Comparison of antimicrobial activity between bacitracin-soaked sutures and triclosan coated suture

Abstract

Background:

With the easily available option for surgeons to soak their suture in antibiotic irrigation solution intraoperatively in mind, this study was designed to evaluate the ability of suture soaked in bacitracin irrigation solution to inhibit the growth of Staphylococcus aureus and Methicillin-resistant Staphylococcus aureus.

Materials and Methods:

Using standard experimental procedure, sterile suture was soaked in Bacitracin, and dried for 10 minutes or 6 hours, incubated for 24 h on inoculated plates, and examined for zone of inhibition around the suture. This was compared to control unsoaked suture and antimicrobial suture (AMS) currently on the market to determine if the minor intraoperative procedural change of placing suture in antibiotic irrigation solution instead of on the sterile table could confer comparable antimicrobial activity.

Results:

The study found the Bacitracin soaked suture (BSS) consistently inhibited the growth of the test organisms. For both organisms, the BSS exhibited a significantly larger zone of inhibition compared to the unsoaked control suture (p < 0.0001). However, both the AMS currently on the market, and a bacitracin aliquot, exhibited significantly larger zones of inhibition compared to both drying times of the BSS (p < 0.0001).

Conclusions:

Placing sutures in a bacitracin irrigation solution intraoperatively instead of directly on the sterile table can achieve some of the in vitro antimicrobial effect seen from AMS currently on the market. This may result in reduced rates of surgical site infections and associated costs without major procedural change and at reduced overhead.

INTRODUCTION

Surgical site infections (SSIs) can be a complication of any surgical procedure. As such, antimicrobial-coated sutures (AMS) represent a strategy developed in an effort to mitigate this risk. Several studies have suggested that AMS reduces SSIs in a wide range of procedures (1–11). Yet, others have suggested they have no such role (12, 13). Staphylococcus aureus is the main pathogen causing SSIs with an incidence of 48% (14) and an incidence of Methicillin resistant S. aureus (MRSA) being 2.33% (15). Currently, the role of AMS in reducing SSIs remains controversial. However, the preponderance of data from large, well-controlled trials indicates a significant reduction of upwards of 30% in reducing the incidence of SSIs across a broad range of surgical procedures (6, 16–19). Despite a considerably increased cost of employing AMS, several economic analyses have concluded that their use is of substantial economic benefit if even 1 SSI is prevented per year across a range of surgical procedures, owing to the exorbitant expense of treating SSIs (18, 20–22). Sutures coated with triclosan are a currently available and widely utilized AMS to combat SSI. Triclosan is a broad-spectrum antibiotic that targets both gram-positive and gram-negative bacteria, thereby inhibiting bacterial colonization of the suture that results in SSI. Therefore, albeit already economically advantageous and clinically effective, the cost-benefit may be further improved via soaking uncoated sutures in an antimicrobial bacitracin irrigation solution intraoperatively.

The aim of this study is to determine if the practice of having the operating room (OR) technician place the uncoated closing suture in the bacitracin irrigation solution at the onset of the case, rather than setting it on the OR table, could result in the conveyance of increased antimicrobial activity to the suture material. The soaked sutures would primarily be utilized for skin closure, with the bacitracin coating aiding to inhibit suture colonization and potentially mitigate SSI occurrence. Bacitracin is an inexpensive antimicrobial currently being used in irrigation solution in the OR and is also effective against Staphylococcus and other Gram-positive bacteria (23). Additionally, the antimicrobial activity of the uncoated bacitracin-soaked suture will be compared to currently utilized AMS in vitro. Overall, this report aims to determine if soaking uncoated sutures in a bacitracin irrigation solution confers comparable antimicrobial activity to market AMS.

MATERIALS AND METHODS

Suture Preparation.

A vial of Bacitracin powder (Pfizer Inc., New York) was reconstituted using sterile technique with 0.9% NaCl Irrigation, USP (Aqualite, Illinois) to create a Bacitracin solution with a concentration of 1000 Units/mL to replicate the intraoperative bacitracin irrigation solution concentration typically used intraoperatively. Ethicon 0 Vicryl (Somerville, New Jersey) was then placed in this Bacitracin solution and soaked for 1 hour to replicate intraoperative soaking time. This is displayed in figure 1. This Bacitracin soaked suture was then placed in a sterile petri dish at room temperature to dry—one group for 10 minutes to replicate intraoperative drying time outside of the solution and a second group for 6 hours to ensure that any measured antimicrobial activity was provided by the prepared suture, not excess Bacitracin solution.

Fig 1. Ethicon 0 VICRYL* sutures soaking in bacitracin irrigation solution intraoperatively.

This displays the ease and feasibility of placing suture packs into an antimicrobial solution during the course of the procedure.

Measuring Antibacterial Activity.

