In this work, contributions
of extracellular polymeric substances
(EPS) to the nanoscale mechanisms through which the multidrug-resistant Acinetobacter baumannii responds to antimicrobial
and hyperosmotic treatments were investigated by atomic force microscopy.
Specifically, the adhesion strengths to a control surface of silicon
nitride (Si3N4) and the lengths of bacterial
surface biopolymers of bound and loose EPS extracted from A. baumannii biofilms were quantified after individual
or synergistic treatments with hyperosmotic agents (NaCl and maltodextrin)
and an antibiotic (tobramycin). In the absence of any treatment, the
loose EPS were significantly longer in length and higher in adhesion
to Si3N4 than the bound EPS. When used individually,
the hyperosmotic agents and tobramycin collapsed the A. baumannii bound and loose EPS. The combined treatment
of maltodextrin with tobramycin collapsed only the loose EPS and did
not alter the adhesion of both bound and loose EPS to Si3N4. In addition, the combined treatment was not as effective
in collapsing the EPS molecules as when tobramycin was applied alone.
Finally, the effects of treatments were dose-dependent. Altogether,
our findings suggest that a sequential treatment could be effective
in treating A. baumannii biofilms,
in which a hyperosmotic agent is used first to collapse the EPS and
limit the diffusion of nutrients into the biofilm, followed by the
use of an antibiotic to kill the bacterial cells that escape from
the biofilm because of starvation.
In this work, the contributions of the pathogenic Listeria monocytogenes cell-wall biomacromolecules to the bacterial mechanics and adhesion to a model inert surface of silicon nitride in water were investigated by atomic force microscopy. Chemical ethylenediaminetetraacetic acid (EDTA) and biological enzymatic trypsin treatments of cells were performed to partially or totally remove the bacterial cell-wall proteins and carbohydrates. Removal of 48.2% proteins and 29.2% of carbohydrates from the cell-wall of the bacterium by the EDTA treatment resulted in a significant decrease in the length of the bacterial cell-wall biomacromolecules and an increase in the rigidity of the bacterial cells as predicted from fitting a model of steric repulsion to the force−distance approach data and classic Hertz model to the indentation-force data, respectively. In comparison, removal of almost all the cell-wall proteins (99.5% removal) and 8.6% of cell-wall carbohydrates by the trypsin treatment resulted in an increase in the elasticity of the bacterial cells, an increase in the extension of the cell-wall biomacromolecules, and a significant decrease in their apparent grafting densities. In addition, adhesion strength of native-untreated L. monocytogenes to silicon nitride in water decreased by 30% on average after the EDTA treatment and further decreased by 60% on average after the trypsin treatment, showing a positive correlation with the% removal of cell-wall proteins by the EDTA and trypsin treatments, respectively.
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