Efficacy of a Sonicating Swab for Removal and Capture of Listeria monocytogenes in Biofilms on Stainless Steel

Abstract

Listeria monocytogenes is of great concern in food processing facilities because it persists in biofilms, facilitating biotransfer. Stainless steel is commonly used for food contact surfaces and transport containers. L. monocytogenes biofilms on stainless steel served as a model system for surface sampling, to test the performance of a sonicating swab in comparison with a standard cotton swab. Swab performance and consistency were determined using total viable counts. Stainless steel coupons sampled with both types of swabs were examined using scanning electron microscopy, to visualize biofilms and surface structures (i.e., polishing grooves and scratches). Laser scanning confocal microscopy was used to image and to quantitate the biofilms remaining after sampling with each swab type. The total viable counts were significantly higher (P ≤ 0.05) with the sonicating swab than with the standard swab in each trial. The sonicating swab was more consistent in cell recovery than was the standard swab, with coefficients of variation ranging from 8.9% to 12.3% and from 7.1% to 37.6%, respectively. Scanning electron microscopic imaging showed that biofilms remained in the polished grooves of the coupons sampled with the standard swab but were noticeably absent with the sonicating swab. Percent area measurements of biofilms remaining on the stainless steel coupons showed significantly (P ≤ 0.05) less biofilm remaining when the sonicating swab was used (median, 1.1%), compared with the standard swab (median, 70.4%). The sonicating swab provided greater recovery of cells, with more consistency, than did the standard swab, and it is employs sonication, suction, and scrubbing.IMPORTANCE Inadequate surface sampling can result in foodborne illness outbreaks from biotransfer, since verification of sanitization protocols relies on surface sampling and recovery of microorganisms for detection and enumeration. Swabbing is a standard method for microbiological sampling of surfaces. Although swabbing offers portability and ease of use, there are limitations, such as high user variability and low recovery rates, which can be attributed to many different causes. This study demonstrates some benefits that a sonicating swab has over a standard swab for removal and collection of microbiological samples from a surface, to provide better verification of surface cleanliness and to help decrease the potential for biotransfer of pathogens into foods.

Keywords: biofilm; capture; removal; sonicating; swab.

FIG 1

Three-dimensional rendered drawing of the sonicating swab adaptor, in front (left) and isometric (right) views. The components of the sonicating swab adaptor included a tightly fitted tube with a locking collar (A), a barbed tube suction port and an adjacent socket (B), a rigid pultruded carbon fiber tube (C), and a swab tip suction head (D).

FIG 2

Box plot of L. monocytogenes recovery from biofilm-covered SSCs with a standard cotton swab and the sonicating swab, in three separate trials.

FIG 3

Representative images of L. monocytogenes biofilms on SSC surfaces, examined with SEM and LSCM, after swabbing with a standard cotton swab and the sonicating swab, in comparison to an unswabbed control. (A to C) SEM images (insets are magnified views of the SSC surfaces and biofilm). Magnification, ×1,000. (A) Unswabbed control SSC. (B) SSC swabbed with a standard cotton swab. (C) SSC swabbed with the sonicating swab. (D to F) LSCM images of AO-stained L. monocytogenes biofilms on SSCs. Magnification, ×20. (D) Unswabbed control SSC. (E) SSC swabbed with a standard cotton swab. (F) SSC swabbed with the sonicating swab.

FIG 4

Box plot of percent areas of L. monocytogenes biofilms remaining on SSCs after swabbing with a standard cotton swab and the sonicating swab, in comparison with an unswabbed control. Boxes followed by different letters are significantly different (P ≤ 0.05).