Using standard experimental procedure, Staphylococcus aureus (ATCC 6538) and Methicillin Resistant Staphylococcus aureus (MRSA) (N315) were grown in Luria Broth (LB) (BD Difco, New Jersey) at 37°C to a MacFarland Turbidity of 1 (5 × 10⁸ CFU/ml). Subsequently, 100ul of each broth was then inoculated on 27 separate 1.5% agar plates. For each group of 27 agar plates: 3 were plated with one cm of the unsoaked Ethicon 0 VICRYL^® Suture as a control, 6 with a 10μl aliquot of the 1000 U/mL Bacitracin solution as an additional control, 6 with one cm sections of BSS dried for 10 minutes, 6 with one cm sections of BSS dried for 6 hours, and 6 with one cm sections of the Ethicon Coated 0 VICRYL^® Plus Antibacterial (polyglactin 910). The growth plates were incubated at 37°C for 24hours, after which they were examined for the presence or absence of a zone of inhibition. Work was repeated in triplicates.

Statistical Analysis.

Statistical analysis was performed using GraphPad Prism 7.02 (GraphPad Software, California). Data was found to be normally distributed and followed a gaussian distribution therefore a one-way ANOVA was used followed by Tukey’s multiple comparison test to evaluate the mean difference of the inhibition zone. Student’s t-test was used for individual comparisons. Data presented in Table 1 is of the mean value for the zone of inhibition ± standard deviation. Statistical significance was determined at p < 0.05.

Table 1.

Comparison of zone of inhibition of Staphylococcus aureus and Methicillin Resistant Staphylococcus aureus (MRSA).

	0 vicryl® (Negative Control)	10 Minute Dried BSSa	6 Hour Dried BSSa	Coated 0 VICRYL® Plus Antibacterial	Bacitracin Solution Aliquot (Positive Control)
Zone of Inhibition v. S. aureus	0 mm	7 mm + 1.41	7 mm ± 1.79	19.5 mm ± 2.07	18.2 mm ± 0.84
Zone of Inhibition v. MRSA	0 mm	5 mm ± 1.22	6.83 mm ± 0.75	17.67 mm ± 1.37	15.5 mm ± 1.05

This displays the differing sizes of zones of microbial inhibition across the suture trials. Data presented in the table is of the mean value for the zone of inhibition ± standard deviation. From left to right comparisons - sutures devoid of any bacitracin solution (Negative Control), 10-minute dried BSS sutures, 6-hour dried BSS sutures, and the pre-coated antibacterial suture, an aliquot of bacitracin solution itself (Positive Control).

BSS^a = Bacitracin solution-soaked Ethicon 0 VICRYL^® Suture

RESULTS

The size results of the zone of inhibition assays are listed in Table 1. A zone of inhibition was present for all replicates of the Bacitracin solution aliquot, both drying times of the BSS, and Ethicon Coated 0 VICRYL^® Plus Antibacterial suture. As expected, the non-antibiotic Ethicon 0 Vicryl failed to exhibit any zone of inhibition. Typical zones of inhibition are shown in Table 2.

Table 2:

Zone of inhibition petri dish images

A: Image showing (from left to right) Staphylococcus aureus in LB broth grown on 1.5% agar plates grown in the presence of1 cm 0 Vicryl® Sutures (Negative Control), BSS* dried for 10 minutes, BSS* dried for 6 hours, Coated 0 VICRYL® Plus Antibacterial and a10 μl aliquot of 1000U/ml Bacitracin solution (Positive Control), after incubation at 37°C for 24hours. B: Image showing (from left to right) MRSA in LB broth grown on 1.5% agar plates grown in the presence of 1 cm 0 Vicryl® Suture (Negative Control), BSS* dried for 10 minutes, BSS* dried for 6 hours,Coated 0 VICRYL® Plus Antibacterial and a 10 μl aliquot of 1000U/ml Bacitracin solution (Positive control) after incubation at 37°C for 24hours.

Using a one-way ANOVA, the average zone of inhibition was compared between all groups of the study. There were no significant differences between the different drying times using BSS. There were no significant differences detected between Ethicon Coated 0 VICRYL® Plus Antibacterial suture and the Bacitracin solution aliquot (p=0.58). For S. aureus and MRSA, both drying times of the BSS exhibited a significantly larger zone of inhibition than the control unsoaked Ethicon 0 Vicryl (p < 0.0001). However, for S. aureus and MRSA, the Ethicon Coated 0 VICRYL^® Plus Antibacterial suture and Bacitracin solution aliquot had significantly larger zone of inhibitions than both drying times of the BSS (p < 0.0001). Overall, these results demonstrate that both drying times are similarly effective, suggesting feasibility of use throughout the entirety of a procedure. The BSS displays robust and statistically significant zones of inhibition, indicating antibacterial activity. The zones of inhibition of the Ethicon Coated 0 VICRYL^® Plus Antibacterial suture was statistically significantly larger than the BSS groups.

