Inhibition of Bacterial Growth In Vitro Following ... - Physical Therapy

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Controls Seeded plates with the wire electrodes in place were incubated without exposure to HVPC to determine whether the wires themselves, the medium, or a combination of both would be inhibitory to bacterial growth. Plates were also prepared with sterile medium (ie, without organisms added) to determine whether wire-insertion preparation introduced contamination. The presence of potential toxic electrochemical end products from the interaction of current, wire, and medium was determined by sending current through sterile medium without organisms. Extreme conditions (ie, beyond a normal therapeutic range) of 500 V for either 30 minutes or 2 hours were used to allow maximum potential toxin production. Overlays of medium containing either S aureus or E coli were poured into the plates, allowed to solidify, and then incubated and evaluated as previously described. Data Analysis Significance of zone width, as a function of the organism, was analyzed by a two-way analysis of variance (ANOVA). 20 A regression analysis was performed using the MIDAS (Michigan Interactive Data Analysis System) program on the Michigan Terminal System. Results Inhibition of growth of S aureus, E coli and P aeruginosa at the cathode (negative electrode) following exposure to HVPC is shown in Figure 4. Each line represents pooled data from three organisms, three replicates each, at 300, 250, 200, and 150 V for 1 to 4 hours of exposure. The ANOVA of the data at 4 hours of exposure revealed that differences using different microorganisms were not significant (F = 2.18; df = 2,18; p < .10), whereas varying the voltage resulted in highly significant differences (F = 152.98; df= 2,18; p
The current waveform from the instrument changed greatly as it flowed through the test medium. The resistance to flow varied, making it impossible to measure actual current flow. Figure 2 shows a reduction in total current and resistance to flow,as indicated by the baseline not returning to zero, as the waveform leaves the stimulator and passes through the medium. Discussion High voltage pulsed current can be effective in killing common woundinfecting bacteria in vitro. All organisms tested were equally affected by 2 hours of exposure to HVPC above 250 V. The data analysis revealed a strong positive linear relationship between the voltage and the duration of exposure to HVPC. Exposure to HVPC at the cathode accounted for most or all killing of bacterial cells. The increasing pH observed at the cathode was transient and probably did not reach levels extreme enough to directly kill bacterial cells. No effect on skin pH following a 30-minute application of HVPC at 100 V was reported by Newton and Karselis. 20 Such a rise in pH could have a static effect on growth that, combined with the lethal effect of HVPC, would help to keep bacterial population levels down and enable body defenses to fightoff the infection. Effects of HVPC at the anode were complicated by production of some toxic electrochemical end products created by passing current through the wire. Zones of inhibition around the cathode were recolonized by motile bacteria, suggesting that no permanent change had occurred there. In contrast, organisms were unable to recolonize the zone of discoloration around the anode, suggesting that lethal end products had accumulated and persisted. These results are comparable to those reported by Barranco et al. 14 Caution must be exercised when attempting to extrapolate the findings of in vitro studies to predict results Fig. A. Width of zone of inhibition at cathode after exposure to high voltage pulse current at 300, 250, 200, and 150 V versus duration of exposure. Points represent mean (± range) of pooled data from Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa. when applying the same intervention to infected wounds in human subjects. Present treatment protocols for use of HVPC on infected wounds, however, generally indicate a treatment duration of 20 to 45 minutes once or twice a day, with voltage amplitude adjusted to a subthreshold level for muscle contraction. Our study used a much higher voltage setting for a much longer exposure duration than those used in current clinical practice. Human subjects may not be able to tolerate such high voltage applications. Alternatively, the actual current flow through the petri plates was very low because of resistance from the test medium. If there is less resistance to current flow in human skin, then lower settings might be bactericidally effective in a clinical setting. Future avenues of investigation include in vivo application of HVPC to infected wounds as well as the application of other types of electrical current to microorganisms in vitro. If future studies indicate that the exposure of infected wounds to electrical current results in decreased infection, then it may be possible to effectively treat infected wounds on a home-care basis with portable electrical stimulators, thereby making wound management more cost effective. Conclusion Some clinicians use HVPC to inhibit bacterial growth in infected wounds. The results of this study indicate that, although there is inhibition or killing of bacteria in vitro at the cathode with the application of HVPC, either the voltage applied or duration of treatment, or both, may need to be substantially increased to achieve healing of infected wounds. Studies conducted in vivo are necessary to 32/654 Physical Therapy/Volume 69, Number 8/August 1989 Downloaded from http://ptjournal.apta.org/ by guest on January 11, 2014
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