Biofilms on Equipment

Attachment of cells to the plastic materials was analyzed with a scanning electron microscope. Like other microscopy techniques, scanning electron microscopy (SEM) is used to study particles (biological or synthetic) that cannot be visualized by the naked eye. SEM differs from other microscopy techniques in that a beam of electrons are emitted onto the surface or sample of interest. This causes secondary electrons to be emitted from the sample, and these secondary electrons are captured by detectors inside the scanning electron microscope, which ultimately transmits the signal into an image that can be viewed on a computer. SEM essentially assesses the topography of a surface as well as the materials on the surface being viewed. Although surfaces often times appear to be smooth, SEM reveals the complex structure of surfaces (see Image 2).

Image 2: Scanning electron microscopy image of plastic material prior to exposure of bacterial cells.
Elizabeth M. Martin, PhD, University of Arkansas

Since it is possible that these bacterial cells could be attached but dead, the plastic materials with attached cells were transferred to a nutritionally rich broth to support the growth of live bacterial cells. This procedure is commonly used to increase the number of bacterial cells that may be in a sample—once the enrichment procedure is performed, any live cells will theoretically be present at a population level, which can be evaluated by detection procedures. Once the plastic materials were enriched, the commonly used detection method PCR was performed. In short, specific DNA sequences (primers) are used to amplify target DNA (Salmonella in this instance) in a sample. Following PCR, real-time instruments can assess whether or not samples consist of the target DNA, and in this instance, thus providing information on which samples consisted of living cells. For each of the samples analyzed, all PCR reactions were positive for Salmonella.

Upon first glance at SEM images, a viewer would reason that these images resemble the terrain of a moon, and these cracks and crevices provide the opportunity for bacteria to set up shop and become established. Container surfaces are just one example of how easily bacteria could become widespread on surfaces throughout a processing plant if established. The potential for bacterial attachment and biofilm formation is a problem for a wide range of surfaces that may contact fresh produce.

What measures involve the most appropriate method of food transportation, primarily for fresh produce? Unfortunately, these answers are not fully understood, but as more research is conducted (and new methods work around a limited water supply), food safety measures will be developed to limit contamination on surface materials.

Spray of sanitizers, phage-particles, or even competitive biofilms of non-pathogenic bacteria may be useful for limiting the attachment of cells to surfaces. Our research indicates that the sanitizers used to sanitize plastic materials are not sufficient in removing attached cells visible by SEM, and much more research will be needed to determine the best approach for sanitization. As more research is conducted pertaining sanitization treatments on biofilms or single attached cells, the ultimate decision on cleaning approaches will require evaluating cost of effectiveness versus the level of acceptable risk. In short, there continues to be many unknowns with biofilms, and everyone needs to be careful with the claims they state since existing data sets may fail to tell the full story. Added to this dilemma will be the requirement that future food safety practices implemented will potentially have to work around water and other environmental limitations for equipment sanitizer treatments, as well as consumer acceptance.

Baker is a graduate research assistant in the Department of Food Science at the Center for Food Safety, University of Arkansas. Reach him at [email protected]. Dr. Ricke, Donald “Buddy” Wray Chair in Food Safety, is director at the Center for Food Safety in the Institute of Food Science and Engineering. Reach him at [email protected].