Emergence of a mutL Mutation Causing Multilocus Sequence Typing-Pulsed-Field Gel Electrophoresis Discrepancy among Pseudomonas aeruginosa Isolates from a Cystic Fibrosis Patient

A multilocus sequence type (MLST) shift (from ST242 to ST996) was detected in Pseudomonas aeruginosa isolates with a uniform pulsed-field gel electrophoresis (PFGE) pattern obtained from a chronically colonized patient. MLST mutational change involved the mutL gene with the consequent emergence of a hypermutable phenotype. This observation challenges the required neutrality of mutL as an appropriate marker in MLST and alerts researchers to the limitations of MLST-only-based population studies in chronic infections under constant antibiotic selective pressure.