In the study bacteria in pure culture on agar were exposed to air ions in a test rig, as shown in Figure 1. Experimentation was undertaken on sessile cultures of Staphylococcus aureus (NCTC 10399/ATCC 13709), Mycobacterium parafortuitum (NCTC 10410/ATCC 19687), Pseudomonas aeruginosa (NCIMB 10848), Acinetobacter baumanii (NCTC 12156/ATCC 19606), Burkholderia cenocepacia (NCTC 10744/ATCC25609), Bacillus subtilis (NCTC 10106/ATCC33234) and Serratia marcescens (NCTC 1377/ATCC 274. Cultures were exposed to positive air ions and negative air ions in separate experiments. In order to ensure uniform ionic exposure the agar plates were earthed and placed directly below a 7-pin electrode set, with the surface of the agar being 25 mm below the electrode tips. A high voltage generator (Brandenburg Alpha III, Brandenberg, UK) was used to apply a DC potential between the pin electrodes and the earthed agar plate. During experimentation an ammeter located on the earth conductor measured the current associated with the ions impinging on the agar plate.
In order to distinguish between effects arising from: (i) the action of the air ions; (ii) the action of the electric field, and (iii) the action of any ozone produced as a by-product of the corona discharge, two interventions were made to the experimental procedure. The first intervention involved placing a thin mica sheet between the 7-pin electrode set and the agar plate, which while leaving the electric field largely unaltered, prevented the air ions from reaching the bacteria. It should be noted that with free charges, the mica plate may have altered the space charge generated by the corona and thus may have modified the field distribution slightly. In addition, the mica plate prevented ozone generated by the electrical discharge from reaching the bacteria. The second intervention involved placing an earthed wire mesh 20 mm below the electrode set, and above the agar plates. This had the effect of greatly reducing (i.e. to negligible levels) the electric field strength and the number of air ions to which the bacteria were exposed. However, ozone generated by the discharge could still reach the agar plate.
All the ion exposure experiments were undertaken under ambient room conditions. All seven bacterial species were exposed to negative air ions (with an electrode potential of -10 kV) for periods of 5, 10 and 15 minutes, with five replicates taken on each occasion. The process was then repeated for samples exposed to positive ions, with an electrode potential of +10 kV. The negative ion exposure experiments were repeated twice more, first with a mica sheet placed above the agar plate and then with an earthed wire mesh located as described above. During experimentation the earth current and moisture loss from the agar plates were recorded. Ozone levels were also monitored using a portable detector (Model A-21ZX, Eco Sensors, USA).
The results of the negative air ion experiment (i.e. without any interventions) are presented in Figure 2, which shows the survival fraction verses exposure time for each bacterial species. From these it can be seen that in every case exposure to negative air ions was associated with a marked reduction in colony count. Statistical analysis using a T-test (two-tailed with equal variance) reveals that for all bacteria species this reduction was significant (p
The results of the positive air ion experiment are presented in Figure 3. In marked contrast to the negative ion results, it can be seen that exposure to positive ions produced a dissimilar effect in the various test bacteria. A substantial reduction was observed for Mycobacterium parafortuitum (96.0% after 15 minutes exposure) (p Bacillus subtilis (70.8% after 15 minutes) (p Pseudomonas aeruginosa (31.4% after 15 minutes) (p Acinetobacter baumanii (31.2% after 15 minutes) (p Burkholderia cenocepacia (32.1% after 15 minutes) (p = 0.059) and Serratia marcescens (24.2% after 15 minutes) (p = 0.002). However, no bactericidal effect was observed for Staphylococcus aureus (p = 0.345). With the exception of Mycobacterium parafortuitum the bactericidal effect produced when the bacteria were exposed to positive ions was much less than that achieved by the negative ions.
The results of the mica plate experiment, revealing the effect of the electric field alone, are presented in Figure 4. Comparison of these results with those in Figure 2 reveals that the intervention of the mica plate had a marked effect on the bactericidal action of the negative ions. This suggests that for most of the bacteria species tested, the bactericidal action observed in Figure 2 was not primarily due to the action of the electric field. However, from Figure 4 it can be seen that for one species in particular, Mycobacterium parafortuitum, the action of the electric field alone appears to have resulted in a strong bactericidal effect, with a 94.9% reduction (p Burkholderia cenocepacia (48.7% after 15 minutes) (p Acinetobacter baumanii (44.0% after 15 minutes) (p Staphylococcus aureus (32.3% after 15 minutes) (p = 0.005), with only marginal reductions occurring for Bacillus subtilis (14.9% after 15 minutes) (p = 0.516) and Pseudomonas aeruginosa (9.3% after 15 minutes) (p = 0.192).
The results of the wire mesh experiment, in which the action of the electric field and the air ions were reduced to negligible levels, are presented in Figure 5. All the bacterial species exhibited marked reductions (p
The mean currents (together with the maximum and minimum values) recorded during the various experiments are presented in Table 1. These values for the positive and negative ion experiments (i.e. columns 2 and 3) indicate that although the potential of the electrode was constant for the each experiment, there was some variation in the current produced. This phenomenon was probably due to variations in the conductivity of the agar, and day-to-day variations in temperature, humidity and air movement in the laboratory. With regard to the mica plate experiment, it can be seen that the current flow was negligible, indicating the lack of any ion flow to the agar plate. Similarly, the earthed wire mesh prevented any current flowing to the agar plate.
During production of negative ions the ozone concentration in the vicinity of the agar plate was found to be 2.3 ppm. However, during positive ion production this value was found to be considerably lower, about 0.8 ppm.
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