(193x) Multidrug-Resistant Escherichia coli: Measurement of Membrane Mechanical Properties, Nanoscale Adhesion, and Biofilm Formation

A report from the United States’ Center for Disease Control and Prevention (CDC) in 2013 estimated that at least two million people get antibiotic-resistant infections yearly and more than 23,000 die because of such infections. Rapid detection of multidrug resistance (MDR) in bacteria is a clinical challenge and pose a serious task to effective clinical management and control of MDR-related infections. To combat resistance, fundamental understanding of how microbial cell morphology and surface properties (elasticity, charge, and adhesion) differ among bacterial strains with variable drug resistances and abilities to form biofilms in response to antibiotics is needed. Here, we employed atomic force microscope (AFM) to characterize cell morphology and surface properties of wild-type multidrug-resistant Escherichia coli (MDR-E.coli) strains with various biofilm capacities before and after exposure to ampicillin at different minimum inhibitory concentrations (MICs). Forces between the hydrophobic or hydrophilic AFM cantilever and at least 15-25 locations on the bacterial surface were quantified. The MIC was determined using the broth dilution method.

Our results showed that individual E. coli strains with variable biofilm abilities responded differently to ampicillin as observed from morphological changes of their surfaces as well as quantified mechanical and adherence properties of their cells. For example, ampicillin at a MIC of 45 μg/ml and above the Clinical and Laboratory Standards Institute (CLSI) breakpoint concentration of susceptibility (8 μg/ml ) induced morphological changes to the weak and moderate biofilm former strains in forms of cellular elongation as well as surface roughness and membrane disorientation within 1-8 hours of exposure. On the contrary, no morphological changes were observed in strong biofilm or no biofilm former E. coli strains.The elongated cells recovered after some time and maintained their normal morphology thereafter. When quantified, Young’s moduli of elasticity of the moderate and the strong biofilm former E. colistrains were not changed from those of the untreated cells upon exposure to ampicillin at MIC of 50 μg/ml and 45 μg/ml, respectively. It is, therefore possible that ampicillin did not disrupt cell wall synthesis in these strains. However, ampicillin at MIC of 50 μg/ml significantly reduced the adhesion strength of the strong biofilm former strain while increased the adhesion force for the moderate biofilm former strain.

When all results are combined, weak biofilm former E. coli strains to elongate their cells and make their surfaces rougher to adapt to antibiotics. On the other hand, strong biofilm former strains rely on their biofilms to help them resist antibiotics more than on induced changes to their mechanical or morphological properties. This investigation provides a proof-of-concept that AFM can be used on the basis of detection of ampicillin resistance.

Keywords: AFM, multidrug-resistance, E. coli, surface properties, adhesion force and biofilm.