 24, 2021

Comparison Of HOCl And HOBr For Efficacy Of Inactivation Of Scrapie Prions, MS2 Bacteriophage And Various Microbes

Expectations of chemical decontamination measures for medical instrumentation and surfaces in hospitals have been eroded by accumulating evidence that certain commonly used formulations are not reliably effective for the full spectrum of contemporary infection control challenges (e.g., HPV, infectious proteins, Candida auris). Hypochlorous acid (HOCl) has been shown to rapidly inactivate infectious prions and MS2 bacteriophage. However, hypobromous acid (HOBr) containing mixtures have been reported to be more potent biocides. To directly compare the biological properties of HOCl and HOBr we characterized the hypohalous acids using analytical spectroscopy and spectrophotometry prior to their use. HOCl and HOBr were tested for efficacy against Scrapie Prions, MS2 bacteriophage, E. coli, S. aureus­ and A. niger.

Characterization of HOCl and HOBr Solution
Figure 1.  Characterization of HOCl and HOBr by UV-Visible and Raman spectroscopy at 22 °C. A). UV-Vis spectra of 147 ppm HOCl at pH 3.7, -OCl at pH 11.8 and a mixture of HOCl and -OCl at pH 7.0.  B). UV-VIS spectra of HOCl (pH 4.0, 3.0 mM),  HOBr (pH 6.2, 3.0 mM) , and OBr- (pH 11.8, 3.0 mM).  C). Raman spectra of HOCl (5.3 mM).  D). Raman spectra of HOBr (1.8 mM). Results/Conclusions:
  • HOCl and HOBr are detected at 728 and 626 cm-1 by Raman Spectroscopy
  • No other chlorine compounds are detectable ​

  • Stability of HOCl and HOBr Solution
    Figure 2.  Stability of HOCl. A). Pseudo first order plot of the temperature dependence of HOCl at 22 °C, pHinitial = 4; 52 °C, pHinitial = 4; 70 °C, pHinitial = 4; and 70 °C, pHinitial = 7.   B). ORP measurements of HOCl solutions at 22 °C, pHinitial = 4; 52 °C, pHinitial = 4; 70 °C, pHinitial = 4; and 70 °C, pHinitial = 7.  C). pH values of HOCl solutions at 22 °C, pHinitial = 4; 52 °C, pHinitial = 4; 70 °C, pHinitial = 4; and 70 °C, pHinitial = 7.
    Figure 3.  Stability of HOBr. B). UV-Visible spectroscopy for HOBr at 22 °C over 9 days;  C). UV-Visible spectra measured at 260 nm of HOBr at 22 °C for 4 h. 
  • Whereas HOCl remained stable for the period of study (6months) HOBr is very short lived (hours) at RT
  • The ORP and pH of HOCl remained constant over the period of study
  • A slight decrease in concentration of active chlorine <20% is seen at elevated temperature

  • Efficacy of HOCl and HOBr Against Microbes, MS2 Bacteriophage and Scrapie Prions
    Figure 4.  Antimicrobial efficacy comparison of HOBr and HOCl. A). E. coli, 20 s exposure.  B). S. aureus, 40 s exposure.  C). A. niger, 40 s exposure. HOBr = blue; HOCl = black.
    Figure 5. Mean Log10 Reduction values (±SD) disinfection of dried MS2 bacteriophage on stainless steel coupons at varied concentrations of HOCl and HOBr after 5 minutes of contact time.
  • HOBr is more effective than HOCl against MS2, E. coli and S. aureus
  • HOCl is more effective against A. niger
  • HOBr and HOCl had similar efficacies against Scrapie Prions after 5 minutes of incubation with HOCl in the RT-QuIC assay.
  • These results are likely due to differences in the reactivity of the two hypohalous acids and their relative abilities to modify key compounds affecting  survival of the microbial targets studied.
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