Impaired virulence and in vivo fitness of colistin-resistant Acinetobacter baumannii

Abstract

Acinetobacter baumannii (American Type Culture Collection strain 19606) acquires mutations in the pmrB gene during the in vitro development of resistance to colistin. The colistin-resistant strain has lower affinity for colistin, reduced in vivo fitness (competition index, .016), and decreased virulence, both in terms of mortality (0% lethal dose, 6.9 vs 4.9 log colony-forming units) and survival in a mouse model of peritoneal sepsis. These results may explain the low incidence and dissemination of colistin resistance in A. baumannii in clinical settings.

Figure 1.

Differential binding of dansyl polymyxin B (DPXB) to bacterial cells of American Type Culture Collection (ATCC) strain 19606 and colistin-resistant mutant strain RC64 of Acinetobacter baumannii. A, Variation of DPXB binding with concentration bacterial density of 2 × 10⁶ colony-forming units (CFUs)/mL (diamonds and dotted line, ATCC 19606; filled circles and solid line, RC64). The inset shows the inner membrane permeabilization as assessed by entrance of the vital dye SYTOX green, using 5 μmol/L of CA(1–7)M(2–9) as a positive control (100% permeabilization; black bars, ATCC 19606; white bars, RC64). B, Fluorescence microscopy of ATCC 19606 cells incubated with DPXB (20 μg/mL). C, Fluorescence microscopy of RC64 cells incubated with DPXB (20 μg/mL).

Figure 2.

In vitro (A) and in vivo (B) growth of American Type Culture Collection (ATCC) strain 19606 and colistin-resistant mutant strain RC64 of Acinetobacter baumannii, separately and in competition, and time of survival (evaluated every 4 h) with the 100% lethal dose of A. baumannii strains ATCC 19606 and RC64 (C). CFU, colony-forming unit.