Co-occurrence of carbapenemase encoding genes in Acinetobacter baumannii, a dream or reality?

Acinetobacter baumannii is an important opportunistic pathogen that is rapidly evolving towards multidrug resistance and is responsible for life-threatening infections. Carbapenems are commonly used to treat A. baumannii infections but the emergence of carbapenemase encoding genes, such as bla_OXA-23-like, bla_OXA-24-like, bla_OXA-58-like, and bla_NDM has been reported. Moreover, several studies have reported the co-occurrence of two distinct carbapenemases in some isolates. The aim of the present study is to demonstrate whether the phenomenon of co-occurrence of two distinct carbapenemase encoding genes in a single isolate still exists.

We studied six strains of A. baumannii including one harboring bla_OXA-23-like and bla_OXA-24-like genes and five with bla_OXA-23-like and bla_NDM genes. One colony of each strain was inoculated in sterile water and diluted ten-fold. Each dilution was cultivated on trypticase soy agar plates for 24 h at 37 °C and the isolated bacteria were analyzed. For two of the six tested strains, we identified two different populations of A. baumannii, each with a different carbapenemase, genes encoding aminoglycoside modifying enzymes, resistance phenotype, and clonal type. In addition, the two different populations had the same aspect on the agar plate.

Here, we demonstrate that A. baumannii infections could be linked to multiple clones harboring different carbapenemase encoding genes in the same sample. In addition, we describe an easy method of verifying the presence of co-occurrence of carbapenemase in one isolate.