Alteration of susceptibility to EDTA, polymyxin B and gentamicin in Pseudomonas aeruginosa by divalent cation regulation of outer membrane protein H1

Induction of outer membrane protein H1 in Pseudomonas aeruginosa results in decreased susceptibility to aminoglycosides, polymyxin B, and EDTA. We have previously shown that protein H1 can become the major cellular protein in cells grown in low (0.02 mM) Mg²⁺. The induction of protein H1 was prevented by supplementation of low Mg²⁺ medium with Mg²⁺, Ca²⁺, Mn²⁺, or Sr²⁺ (each at 0.5 mM), but not with Zn²⁺, Ba²⁺, Sn²⁺, A1³⁺ or Na⁺ (each at 0.5 mM). Only cells grown in the presence of those cations which failed to prevent H1 induction were resistant to the cationic antibiotics, polymyxin B and gentamicin, and to chelators of divalent cations. Cells grown in Ca²⁺, but not in Mg²⁺, were susceptible to outer membrane permeabilization by the Ca²⁺ specific chelator EGTA, whereas both were susceptible to EDTA. In agreement with this, cells grown in Mg²⁺, Ca²⁺, Mn²⁺, or Zn²⁺ showed enhanced levels of these cations respectively as their major cell envelope-associated cation. When cells were shifted from low to high Mg²⁺ medium, the time course of the decrease in the levels of protein H1 correlated well with the increase in sensitivity to EDTA and polymyxin B. These results support the hypothesis that protein H1 acts to replace divalent cations at a critical outer membrane site attacked by cationic antibiotics and chelators of divalent cations, and suggest that only a small proportion of the total divalent cation-binding sites in the outer membrane are susceptible to attack by these agents.