Impact of Biofilm Formation and Composition on Antibiotic Resistance in Environmental Isolates of Escherichia coli and Salmonella spp.

Escherichia coli and Salmonella spp., like most bacteria, prefer to grow in biofilms. These biofilms provide bacteria with protection from harsh environmental factors (such as desiccation and changes in pH), aid in the evasion of host immune responses and provide increased antibiotic resistance. Biofilms are present in non-host environments (e.g. water pipes) as well as in mammalian hosts (in the healthy gastrointestinal microbiota and in over 65% of nosocomial infections). Two important components utilized by E. coli and Salmonella spp. to form biofilms are cellulose and curli fimbriae. Curli fimbriae mediate the attachment of bacteria to abiotic surfaces and host epithelial cells. The other component, cellulose, is an exopolysaccharide that provides many benefits such as water retention, tensile strength to the structure and masking of bacterial antigens from host lymphocytes. This research aims to better elucidate the association between host and non-host biofilms produced by E. coli and Salmonella spp.. Firstly, environmental isolates of E. coli and Salmonella spp. were profiled for biofilm formation and survival in host and non-host conditions. Then, biofilm composition (curli fimbriae and cellulose) was monitored under varying conditions in order to understand the correlation between expression of components and biofilm formation in host and non-host conditions. The isolates were examined for antibiotic resistance and acid tolerance in synthetic gastric juice. It was found that over 98% of isolates were able to form biofilms. Isolates produced the highest proportion of moderate biofilms at 23°C and 28°C with 38% and 42% of total isolates, respectively. Some biofilm-formers expressed curli fimbriae and cellulose components, with the highest proportion of components expressed at 37°C. Overall, the presence of biofilms increased isolates’ ability to survive pH stress and antibiotic resistance. These results show that environmental bacteria possess characteristics that may allow them to infect a host