Impact of Antimicrobial Gauze on Bacterial Colonies in Wounds that Require Packing
Heavy microbial contamination has a negative effect on wound healing. Polymicrobial colonization and the presence of antibiotic-resistant bacteria may impede the healing of delayed closure surgical wounds, pressure ulcers, and diabetic foot ulcers.
For this reason, a clinical case series was conducted to determine whether using a new antimicrobial gauze dressing containing polyhexamethylene biguanide (PHMB) in wounds of these types that require packing would result in a greater reduction of bacterial colony counts than when using the gauze without PHMB (control), as measured by quantitatively comparing the bacterial colonies cultured.
Heavy microbial contamination has been associated with delayed wound healing and infection in both acute and chronic wounds.1 In one study, researchers concluded that although the precise role of micro-organisms in recalcitrant wounds has not been fully elucidated, their presence in large numbers is seldom beneficial.2 Many micro-organisms commonly found in wounds reside in moist sites such as the ears and the intestines. In addition, the skin has resident microflora available to the wound. Any of these organisms can become pathogenic, reproduce, and invade a wound.3
Delayed closure surgical wounds will not heal when large numbers of micro-organisms (usually >105) are present.4 Furthermore, a growing body of evidence shows that microbial load plays an important role in the delayed healing of many chronic wounds. Some researchers have proposed that chronicity begins with bacteria.5 Although all chronic wounds have some micro-organism component and none are sterile, many chronic wounds will harbor three or more organisms, including Gram-negative and Gram-positive aerobes and anaerobes.6 In some wounds, these organisms may result in a fulminate infection or in delayed wound healing.
The presence of infection should be considered in any wound that fails to show signs of healing, despite "ideal" conditions.7 Wound infections result from an alteration of the dynamic balance between the host and the offending organism. Thus, wound management should focus on decreasing the level of contamination in a wound while simultaneously establishing a supportive environment for healing.
Multi antibiotic-resistant bacteria continue to pose significant threats to patients in all clinical settings. Hospitals and other healthcare environments face ongoing battles with resistant organisms that are difficult to isolate and treat. Infections occurring in healthcare settings are often caused by bacteria that are resistant to commonly used antibiotics.8 The spread of resistant bacteria results from several factors, including misuse of antibiotics in human and animal medicine.
In the decades since antibiotics were first introduced, strains of bacteria that were once sensitive to certain drugs acquired mechanisms of resistance, rendering certain agents ineffective against them. The increase in antibiotic-resistant strains has led to a corresponding increase in morbidity and mortality. A recently published series on infection cases reported that deaths linked to hospital infections represent the fourth leading cause of mortality among Americans.9 Epidemiologists warn of escalating rates of infections inside specialized long-term care centers, nursing homes, and outpatient clinics. Preventing the transmission of these organisms is critical.
A prospective, randomized, 5-week controlled, open-label, multicenter study was conducted to compare a woven gauze dressing containing an antimicrobial agent (Kerlix® A.M.D. Gauze Dressing, Tyco Healthcare Group LP, Mansfield, Mass.) to the study control, an identical, sterile, woven gauze dressing without an antimicrobial component.
The antimicrobial gauze dressing is a cotton blend of woven gauze washed twice, mechanically crimped, and fluff-dried in a relaxed state. It is impregnated with polyhexamethylene biguanide (PHMB) (Cosmocil CQ, ZENECA Biocides, Wilmington, Del.), a hetero-disperse mixture of polymers whose activity as an antimicrobial agent is well accepted. The lethal action of PHMB is an irreversible loss of essential cellular components as a direct consequence of cytoplasm membrane damage. Micro-organisms cannot adapt; no known resistance to PHMB has been documented.
The PHMB dressing is proven to resist bacterial colonization within the dressing and reduce bacterial penetration through the dressing.10 This is largely attributed to the antimicrobial agent (PHMB), which provides a broad spectrum of activity against a wide range of micro-organisms, fungi, and yeast, regardless of the presence of organic matter, and is highly active against Gram-negative bacteria. Furthermore, PHMB has minimal to no odor, is non-foaming, chemically stable and non-volatile, and has low mammalian toxicity. The toxicity of PHMB has been studied extensively in mammalian systems to support its use in medical products.10,11 The FDA has reviewed PHMB as an antimicrobial component in gauze dressings and has cleared its use in such devices under the pre-market notification (510k) process.
