The Impact of Noncontact, Nonthermal, Low-Frequency Ultrasound on Bacterial Counts in Experimental and Chronic Wounds
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Preventing wound infection and the development of resistant bacteria are important concerns in wound management. To determine if noncontact, nonthermal, low-frequency ultrasound therapy is effective in controlling wound bacterial colony counts, a series of four related experiments was conducted. First, ultrasound penetration in both wounded and intact skin was assessed in vitro. Compared to sham, noncontact ultrasound penetrated farther into both wounded (3 mm to 3.5 mm versus 0.35 mm to 0.50 mm) and intact (2.0 mm to 2.5 mm versus 0.05 mm to 0.07 mm, respectively) pig skin. Second, using an in vitro model to stain and count live/dead bacteria, 0% of sham treated and 33% of Pseudomonas aeruginosa, 40% of Escherichia coli and 27% of Enterococcus faecalis were dead after one ultrasound application. Minimal effects on methicillin-resistant Staphylococcus aureus and S. aureus were observed. Third, using an in vivo model, after 1 week, while differences between different bacterial species were observed, overall bacterial quantity decreased with ultrasound treatment (from 7.2 ± 0.79 to 6.7 ± 0.91 colony forming units per gram of tissue [CFU/g]) and silver antimicrobial dressings (from 7.2 ± 0.79 to 5.7 ± 0.6 CFU/g) but increased to 8.6 ± 0.15 CFU/g for sham and 8.6 ± 0.06 CFU/g for water-moistened gauze. Fourth, 11 patients (average age 60 years) with pressure ulcers containing bacterial counts >105 CFU/g of tissue received 2 weeks of noncontact ultrasound therapy. The quantities of seven bacterial organisms were reduced substantially from baseline to 2 weeks post treatment. None of the wounds exhibited signs of a clinical infection during the treatment period and no adverse events were observed. Taken together, these four studies indicate that noncontact ultrasound can be used to reduce bacterial quantity. Controlled clinical studies are warranted to ascertain the efficacy of this treatment and to further elucidate its effects on various Gram-negative and Gram-positive bacteria.
All chronic wounds are contaminated with bacteria; however, bacteria virulence and quantity, together with the host’s immune response, determine the clinical response and, ultimately, if infection is present.1 A small bioburden in an immune-competent host may have no impact on healing; a large bioburden or an inadequate immune response can manifest clinically as either gross wound infection or as a “stunned,” nonhealing wound.2 Optimal diagnosis and treatment of wound bioburden are subject to debate and no current standard of care exists. Many clinicians have adopted a universal policy of bioburden elimination through the routine use of antibiotics and/or local antiseptics. This approach has led to a widespread increase in antibiotic-resistant organisms and other untoward effects such as an increase in Clostridium difficile infections. 3
The presence of biofilms within chronic wounds has further reduced the effectiveness of systemic antibiotic use. 4 In addition, there is concern that antimicrobials will not achieve tissue levels in ischemic wounds and wounds with heavily fibrotic granulation tissue adequate to be sufficiently effective, 5 underscoring a pressing need for a locally administered, nontoxic method to reduce bacterial counts within a wound bed that does not increase bacterial resistance.
Noncontact, nonthermal, low-frequency ultrasound (MIST Therapy® System, Celleration, Inc., Eden Prairie, MN) — hereafter “noncontact ultrasound” — delivers ultrasound energy through a sterile saline mist.
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