A novel insertion sequence, ISAba10, inserted into ISAba1 adjacent to the bla_OXA-23 gene and disrupting the outer membrane protein gene carO in Acinetobacter baumannii

We investigated an outbreak caused by carbapenem-resistant Acinetobacter baumannii carrying the bla_OXA-23 gene. A novel insertion sequence (IS), named ISAba10, was found to be inserted into the ISAba1 element preceding the bla_OXA-23 gene in a group of isolates showing higher carbapenem MICs. The presence of ISAba10 was associated with increased OXA-23 expression, likely by providing additional promoter sequences. ISAba10 was also inserted into the carO outer membrane protein gene in most of these isolates.