Antibiotic selection leads to inadvertent selection of nfxC-type phenotypic mutants in Pseudomonas aeruginosa.

Spontaneous nfxC-type phenotypic mutants of Pseudomonas aeruginosa are characterized by chloramphenicol resistance and increased expression of the multidrug efflux pump, MexEF-OprN. These mutants frequently arise during the propagation of bacteria in laboratory conditions. This study shows that some of the discrepancies in transcriptomic/phenotypic profiles of two rsmA mutants, PAZH13 (rsmA mutant in strain PAO1) and PAKΔrsmA, are due to the nfxC-type nature of PAZH13. This indicates that awareness of the possible occurrence of these spontaneous nfxC-type phenotypes is necessary. Screening of mexE levels could provide clarity when an nfxC-type phenotype is suspected.