Participants. All subjects in this multicenter clinical evaluation were initially enrolled while in a hospital or subacute care facility and, when discharged, followed for the study duration in the subacute care, long-term care, or home care setting. Participants were eligible if they met the criteria (see Table 1). Wounds that required packing were targeted and included delayed closure surgical wounds; diabetic neuropathic foot ulcers (with adequate blood supply as determined by the investigators); pressure ulcers; and wounds with tunneling, fissures, or cavities. Clinically, subjects could not be receiving topical or systemic antibiotics and were required to have no known sensitivity to PHMB.
After receiving approval from an independent Institutional Review Board (IRB), the principal investigators enrolled patients who met inclusion criteria and provided written informed consent. The consent included a description of the study methodology, possible risks, potential benefits, alternative therapy, associated costs, compensation for participation or injury, legal rights, and the right to withdraw. To ensure confidentiality, each subject was assigned a number to track study results, no names were printed on data collection instruments or study forms, and researchers kept the list of subject names in a locked file.
Subjects were randomized according to a schedule created using the Microsoft® Excel 2000 computer program. Twenty-four subjects were enrolled with the assignment of 12 to each study arm (see Table 2).
Procedure. For daily dressing changes, all caregivers were trained on proper dressing technique by the researchers at the first visit by demonstration/return demonstration. Dressing changes included wound irrigation with a minimum of 100-cc normal (0.9%) saline at 4 to 15 psi and lightly packing the wound with either the PHMB antimicrobial gauze treatment dressing or the control, using clean technique. A secondary dressing (either TELFA™ Island Dressing or TELFA™ Plus Dressing, Tyco Healthcare Group LP, Mansfield, Mass.) was applied to all wounds in both the treatment and control groups, the choice dependent on whether exudate was light (island dressing) or moderate to heavy (plus dressing).
Before applying the first study-related dressing, the principal investigator photographed the wound and the periwound area. At baseline and during each weekly study-related dressing change, researchers measured wound size and documented 13 wound assessment parameters using the Wound Assessment Scoring Parameter Tool (WASPT).12 This clinically validated method uses sequential scoring that correlates to the actual process of wound healing. Progress reporting is streamlined and concise using objective, measurable data. The WASPT tool comprises a scoring system from one (1) to four (4) for each of 13 wound assessment parameters (see Table 3).
Throughout the study, wound variables dictated the frequency of dressing changes - eg, daily or on an "as needed" basis if loss of integrity or excessive drainage occurred. Caregivers performed changes as they had been instructed by the principal investigator and in compliance with the randomization schedule.
Cultures. Wound swab cultures were collected and laboratory results were used to analyze bacterial colony counts. This has been found to be a reliable method for quantifying the number of viable bacteria colonizing an open wound.13,14 Researchers have indicated that occasional monitoring by wound swab culture provides baseline information and shows trends in the efficacy of wound therapy.2
The swab cultures were collected using aseptic technique at baseline (before initial application of either the study or control dressing during the first visit) and once per week during the study-related dressing change for a total of five wound swab cultures per subject. The principal investigator collected all cultures by: 1) cleansing the wound with at least 100 cc normal saline and an irrigating device providing 4 to 15 psi of pressure; 2) rotating a rayon-tipped swab applicator over the healthy-appearing granulation tissue in a 10-point of contact, zigzag pattern; and 3) inserting the tip of the swab into a sterile round bottom polypropylene tube containing a non-nutritive, highly conductive transport medium (Starswab II Bacteriology Culture Collection and Transport System, Medegen Medical Products, Gallaway, Tenn.). All culture samples were immediately sent to a CLIA-certified laboratory for susceptibility testing and identification of microbes. Data for individual subjects were reported weekly. Investigators recorded results by study group until reports on all cultures were completed.
To determine if the PHMB antimicrobial gauze dressing affected colony counts, researchers analyzed bacterial log counts. The highest log count (cfu/mL) for any microbe present at each weekly study-related dressing change was compared to the highest log count in the wound at baseline. The differences in the log colony counts for each group were totaled per study week and compared.
Twenty-four individuals, 10 men and 14 women (mean age 68.5 years, range 48 to 98), were enrolled in the study. Of these, 21 (12 from the antimicrobial dressing treatment group and nine from the control group), completed the study to conclusion. Of the three subjects who were not evaluated, one voluntarily withdrew and two others were re-hospitalized and lost to follow-up (see Table 2).
