title: Different surface charge of colistin-susceptible and -resistant Acinetobacter baumannii cells measured with zeta potential as a function of growth phase and colistin treatment.
abstract: OBJECTIVE
  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.
authors.affiliation: Facility for Anti-infective Drug Development and Innovation, Drug Delivery, Disposition and Dynamics, Monash Institute of Pharmaceutical Sciences, Monash University, 381 Royal Parade, Parkville 3052, Victoria, Australia.
authors.forename: Rachel L
authors.initials: RL
authors.lastname: Soon
authors.forename: Roger L
authors.initials: RL
authors.lastname: Nation
authors.forename: Stewart
authors.initials: S
authors.lastname: Cockram
authors.forename: Jennifer H
authors.initials: JH
authors.lastname: Moffatt
authors.forename: Marina
authors.initials: M
authors.lastname: Harper
authors.forename: Ben
authors.initials: B
authors.lastname: Adler
authors.forename: John D
authors.initials: JD
authors.lastname: Boyce
authors.forename: Ian
authors.initials: I
authors.lastname: Larson
authors.forename: Jian
authors.initials: J
authors.lastname: Li
mesh_terms: D000151:Acinetobacter Infections / Q000188:drug therapy / Q000382:microbiology*; D040981:Acinetobacter baumannii / Q000187:drug effects* / Q000302:isolation & purification; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D003091:Colistin / Q000494:pharmacology*; D024901:Drug Resistance, Multiple, Bacterial; D004560:Electricity; D005798:Genes, Bacterial; D006801:Humans; D008070:Lipopolysaccharides / Q000096:biosynthesis / Q000737:chemistry; D008826:Microbial Sensitivity Tests; D009154:Mutation; D055672:Static Electricity
chemical_list: D000900:Anti-Bacterial Agents; D008070:Lipopolysaccharides; D003091:Colistin