Degradation and Deactivation of Bacterial Antibiotic Resistance Genes during Exposure to Free Chlorine, Monochloramine, Chlorine Dioxide, Ozone, Ultraviolet Light, and Hydroxyl Radical

This
work investigated degradation (measured by
qPCR) and biological deactivation (measured by culture-based
natural transformation) of extra- and intracellular antibiotic resistance
genes (eARGs and iARGs) by free available chlorine (FAC), NH2Cl, O3, ClO2, and UV light (254 nm), and of
eARGs by •OH, using a chromosomal ARG (blt) of multidrug-resistant Bacillus subtilis 1A189.
Rate constants for degradation of four 266–1017
bp amplicons adjacent to or encompassing the acfA mutation enabling blt overexpression increased
in proportion to #AT+GC bps/amplicon, or in proportion to #5′-GG-3′
or 5′-TT-3′ doublets/amplicon, with respective values
ranging from 0.59 to 2.3 (×1011 M–1 s–1) for •OH, 1.8–6.9
(×104 M–1 s–1)
for O3, 3.9–9.2 (×103 M–1 s–1) for FAC, 0.35–1.2­(×101 M–1 s–1) for ClO2, and 2.0–8.8 (×10–2 cm2/mJ) for UV at pH 7, and from 1.7–4.4 M–1 s–1 for NH2Cl at pH 8. For FAC, NH2Cl, O3, ClO2, and UV, ARG deactivation paralleled degradation of amplicons approximating
a ∼800–1000 bp acfA-flanking sequence
required for natural transformation in B. subtilis, whereas deactivation outpaced degradation for •OH. At practical disinfectant exposures,
eARGs and iARGs were ≥90% degraded/deactivated by FAC, O3, and UV, but recalcitrant to NH2Cl and ClO2. iARG degradation/deactivation always lagged cell inactivation. These findings
provide a quantitative framework for evaluating ARG fate during disinfection/oxidation,
and support using qPCR as a proxy for tracking ARG deactivation under carefully selected circumstances.