Researchers have discovered that common disinfectants face an uphill battle killing Salmonella once it has had the time to form a biofilm – a community of cells that attach to each other and a surface, increasing the density of bacterial growth and providing support from harsh environments. Researchers at the National University of Ireland, Galway, allowed Salmonella enterica cells to grow for seven days before applying three types of disinfectant — sodium hypochlorite, sodium hydroxide and benzalkonium chloride. They found that none of the disinfectants was able to kill the cells after that amount of time. Even soaking the biofilms in disinfectant for an hour and a half failed to kill them. “Once Salmonella cells are allowed to become established on a surface, the number of cells will increase over time, resulting in difficulty – if not impossible – to completely eliminate or kill all cells once part of a mature biofilm,” said Mary Corcoran, one of the authors of the study published in the journal Applied and Environmental Microbiology. The strains she tested were able to form a biofilm on glass, steel, polycarbonate plastic, glazed tile and concrete. “In terms of ‘real world’ environments, it is estimated that most organisms are capable of this, and that a high percentage of micro-organisms will form a biofilm to optimize growth and survival,” Corcoran said. Once biofilms grow, it is possible that abrasive cleaning or higher concentrations of disinfectants could reduce the pathogen count, Corcoran said, but “the recommended concentrations – frequently used in industry – did not kill the biofilm cells, particularly, after seven days biofilm growth.” To help put the issue of biofilms into context, the study referenced two outbreaks of the Agona serotype of Salmonella where the pathogen remained in the food processing facility for 10 years “despite intensive cleaning and decommissioning of contaminated equipment.” Corcoran’s tests used three Salmonella serotypes – Agona, Typhimurium and Enteritidis – and a Pseudomonas aeruginosa organism. “However, it is most likely that a similar pattern is true for other serotypes of Salmonella and other organisms such as E. coli,” Corcoran said. She also called the results an opportunity for food manufacturers to become more aware of possible biofilm formation. “People should be aware that cleaning with disinfectants may not eliminate all cells – particularly if they have been given an opportunity to form a biofilm over time,” Corcoran said. To head off an issue of resistance, Corcoran strongly recommended “appropriate and frequent cleaning to prevent the buildup of bacteria on surfaces” and “improving handling practices such as ensuring raw food is prepared in a separate area from cooked food” to reduce the risk of cross-contamination.