The pencil eraser swab technique to quantify Cutibacterium acnes on shoulder skin

Abstract

Introduction: Cutibacterium acnes is the most common cause of postoperative infections in orthopaedic shoulder surgery and is hard to eradicate with current measures. Newer strategies focus on reducing bacterial load on the skin before surgery. Several previous studies have used a large number of both described and undescribed sampling techniques. The purpose of this study was to compare three previously described swab techniques to obtain bacterial cultures: Levine's (L) technique, the Z technique and the pencil eraser swab (PES) technique. Methods: Three consecutive skin swabs were collected from the right shoulder, on 15 healthy male volunteers, using Levine's technique, Z technique and PES technique from each participant. To determine the number of living bacteria, serial dilutions were made, and after culturing for 5 d, viable count (VC) was expressed as CFU/mL (with CFU representing colony-forming unit). Results: The PES technique yielded significantly higher VC than the two others. PES: median 3700 CFU/mL, L: 200 CFU/mL and Z: 220 CFU/mL ( $p=0.003$ ). There was no significant difference between the methods regarding the number of positive cultures. PES: 14/15, L: 11/15 and Z: 12/15. Conclusions: There is a need to harmonise sampling techniques of C. acnes in order to compare the efficacy of different measures to reduce the bacterial load on the skin before and during surgery. Of the three tested methods, the PES technique is simple and produces the highest bacterial counts.

Figure 1

The three different templates. Z: Z technique, L: Levine's technique and PES: pencil eraser swab technique. Each rectangle has the dimensions of $\mathrm{1}\mathrm{cm}\times \mathrm{10}$  cm, the square measures $\mathrm{1}\mathrm{cm}\times \mathrm{1}$  cm, and there is 1 cm between each template.

Figure 2

PES technique: rub the swab with an oscillating movement – like using a pencil eraser – going down over a 10 cm line and then in the same manner up again for a total of 15 passages (https://doi.org/10.5446/55554; Scheer, 2021a).