Growth Dynamics, Antibiotic Susceptibility and the Effect of Sublethal Ciprofloxacin Concentrations in Susceptible and Resistant Escherichia coli in Biofilm

Abstract: Instead of planktonic growth in nature, many species of bacteria form biofilm to survive in harsh conditions. Although many chronic bacterial infections are caused by bacterial species in a biofilm lifestyle, previous research has focused on studying antibiotic resistance in planktonic growth. Here we used a modified MBEC assay, i.e. biofilm growth on pegs, to determine Escherichia coli biofilm inhibitory concentrations (BIC) of ciprofloxacin, streptomycin and rifampicin and to study the minimal selective concentration (MSC) for ciprofloxacin in E. coli biofilm. We could observe high inhibitory concentrations for all antibiotics in the biofilm pre-formed in media without antibiotics compared to the biofilm formed in antibiotics. We also show preliminary result indicating that sublethal concentrations of ciprofloxacin lead to the selection of ciprofloxacin resistant mutants in biofilm and that the selection level is lower than what was observed in planktonic growing E. coli. With more knowledge in how the biofilm formation precedes in different antibiotic settings, the treatment for chronic biofilm infections used today could be evaluated and changed so that the infections could be eradicated.