Purpose: This work determined the ability of the top seven STEC strains to form biofilms on polystyrene and stainless steel; assessed their survival within dry biofilms; and quantified STEC transfer to lettuce pieces.
Methods: The ability of 14 STEC strains to form biofilms and survive on six well polystyrene (POL) plates and on two cm diameter 304-2B disk-shaped stainless steel (SS) coupons was examined. Biofilms on POL and SS were allowed to grow in low salt LB broth at 25°C for six days, washed three times with phosphate buffer, and dried for four hours. POL dry biofilms were stored for 2, 4, 6, and 30 days. SS dry biofilms were storage for 30 days. At each interval, dried lettuce pieces were placed over the dry SS and POL biofilms for two minutes. Lettuce pieces were collected and STEC enumerated. Thirty day old dry biofilms, negative for bacterial transfer, were enriched 24 hours to investigate bacterial survival. Scanning electron microscopy was used to evaluate dry biofilm formation on SS coupons. Experiments were conducted three times in duplicate.
Results: All STEC strains were able to transfer one to six log from POL dry biofilms to lettuce at two, four, and six days. Almost all serotypes showed decreased detachment with time. After 30 days on SS, all serotypes, but O26, detached and transfer to lettuce (0.3 to 3 log). After enrichment, all seven STEC serotypes were recovered from 30 day-old dry biofilms. Significant differences in transfer and survival were found among serotypes and with time (P=0.001).
Significance: Results showed that STEC can detach from dry biofilms and transfer to dried, fresh lettuce; and, after rehydration/enrichment, bacteria were able to fully recover. This suggested that dry biofilms could be an important factor in STEC cross-contamination in dry and wet food processing environments.