Members of this family are found mainly in Acinetobacter and some of them are implicated in the resistance against imipenem. This 34-kDa family is distantly related to the CarO family. In Acinetobacter baumannii, the X-ray crystal structure of Omp33 at a resolution of 2.1 Å reported that Omp33 forms a 14-ß-stranded barrel. without stable extracellular loop constrictions. Instead, an extended and unusual periplasmic turn connecting ß-strands 2 and 3 is present, which folds into the pore lumen and completely blocks the aqueous channel.