The Impact of Noncontact, Nonthermal, Low-Frequency Ultrasound on Bacterial Counts in Experimental and Chronic Wounds
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It is important to note that biofilms have been found to be present at this depth, although biofilms were not investigated in these studies.
An investigation into whether the observed antimicrobial action in vitro could be effected in a porcine wound model showed a reduction in the overall quantity of bacteria present in a wound after a single week (four treatments) of noncontact ultrasound therapy. Noncontact ultrasound produced a similar, although less pronounced, reduction in total bioburden compared to a silver-impregnated dressing; whereas, bioburden in sham-treated and control animals increased during a similar time frame. In brief, these preclinical studies demonstrated that noncontact ultrasound can produce an antimicrobial effect.
Similar to the in vitro results, in the clinical study a difference was noted in the response of Gram-negative and Gram-positive bacteria to administration of noncontact ultrasound. Quantitative biopsy values for Staphylococcus species, including MRSA, decreased substantially between pre- to post-procedure sampling. However, Streptococcus G showed only a modest reduction and Streptococcus A counts increased in a single individual. Ultimately, healing progression was evident, with 26% reduction in area and 20% reduction in volume during the 2-week study.
Like most early-stage research, this series of experiments to elucidate the effects of noncontact ultrasound on bacterial counts has limitations and raises as many questions as it answers. Most notably, without a sham control in the human trial, it is impossible to discern whether the observed healing was the result of a reduction in bacteria alone or some other mechanisms. Larger human trials utilizing a sham control are planned to further investigate bacteria reduction. Additionally, although the treatment times (minutes) and frequencies (treatments per week) used in the laboratory studies were based on typical clinical application, it is not known whether these treatment parameters are in fact comparable for in vitro and animal application. Finally, further research is needed to investigate the differential effects of noncontact ultrasound on the various bacterial organisms observed in the animal and human models.
When considered together, the results of this four-part study to assess the effect of noncontact low-frequency ultrasound on wound bacterial counts suggest this therapy may reduce bioburden in the wound bed and have prompted additional research into the effectiveness of noncontact ultrasound therapy on the mechanical disruption of a biofilm. Chronic wounds are frequently characterized by high bioburden, the presence of senescent cells, nonviable slough, and scar tissue. 19
Noncontact ultrasound is currently used in the treatment of chronic wounds and indicated for debridement of yellow slough, fibrin, tissue exudates, and bacteria. The bacteria reduction observed in this series of studies suggests that noncontact ultrasound may play a role in reducing overall bacterial burden. Moreover, there is evidence that wounds with >105 CFU/g of tissue may not exhibit clinical signs of infection.20 As in most biological systems, the goal is balance, not necessarily extreme wound sterility that is achieved through antisepsis. 21 Future studies will need to focus not only on quantitative biopsy results, but also on the fingerprint of specific bacteria present in the wound. Techniques such as rapid polymerase chain reaction (PCR) analysis should facilitate this type of investigation. Certainly, nontoxic, effective therapy without the risk of developing bacterial resistance would be a promising option for preventing infection and healing infected wounds.
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