aeruginosa increasing the number of
active sites able to bind carbapenems even if they are
inefficient at hydrolysing them or by loss of porins in
A. baumannii. However, it is their capability to accept
transferable b-lactamase genes that confer carbapenem
resistance at a level that will result in clinical failure that
causes most concern. In the mid 1990s, P. aeruginosa
strains isolated in Japan were found to harbour a transferable
class B metallo-b-lactamase called IMP-1 [19]. This
enzyme could increase the MIC to greater than 128mg/L
depending on the permeability characteristics of the
strain. This gene was thought originally to be confined
to southeast Asia but it and a myriad of similar genes
(blaIMP-2 – blaIMP-12) have now been isolated in P. aeruginosa
and other Gram-negative bacteria from around the
world. A similar class of b-lactamases (VIM 1–4) have
been found in Asia and Europe, almost exclusively in
P. aeruginosa [20]. The extent and diversity of these
enzymes suggests that they will be very difficult to
overcome.