Microbial biofilm detection on food contact surfaces by macro-scale fluorescence imaging

Hyperspectral fluorescence imaging methods were utilized to evaluate the potential detection of pathogenic bacterial biofilm formations on five types of food contact surface materials: stainless steel, high-density polyethylene (HDPE), plastic laminate (Formica), and two variations of polished granite. The main objective of this study was to determine a minimal number of spectral fluorescence bands suitable for detecting microbial biofilms on surfaces commonly used to process and handle food. Spots of biofilm growth were produced on sample surfaces by spot-inoculations of pathogenic Escherichia coli O157:H7 and Salmonella followed by room temperature storage for 3 days. Subsequently, hyperspectral fluorescence images were acquired from 421 to 700 nm using ultraviolet-A excitation. Both E. coli O157:H7 and Salmonella biofilms emitted fluorescence predominantly in the blue to green wavelengths with emission maxima at approximately 480 nm. A single-band image at 559 nm was able to detect the biofilm spots on stainless steel. On HDPE and granite, algorithms using different two-band ratios provided better separation of the biofilm spots from background areas than any single-band images did. The biofilm spots on stainless steel, HDPE, and granite could be detected with overall detection rate of 95%. On Formica, too many false positives were present to accurately determine an effective biofilm detection rate. This may have been due to the lower cell population density that was observed for the biofilm spots grown on Formica (approximately 4.3–6.4 log cfu cm⁻²) as compared to the other surfaces. These findings can be incorporated into developing portable hand-held imaging devices for sanitation inspection of food processing surfaces.