FIG 2.

Frequency of emergence of colistin-resistant A. baumannii (type strain, ATCC 19606) strains and changes in mutation tendency in combination with meropenem. (A) The frequency of colistin resistance was analyzed by plating 0.2 mL of ATCC 19606 culture (OD₆₀₀ of 1.0) on LB agar containing 5× MIC (10 μg/mL) of colistin for 24 h at 37°C. The frequency of resistance to colistin, when combined with 1/5× MIC (0.1 μg/mL) of meropenem, was determined by plating 0.1 mL of ATCC 19606 culture (OD₆₀₀ of 10) on LB agar containing 10 μg/mL of colistin and 0.1 μg/mL of meropenem. Data are shown in a box plot; 20 independent assays were compared by the Mann-Whitney U test. ***, P < 0.001. (B) Mutation site analysis of colistin-meropenem-resistant ATCC 19606 strains isolated by direct plating on LB agar plates containing 5× MIC (10 μg/mL) of colistin and 1/5× MIC (0.1 μg/mL) of meropenem. Sanger sequencing was performed on the genomic regions containing the pmrAB genes. In the boxes, base changes are shown on top, while amino acid changes are shown below. The numerical values are the numbers of isolated strains, and numbers in parentheses are the numbers of independent events.