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Abstract
Introduction: Chronic wounds are costly to treat and are an increasing health problem. The aetiology of chronic wounds is multi-factorial, but recent evidence suggests that bacteria play an important role and be responsible for a majority of chronic infections. S. epidermidis and P. aeruginosa are commonly associated with chronic wounds. A major part of the virulence is their ability to form biofilms. Biofilms are a complex community of bacteria adhering to surfaces and imbedded in a biopolymer matrix that aids the bacteria in surface attachment, has water, nutrient and oxygen channels and protects the bacteria from host and antimicrobial agents. This high resistance of biofilms presents challenges to the medical community for fighting infections. Recently, topical negative pressure (TNP) dressings have been used in chronic wounds to aid healing. Although there is some research evaluating the effect of TNP dressings on biofilms, there is no research on the efficacy of a combination of TNP dressings and antiseptic instillation.
Aims: Our aims were to identify the minimum eradication concentration (MEC) and the minimum biofilm eradication concentration (MBEC) of S. epidermidis and P. aeruginosa against a range of antiseptics and use sub-inhibitory concentrations of antiseptics in an in vitro wound model to assess the efficacy of TNP dressings in combination with antiseptic instillation.
Method: Biofilms were generated in a biofilm generator (24 coupons at a time) with a 24 hour batch phase and a 24 hour flow-through phase. Coupons were then transferred to an in vitro wound chamber. Each wound chamber had 6 coupons. Wound chambers were subjected to various frequency of antiseptic instillation with and without TNP. 5 coupons were prepared for quantitative measurements using standard microbiological methods, while 1 coupon was prepared for scanning electron microscopy.
Results: S epidermidis: MBEC of PI against S. epidermidis lay between 0.078 mg/ml and 0.156 mg/ml while the MEC was 4.883 μg/ml. The MBEC of HA against S. epidermidis was between 0.025 mg/ml and 0.05 mg/ml while the MEC was 3.125 μg/ml. Increasing frequency of antiseptic instillations without TNP resulted in a reduction in bacterial load ranging from 0.5 to 1.5 log10 reduction. Combining antiseptic instillations with TNP resulted in a greater decrease in bacterial numbers, ranging from 0.5 to almost 2.5 log10 reduction. PI instillation 12 times per day in combination with TNP was the most effective and resulted in an almost 2.5 log10 reduction in bacterial numbers. The SEM images complemented the results observed with colony counts.
P aeruginosa: MBEC of PB against P. aeruginosa lay between 0.05 mg/ml and 0.20 mg/ml while the MEC was 0.2mg/ml. The MBEC of HA against P. aeruginosa was > 0.05 mg/ml while the MEC was < 3.125 μg/ml. Similar to the results observed with S. epidermidis, increasing frequency of antiseptic instillation resulted in an increased reduction in bacterial numbers. Combining TNP with PI instillation resulted in a greater decrease in bacterial numbers (7 log10 reduction) compared with PI instillation alone (5 log10 reduction). 3 times per day PB instillation resulted in approximately 0.5 log10 reduction in bacterial numbers. Addition of TNP to 3 times per day PB instillation resulted in a greater decrease in bacterial numbers (1.5 log10 reduction). Although higher frequency of PB instillation resulted in a greater decrease in bacterial load, the additive effect of TNP to PB instillation was not present. The SEM complemented these findings.
Conclusion: MBEC of PI against S. epidermidis lay between 0.078 mg/ml and 0.156 mg/ml while the MEC was 4.883 μg/ml. The MBEC of HA against S. epidermidis was between 0.025 mg/ml and 0.05 mg/ml while the MEC was 3.125 μg/ml. MBEC of PB against P. aeruginosa lay between 0.05 mg/ml and 0.20 mg/ml while the MEC was 0.2mg/ml. The MBEC of HA against P. aeruginosa was > 0.05 mg/ml while the MEC was < 3.125 μg/ml. The use of TNP in combination with antiseptic instillation resulted in a greater reduction of bacterial numbers when compared with instillation alone.