Transfer of bacteria between stainless steel and chicken meat: A CLSM and DGGE study of biofilms

This study aimed to assess the interaction between bacteria and food processing surfaces using novel methods. Microbial cross contamination between stainless steel, a common food processing material, and raw chicken was studied using microbiological culture, specialized microscope and molecular techniques. Confocal laser scanning microscopy (CLSM) allowed the visualization of biofilms containing single or dual species of Escherichia coli O157: H7, Salmonella typhimurium, Bacillus cereus, Staphylococcus aureus and Pseudomonas aeruginosa, formed after 6 days' incubation on stainless steel or 4h on raw chicken. The results provided information on intra-biofilm location and stratification of species within dual species biofilms. Top-to-bottom Z-stack images revealed that, on both materials, S. typhimurium and E. coli attached concurrently, the former in greater numbers. E. coli and B. cereus segregated on steel, E. coli more frequent near the metal surface, B. cereus almost the only species in outer layers. Few cells of S. aureus, found at all depths, were seen in the 2.9 mu m thick biofilm on steel with E. coli. Greatest attachment was shown by P. aeruginosa, followed by S. typhimurium, E. coli and finally Gram positive species. Large amounts of EPS in P. aeruginosa biofilms made visualization difficult on both materials, but especially on chicken meat, a limitation of this technique. Nevertheless, CLSM was useful for determining time sequence of adhesion and species makeup of thin biofilms. The technique showed that five min contact between bacterially-contaminated chicken and sterile steel resulted in greatest transfer of P. aeruginosa, followed by S. typhimurium. This was confirmed using DGGE. Gram positive bacteria transferred poorly. A biofilm containing 2.3 x 10(5) cfu.cm(-2) B. cereus on steel transferred an undetectable number of cells to chicken after 5 min contact. This species was unable to form biofilm on chicken when incubated for 4 h in growth medium. S. typhimurium and P. aeruginosas were most efficiently transferred from contaminated steel to raw chicken within 5 min contact, with 20-30% transfer from single species biofilms. All other species, and all cells in dual species biofilms, showed less than 2% transfer. CLSM and DGGE were shown to be useful techniques for the study of bacterial adhesion to stainless steel.