Positive and Negative Ions Potently Inhibit the Viability of Airborne Gram-Positive and Gram-Negative Bacteria

Abstract

Positive and negative ions (PAIs and NAIs, respectively) generated by air ionizers curb indoor spread of airborne pathogens through cellular oxidative damage. Thus, here, we asked whether ion exposure of Staphylococcus aureus and Escherichia coli bacteria-either plated on agar or trapped in air filters-would affect their viability and whether this effect would be influenced by variations in bacterial type and load, action area, distance from the ion generator, exposure time, or filter type. We selected these two vegetative bacterium species because, besides being representative of Gram-positive and Gram-negative strains, respectively, they are widely recognized as the two most common airborne pathogens. We observed a robust ion inhibitory effect on the viability of free bacteria regardless of the experimental condition employed. Specifically, 12-h ion exposure of plated S. aureus and E. coli, at either 5 cm or 10 cm from the ion source, reduced bacterial viability by ∼95% and 70%, respectively. Furthermore, 3-h ion exposure was sufficient to reduce the viability of both bacterial species trapped in filters. Our results showing a strong antibacterial activity of PAI and NAI under all experimental conditions tested further support the use of air ionizers for preventing and/or containing airborne infection in domestic and nondomestic settings. IMPORTANCE Indoor air is a well-established vehicle for direct and indirect spread of a wide variety of human pathogens-as bioaerosols are composed of bacteria, viruses, fungi, and other types of organisms-that may trigger some pathologies. Plasmacluster ionizers are known for their ability to generate positively or negatively charged air ions (PAIs and NAIs, respectively) that can kill/inactivate indoor airborne pathogens, through oxidative stress-induced damage, in various environments. Given these premises, the aim of this study was to evaluate the viability of Gram-positive and Gram-negative bacteria exposed to PAI and NAI under different experimental variables such as bacterial type and load, action area, distance from the ion generator, ion exposure time, and filter type. Altogether, our findings, demonstrating a remarkable PAI and NAI antibacterial activity, stress the importance of using air ionizers to prevent indoor airborne infection.

Keywords: Escherichia coli; Staphylococcus aureus; air filters; air ionizer; antibacterial activity.

FIG 1

(A and B) Direct ion effect on the viability of S. aureus (A) and E. coli (B) plated at 10⁴ CFU/ml on 150-mm petri dishes, with the ionizer being placed at a distance of 5 or 10 cm. n ≥ 3 replicate experiments. *, P < 0.05, Student’s t tests.

FIG 2

(A and B) Ion effect on the viability of S. aureus trapped in PP (A) or PET (B) filters at 10⁴ CFU/ml, with the ionizer being placed at a distance of 5 or 10 cm. n ≥ 3 replicate experiments. *, P < 0.05, Student’s t tests.

FIG 3

(A and B) Ion effect on the viability of E. coli trapped in PP (A) or PET (B) filters, at 10⁴ CFU/ml, with the ionizer being placed at a distance of 5 or 10 cm. n ≥ 3 replicate experiments.

FIG 4

Schematic representation of the ionization setup. (a) vertical flow cabinet; (b) battery; (c) ionizer support; (d) ionizer; (e) NA petri dish with seeded bacteria or empty petri dish with bacteria trapped into filters exposed to ions; (f) NA petri dish with seeded bacteria or empty petri dish with bacteria trapped into filters not exposed to ions (controls); (g) container with hot water to keep a constant humidity; (h) humidity/temperature data recorder; (i) UV lamp.