Subjects randomly assigned to the treatment group had a higher number of risk factors that may affect wound healing15 (see Table 4). Wounds in both groups had a median wound size value of 7 cm2 at baseline. At the final assessment (Week 5), two wounds in the PHMB antimicrobial gauze treatment group and one in the control group had healed.
Comparative total WASPT scores at enrollment and at Week 5 are shown for wounds in both treatment (see Table 5) and control groups (see Table 6). Individual WASPT wound assessment parameters documented at baseline and at week 5 are detailed in Table 7. While the number of subjects in treatment and control groups were not equal, the comparison of WASPT scores is designed to show that even with randomization the more severe patients were assigned to the treatment group.
The microbiology findings in the PHMB antimicrobial gauze dressing treatment group demonstrated a greater reduction in both the total number of recovered microbial isolates and log colony counts (cfu/mL). Most notable were the effects on the polymicrobial bioburden (presence of multiple species of organisms) in the PHMB antimicrobial gauze treatment group, especially the culture results after 1 week of dressing application.
During the course of the study, 117 cultures were performed. Many of the same organisms were cultured in both groups, including Proteus mirabilis, Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa , Enterobacter spp., Acinetobacter spp., and Enterococcus spp. In addition, an overwhelming number of Gram-negative rods were recovered. Many other organisms were identified on culture but all had a prevalence of <8% (see Table 8).
The incidence of beta-lactamase response (an indicator of resistant organisms16) was 22%, of which 52% was positive as antibiotic-resistant. The study findings also suggest that a significant number of organisms identified on laboratory data are known to be of nosocomial-transmitted pathology.
At baseline, 15 microbial isolates were recovered and counted in wounds treated with PHMB antimicrobial gauze; 12 isolates were recovered in the control wounds at baseline. After 1 week of PHMB dressing application, the number of micro-organisms recovered in the study group dropped from 15 to six isolates. The control group had a decrease of two isolates (from 12 to 10 micro-organisms). Only in Week 3 were fewer micro-organisms recovered in the control group than in the study group. Overall, the PHMB group exhibited a greater reduction in the total number of micro-organisms recovered throughout the study duration (see Figure 1).
The most notable laboratory result reported was the change in the polymicrobial bioburden, particularly during the first half of the study. At baseline, five polymicrobic cultures were recovered for subjects randomized to the PHMB antimicrobial gauze dressing treatment group and two polymicrobial cultures were recovered for subjects in the control group. By Week 1, wounds randomized to the PHMB antimicrobial gauze treatment group demonstrated a 100% reduction in the polymicrobial bioburden level by dropping to a count of zero, in contrast to the 50% reduction exhibited by the control group. Hence, the number of polymicrobial isolates recovered in the PHMB antimicrobial gauze dressing treatment group decreased dramatically after 1 week of the dressing application. Moreover, this group's polymicrobial counts remained reduced for the following study weeks and only returned to baseline at Week 4. However, in the control group, the number of polymicrobial cultures rose to 60% above baseline at Week 4 (see Figure 2).
To determine if the PHMB antimicrobial gauze dressing affected colony counts, the highest log count (cfu/mL) for any microbe present at each weekly study-related dressing change was compared to the highest log count in the wound at baseline. Differences in the log colony counts for each group were totaled per study week and compared. Overall, the results demonstrated a dramatic decrease in the colony counts (cfu/mL) in the PHMB antimicrobial gauze dressing group at Weeks 1 and 2. Further, the PHMB group demonstrated a greater reduction in the overall log colony counts (see Figure 3).
In this prospective, randomized, controlled evaluation, the use of a new PHMB antimicrobial gauze dressing resulted in a larger reduction in the total number of microbial isolates, the number of polymicrobial counts, and the log colony counts (cfu/mL) as compared to the regular gauze dressing. This compares with the findings of Mertz et al,17 who noted that PHMB antimicrobial gauze dressings substantially reduced or eliminated by 4 to 5 logs, the amount of P. aeruginosa that gained access to the wound bed at all assessment times.
The number of subjects and wounds evaluated (n = 21) was too small to complete statistical analyses of the data. However, the observed trends are noteworthy and suggest that a larger subject population study is warranted to determine the impact of PHMB antimicrobial gauze dressing on nosocomial infection rates and the clinical results possible from controlling the polymicrobial bioburden in delayed healing surgical wounds, pressure ulcers, and diabetic foot ulcers.
