To comprehend the resistance of colistin resistance, we investigated the relationships between amino acid alterations and expression of PmrAB and PhoPQ and colistin resistance in 16 colistin-nonsusceptible clinical Pseudomonas aeruginosa isolates. In addition, we obtained induced colistin-resistant mutants and their colistin-susceptible revertants. Expression levels of the pmrA, phoP, parR, cprR, and pmrH genes were determined for them. Nine amino acid substitutions unique to 10 colistin-nonsusceptible P. aeruginosa (CNPA) isolates were identified: 7 in PmrB and 1 each in PmrA and PhoQ. However, 6 CNPA isolates did not show amino acid substitutions compared with colistin-susceptible P. aeruginosa isolates. Among 16 CNPA isolates, 7 and 8 isolates displayed activated expression of pmrA and phoP, respectively. Activated expression of pmrA and/or phoP was identified in 13 isolates of CNPA isolates, but some had no noticeable PmrAB and PhoPQ amino acid substitutions. In addition, in vitro selected colistin-resistant mutants (P5R and P155R) showed higher expression level in pmrA, phoP, and pmrH than their parent strains (P5 and P155) and colistin-susceptible, revertant strains (P5R-rev and P155R-rev). However, expression of the parR and cprR genes was not consistent. Our data may indicate that amino acid substitutions of PmrAB or PhoPQ do not have an immediate connection with decreased susceptibility of colistin in P. aeruginosa isolates, although activated expression of pmrAB and/or phoPQ resulting in overexpression of pmrH may be required for colistin resistance. Expression of pmrAB or phoPQ related with colistin nonsusceptibility may not explained by a single mechanism, which may suggest that colistin resistance appears easily by diverse pathways in clinical settings as well as in laboratory. (C) 2014 Elsevier Inc. All rights reserved.
