Inhibition of aminoglycoside 6'-N-acetyltransferase type Ib by zinc: reversal of amikacin resistance in Acinetobacter baumannii and Escherichia coli by a zinc ionophore

Abstract

In vitro activity of the aminoglycoside 6'-N-acetyltransferase type Ib [AAC(6')-Ib] was inhibited by ZnCl2 with a 50% inhibitory concentration (IC50) of 15 μM. Growth of Acinetobacter baumannii or Escherichia coli harboring aac(6')-Ib in cultures containing 8 μg/ml amikacin was significantly inhibited by the addition of 2 μM Zn(2+) in complex with the ionophore pyrithione (ZnPT).

FIG 1

Effect of zinc on AAC(6′)-Ib activity. (A) KAN acetylating activity of AAC(6′)-Ib in the presence of different cations. Compounds were added at the indicated concentrations, and the activity was compared to that observed in the absence of additions (C). (B) The percentage of acetylating activity of KAN by AAC(6′)-Ib was calculated for reaction mixtures containing different concentrations of ZnCl₂.

FIG 2

Comparison of the effects of ZnCl₂ and ZnPT on resistance to AMK. A. baumannii A155 (panels A to D) or E. coli(pNW1) (panels E to H) were cultured in 100 μl Mueller-Hinton broth in microtiter plates at 37°C, with the additions indicated in the figure, and the OD₆₀₀ was periodically determined. Panels A, C, E, and G show the effect of addition of ZnCl₂ (A and E) or ZnPT (C and G) on cell growth. Panels B, D, F, and H show the effect of addition of ZnCl₂ (B and F) or ZnPT (D and H) on resistance to 8 μg/ml AMK.