Overall, the PHMB group exhibited a greater reduction in the total number of micro-organisms recovered throughout the study duration. Only in Week 3 were fewer micro-organisms recovered in the control group than in the study group (a decrease in the number of isolates from 12 to 10 micro-organisms). This small decrease may have been caused by external circumstances unique to that point in time, as the isolates then increased over the remaining study period.
Study results may be important, particularly in healthcare environments where concern over antibiotic resistance is increasing. Microbial isolates recovered during the weekly wound swab cultures were different from microbial species recovered at baseline. This trend was noted for eight of the 12 subjects in the PHMB antimicrobial gauze dressing treatment group. A number of study variables may explain this result, including wounds of different etiologies, different wounded body sites, changes in clinical care settings, and possible variations in dressing change procedures.
The increase in colony counts and the increase in the number of polymicrobial cultures seen at Week 4 could be directly proportional to changes in certain environmental variables. For example, subjects transferred from the original acute care setting to a long-term care facility or home became exposed to different micro-organisms, temperatures, humidity, dressing change procedures, and caregivers, all of which may help explain this occurrence. Despite a polymicrobial increase at Week 4, wounds in the PHMB antimicrobial gauze dressing treatment group did not deteriorate and evidence of healing continued. Perhaps this group's wounds were better able to respond to environmental challenges, given the earlier reduction in the polymicrobial bioburden.
Clinically, notable results from the PHMB antimicrobial gauze dressing treatment group were observed in an 81-year-old woman with a nonhealing sternal wound following coronary artery bypass graft. The subject had multiple risk factors, including uncontrolled type 1 diabetes mellitus, obesity, anemia, venous insufficiency, and pressure ulcers on the buttocks. On enrollment in the hospital setting, the first culture indicated 300,000 cfu/mL of S. aureus. After 1 week, the colony count was reduced to 50,000 cfu/mL. No growth was noted at Weeks 2 and 3. Overall, the wound size (length x width) decreased each week from 15 cm2 at baseline to 7 cm2 by Week 1, <4 cm2 at Week 2, to healing at Week 4.
Notable results also were observed in an 86-year-old man who demonstrated remarkable wound healing as a member of the PHMB dressing treatment group. He was enrolled with a nonhealing left groin surgical incision secondary to evacuation of a hematoma post femoral popliteal bypass procedure. The patient presented with peripheral vascular disease and a below-knee amputation. On enrollment in the subacute care setting (baseline), the full-thickness wound measured 7 cm2 with 2 cm to 4 cm of undermining and yellow, loosely adherent, stringy slough. The first culture indicated 300,000 cfu/mL of Citrobacter freudii (see Figure 4). After receiving the PHMB antimicrobial gauze dressing for 1 week, the wound was 100% viable bright red tissue (see Figure 5). The wound swab culture taken at Week 2 recovered no bacterial growth (see Figure 6). After 3 weeks, the wound size (length x width) had decreased to <4 cm2 with no undermining present (see Figure 7). By 4 weeks, the wound healed (see Figure 8).
The concept of critical colonization (increased bacterial burden) recently has been introduced to describe wounds moving between the spectrums of colonization (where the host is unaffected) and local infection (in which host injury occurs).18 Clinically, many patients with delayed closure surgical wounds or chronic wounds that require packing may have a complete absence of signs of infection when critical factors impact host response and wound closure. The literature indicates that wound swab cultures may be used to provide baseline information and show trends in the efficacy of topical microbial wound therapy.2
The presence of wound-associated bacteria is not the only factor that indicates infection or potential for impaired wound healing; ascertaining when an increase in microbial load will result in either infection or impaired healing is difficult in clinical practice. Therefore, the results of this preliminary study may have important implications. First, packing wounds with PHMB antimicrobial gauze dressing may be beneficial in reducing the bacterial bioburden in terms of both the total amount of micro-organisms and the number of species. Reducing total numbers of bacteria and polymicrobial bioburden may reduce the risk of infection and simultaneously result in more favorable wound healing. Second, substituting PHMB antimicrobial gauze dressing for regular gauze is a simple solution that does not require a change in existing clinical protocols.
Polyhexamethylene biguanide antimicrobial gauze dressing could be an important adjunct in the control of polymicrobial bioburden in delayed healing surgical wounds, pressure ulcers, and diabetic foot ulcers, as well as a potential tool to reduce nosocomial infection rates. Its use in a wide variety of wounds merits additional research.
The authors wish to thank Jack C. Runner, Vice President, North Coast Clinical Laboratory, Inc., and Kathleen Thimsen, RN, MSN, for their valuable input during this study and preparation of the manuscript.