Influences of biofilm structure and antibiotic resistance mechanisms on indirect pathogenicity in a model polymicrobial biofilm

Indirect pathogenicity (IP), the commensal protection of antibiotic-sensitive pathogens by resistant microorganisms of low intrinsic virulence, can prevent the eradication of polymicrobial infections. The contributions of antibiotic resistance mechanisms and biofilm structure to IP within polymicrobial biofilms were investigated using a model two-member consortium. Escherichia coli ATCC 33456 was transformed with vectors conferring either ampicillin or spectinomycin resistance, creating two distinct populations with different resistance mechanisms. Each strain alone or the consortium was grown as biofilms in flow cells and planktonically in chemostats. Comparisons in survival and activity were made on the basis of MICs and minimum biofilm preventative concentrations, a newly introduced descriptor. In ampicillin-containing medium, commensal interactions were evident during both modes of cultivation, but the sensitive strain experienced a greater benefit in the chemostat, indicating that the biofilm environment limited the commensal interaction between the Amp(r) and Spt(r) strains. In spectinomycin-containing medium, growth of the sensitive strain in chemostats and biofilms was not aided by the resistant strain. However, green fluorescent protein expression by the sensitive strain was greater in mixed-population biofilms (9% +/- 1%) than when the strain was grown alone (2% +/- 0%). No comparable benefit was evident during growth in the chemostat, indicating that the biofilm structure contributed to enhanced activity of the sensitive strain.