Different Surface Charge of Colistin-Susceptible and -Resistant Acinetobacter Baumannii Cells Measured With Zeta Potential as a Function of Growth Phase and Colistin Treatment

J Antimicrob Chemother. 2011 Jan;66(1):126-33. doi: 10.1093/jac/dkq422. Epub 2010 Nov 16.

Abstract

Objectives: electrostatic forces mediate the initial interaction between cationic colistin and Gram-negative bacterial cells. Lipopolysaccharide (LPS) loss mediates colistin resistance in some A. baumannii strains. Our aim was to determine the surface charge of colistin-susceptible and -resistant A. baumannii as a function of growth phase and in response to polymyxin treatment.

Methods: the zeta potential of A. baumannii ATCC 19606 and 10 clinical multidrug-resistant strains (MICs 0.5-2 mg/L) was assessed. Colistin-resistant derivatives (MIC >128 mg/L) of wild-type strains were selected in the presence of 10 mg/L colistin, including the LPS-deficient lpxA mutant, ATCC 19606R. To determine the contribution of LPS to surface charge, two complemented ATCC 19606R derivatives were examined, namely ATCC 19606R + lpxA (containing an intact lpxA gene) and ATCC 19606R + V (containing empty vector). Investigations were conducted as a function of growth phase and polymyxin treatment (1, 4 and 8 mg/L).

Results: wild-type cells exhibited a greater negative charge (-60.5 ± 2.36 to -26.2 ± 2.56 mV) thancolistin-resistant cells (-49.2 ± 3.09 to -19.1 ± 2.80 mV) at mid-log phase (ANOVA, P < 0.05). Opposing growth-phase trends were observed for both phenotypes: wild-type cells displayed reduced negative charge and colistin-resistant cells displayed increased negative charge at stationary compared with mid-logarithmic phase. Polymyxin exposure resulted in a concentration-dependent increase in zeta potential. Examination of ATCC 19606R and complemented strains supported the importance of LPS in determining surface charge, suggesting a potential mechanism of colistin resistance.

Conclusions: zeta potential differences between A. baumannii phenotypes probably reflect compositional outer-membrane variations that impact the electrostatic component of colistin activity.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acinetobacter Infections / drug therapy
  • Acinetobacter Infections / microbiology*
  • Acinetobacter baumannii / drug effects*
  • Acinetobacter baumannii / isolation & purification
  • Anti-Bacterial Agents / pharmacology*
  • Colistin / pharmacology*
  • Drug Resistance, Multiple, Bacterial*
  • Electricity*
  • Genes, Bacterial
  • Humans
  • Lipopolysaccharides / biosynthesis
  • Lipopolysaccharides / chemistry
  • Microbial Sensitivity Tests
  • Mutation
  • Static Electricity

Substances

  • Anti-Bacterial Agents
  • Lipopolysaccharides
  • Colistin