Transfer of microorganisms, including Listeria monocytogenes, from various materials to beef

Abstract

The quantity of microorganisms that may be transferred to a food that comes into contact with a contaminated surface depends on the density of microorganisms on the surface and on the attachment strengths of the microorganisms on the materials. We made repeated contacts between pieces of meat and various surfaces (stainless steel and conveyor belt materials [polyvinyl chloride and polyurethane]), which were conditioned with meat exudate and then were contaminated with Listeria monocytogenes, Staphylococcus sciuri, Pseudomonas putida, or Comamonas sp. Attachment strengths were assessed by the slopes of the two-phase curves obtained by plotting the logarithm of the number of microorganisms transferred against the order number of the contact. These curves were also used to estimate the microbial population on the surface by using the equation of A. Veulemans, E. Jacqmain, and D. Jacqmain (Rev. Ferment. Ind. Aliment. 25:58-65, 1970). The biofilms were characterized according to their physicochemical surface properties and structures. Their exopolysaccharide-producing capacities were assessed from biofilms grown on polystyrene. The L. monocytogenes biofilms attached more strongly to polymers than did the other strains, and attachment strength proved to be weaker on stainless steel than on the two polymers. However, in most cases, it was the population of the biofilms that had the strongest influence on the total number of CFU detached. Although attachment strengths were weaker on stainless steel, this material, carrying a smaller population of bacteria, had a weaker contaminating capacity. In most cases the equation of Veulemans et al. revealed more bacteria than did swabbing the biofilms, and it provided a better assessment of the contaminating potential of the polymeric materials studied here.

FIG. 1.

Contact angles measured on the surfaces of cleaned materials; conditioned materials; conditioned and rinsed materials; biofilms of Comamonas sp., L. monocytogenes, S. sciuri (Staph. sciuri), and P. putida (Ps. putida) on stainless steel, PVC, and PU; and on the surfaces of pieces of beef. Contact angles with water (a), formamide (b), and di-iodomethane (c) are shown. The bars represent the confidence intervals. The results are from two duplicate experiments and 10 measurements per experiment.

FIG. 2.

Total CFU transferred to beef by 12 contacts with biofilms of Comamonas sp. (C sp), L. monocytogenes (Lm), S. sciuri, and P. putida (Ps putida) as a function of the bacterial populations of the same biofilms calculated by using the equation of Veulemans et al. (50) on stainless steel (a), on PU (b), and on PVC (c). The bars represent the confidence intervals of two duplicate experiments. The straight line is the bisector.

FIG. 3.

Percentage of objects in each of the five object area classes defined for analyzing the two-dimensional structure of biofilms of Comamonas sp., L. monocytogenes, S. sciuri (Staph. sciuri), and P. putida (Ps. putida) on stainless steel, PU, and PVC. The results are from two duplicate experiments.

FIG. 4.

Comparison of the logarithm of the concentration of ConA and WGA required to achieve an OD₄₀₅ of 0.8 for biofilms of Comamonas sp., L. monocytogenes, S. sciuri (Staph. sciuri), and P. putida (Ps. putida). The bars represent the individual standard deviations from one duplicate experiment. When exopolysaccharides were not detectable, the threshold value was used to perform the statistical analysis.

FIG. 5.

Differences between the logarithm of the bacterial populations of biofilms of Comamonas sp., L. monocytogenes, S. sciuri (Staph. sciuri), and P. putida (Ps. putida) grown on stainless steel and polymers, calculated by using the equation of Veulemans et al. (50), and the logarithm of the bacterial populations of the same biofilms, calculated from a count after swabbing (VSD). The bars represent the confidence intervals from two duplicate experiments.

FIG. 6.

Slopes of two-phase curves obtained by plotting the logarithm of the number of CFU transferred to beef as a function of the number of serial contacts with biofilms of Comamonas sp., L. monocytogenes, S. sciuri (Staph. sciuri), and P. putida (Ps. putida) grown on stainless steel and polymers. (a) Slope 1 (k₁) characterizes the first 3 contacts, and (b) slope 2 (k₂) characterizes the last contacts. The bars represent the confidence intervals from two duplicate experiments.