Impact of ionizing radiation enhanced when used to treat biofilm bacteria.

Biofilm-associated bacteria are more resistant to antimicrobial treatments than their planktonic counterparts. Far greater concentrations of chemical sanitizers, such as chlorine, trisodium phosphate and iodine, are required to eliminate biofilm-associated cells.

Ionizing radiation has been used to inactivate Salmonella on a variety of products and food contact surfaces, but the efficacy of the process against biofilm-associated cells is not well documented. So USDA-ARS scientists determined the radiation sensitivity of three isolates of Salmonella under planktonic or biofilm conditions. They indicate that the antimicrobial efficacy of ionizing radiation can be preserved or improved when it is used to treat biofilm-associated bacteria. It appears that the biofilm habitat exerts a complex and isolate-dependent influence on the efficacy of ionizing radiation.

In experiments, the ARS researchers grew cultures in tryptic soy broth and biofilms that were allowed to form on sterile glass slides. The broth and biofilm cultures were irradiated to doses of 0.0, which was the control, as well as to 0.5, 1.0, 1.5, 2.0 and 2.5 kGy. Biofilm-associated cells of S. stanley and S. enteritidis were significantly more sensitive to ionizing radiation than were their planktonic counterparts. The D10 values (doses required to reduce a population of bacteria by 1 log10) of S. Anatum were not significantly different for biofilm-associated (0.645 kGy) and planktonic cells (0.677 kGy).

In contrast, the biofilm-associated cells of S. stanley were significantly more sensitive to ionizing radiation than were the respective planktonic cells, with D10 values of 0.531 kGy and 0.591 kGy, respectively. The D10 values of S. enteritidis were similarly reduced for biofilm-associated (0.436 kGy) cells, compared with planktonic (0.535 kGy) cells. Live and dead staining coupled with confocal microscopy and a digital image analysis indicated that irradiation eliminated Salmonella at similar rates throughout the biofilm.

Until recently, much of our knowledge about bacteria came from studies of individual, free-floating cells. Scientists weren't aware of the complexity and prevalence of biofilm lifestyles until the 1970s. Now, many argue that the free-floating, or planktonic, lifestyle of bacteria that's most familiar to laboratory scientists may be nothing more than a way for cells to disperse and colonize new habitats.

Further information. Brendan Niemira, Food Safety Intervention Technologies Research Unit, USDA-ARS Eastern Regional Research Center, Room 3112.2, 600 E. Mermaid Lane, Wyndmoor, PA 19038; phone: [phone]; fax: [phone]; email: [e-mail].