Expression of the multiple antibiotic resistance operon (mar) during growth of Escherichia coli as a biofilm

The multiple antibiotic resistance (mar) operon is a global regulator contr olling the expression of various genes in Escherichia coli which constitute s the mar regulon. Upregulation of mar leads to a multi-drug resistant phen otype, which includes resistance towards structurally unrelated antibiotics , organic solvents and the disinfectant pine oil. Biofilms also display sim ilar decreases in susceptibility to antimicrobial agents. A marO(II)-lacZ f usion strain (SPC105) of E. coli was used to monitor mar expression under v arious growth conditions including batch, continuous and biofilm culture. I n chemically-defined media (CDM), mar expression was maximal in mid-log and declined in the stationary phase. Conversely, in rich media (Luria-Bertani broth), minimal expression in mid-log was followed by an increase in the s tationary phase. In continuous culture, expression was inversely related to specific growth rate (mu = 0.05-0.4 h(-1)). LacZ expression by the marO(II )-lacZ fusion was generally low within the total biofilm population and equ ivalent to that of stationary phase cultures grown in batch culture. When t he expression of mar in CDM batch culture was compared with that in biofilm populations, beta-galactosidase activity was generally higher throughout b atch culture than in the attached population. Overall, these results sugges t that while mar expression will be greatest within the depths of a biofilm where growth rates are suppressed, its probable induction within biofilms cannot explain the elevated levels of antibiotic resistance observed.