DISCUSSION

Since Ethicon gained approval to market their antimicrobial polyglactin 910 with triclosan sutures in 2002, their reception has been mixed with recent trials and meta-analyses providing evidence of their benefit (1–9), despite individual studies initially failing to show a reduction in SSIs (12, 13). In vitro studies of AMS have provided further support for their benefit. Rothenburger et al. used zone of inhibition assays to demonstrate that polyglactin 910 with triclosan sutures inhibit the growth of S. aureus and S. epidermidis even after passes through tissue and aqueous immersion (24). Edmiston et al. demonstrated decreased adherence of Gram positive and negative bacterial species to AMS which lead to decreased local bacterial loads for at least up to 96 hours (25). This latter finding could help explain the findings of a study by Ford et al. that reported less postoperative pain in general pediatric surgical patients in which AMS were used, although interestingly it failed to show a concomitant decrease in SSIs within the group (26).

Most initial studies investigated the utility of AMS in GI procedures, but recent studies have begun to explore broader applications. Rozzelle et al. showed decreased CSF shunt infection rates with the use of AMS (8). Others have demonstrated decreased bacterial loads around implant sites in rat models, suggesting a role for AMS in decreasing implant infections (17). With these added applications, others have attempted to develop sutures for ophthalmological procedures (16).

Despite the increased upfront costs, AMS have demonstrated their cost effectiveness by decreasing overall costs related to the management of wound infections. One study estimated the widespread use of AMS at one center could save approximately $1.5 million in a year by avoiding costs related to SSIs (3). Another report found AMS to significantly reduce the incidence of SSIs and subsequently lead to reduced overall costs, as its analysis found the average cost of SSI management to be $2,310 (7). Indeed, several studies have calculated robust reductions in cost by employing AMS; they have estimated that if even 1 SSI was prevented per 200 spine cases per year (18), per 5,000 shunt cases, per 1,100 sternotomies (22), and generally a single SSI for abdominal (21) and other surgical procedures (20), then an economic advantage was conferred.

Given the reduction in SSIs and subsequent associated costs, our study sought to replicate the antimicrobial activity of marketed AMS at a reduced overhead by having the surgical technician place the sutures in a bacitracin solution that is prepared at the beginning of the procedure for use as irrigation in lieu of directly placing them on the table. An example of this procedural change is shown in Figure 1. Our in vitro antimicrobial zone of inhibition assays suggests that our intraoperatively prepared Ethicon 0 VICRYL® BSS provide superior antimicrobial activity compared to unsoaked Ethicon 0 VICRYL^® suture. This benefit is potentially due to a Bacitracin coating that forms during the soaking process, evidenced by sustained antimicrobial activity even after allowing 6 hours of drying. Yet, our study also suggests that while the soaking process confers some antimicrobial activity, it fails to retain the full antimicrobial exhibited by an aliquot of the Bacitracin irrigation solution itself. Additionally, the current AMS on the market—Ethicon’s Coated 0 VICRYL^® Plus Antibacterial suture—exhibits superior antimicrobial activity to our intraoperatively BSS, likely due to enhanced drug-eluting properties related to their manufacturing process. However, as zones of inhibition signify drug potency, the differing sizes may not be clinically relevant. The BSS displays a consistent antibacterial activity, with the Ethicon Coated 0 VICRYL^® Plus Antibacterial suture demonstrating a greater drug concentration and eluting property as evinced by the greater zone of inhibition sizes. Therefore, given local potency of the BSS in preventing bacterial growth, it is uncertain if there would be clinically meaningful differences in SSI incidence using either suture. Further studies examining the antimicrobial activity of our BSS after passes through tissue and aqueous immersion could help identify the longevity and overall viability of this antimicrobial coating in a clinical setting.

Ultimately, this study suggests that with the minor procedural change of placing sutures in a bacitracin irrigation solution intraoperatively, instead of directly on the table, it can achieve some of the antimicrobial effect of the AMS currently on the market. This may result in reduced rates of SSIs and associated costs without major procedural change and at reduced overhead. However, one limitation of the study that is important to note is that triclosan is an antimicrobial that is effective against both gram-positive as well as gram-negative bacteria, while bacitracin is mainly effective against gram-positive bacteria. Thus, especially in contaminated and clean-contaminated operations undertaken on gastrointestinal, genitourinary, and gynecologic tracts, where gram-negative infections are rife, the efficacy of bacitracin-soaked sutures may be negatively impacted. This quandary warrants further investigation. Future studies should be undertaken utilizing this change to fully ascertain its clinical efficacy and price-reducing effectiveness.

CONCLUSIONS

AMS lower the incidence of SSI and subsequently decrease associated costs despite increased overhead cost. We attempted to develop a novel method that could reproduce the antimicrobial benefits while lowering these overhead costs. Our zone of inhibition assays suggests that our method of intraoperatively soaking sutures in the antibiotic irrigation confers antimicrobial activity against S. aureus and MRSA that is superior to plain suture, yet inferior to the AMS currently on the market. Further studies are needed to determine if our BSS can better replicate the antimicrobial activity of the AMS currently on the market and if the antimicrobial activity observed in our study has clinical implications.