Gram-negative bacteria were the first pathogens responsible
for hospital-acquired infections. The most problematic
of these when ability to treat and pathogenicity are
considered are Acinetobacter baumannii and Pseudomonas
aeruginosa. Both these non-fermentors have few porins,
small in the case of A. baumannii, which have given them a
predisposition to survive in an antibiotic-rich environment.
They are responsible for most hospital-acquired
pneumonia in patients previously treated with antibiotics
[18] as well as causing many other life-threatening infections.
Their ability to rapidly acquire resistance to cephalosporins
and fluoroquinolones has left the carbapenems
as the only remaining viable treatment option. Both
species can become resistant to carbapenems by genetic
mutation, to hyperproduction of the class C chromosomal
b-lactamase in P. 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.
Gram-negative bacteria were the first pathogens responsiblefor hospital-acquired infections. The most problematicof these when ability to treat and pathogenicity areconsidered are Acinetobacter baumannii and Pseudomonasaeruginosa. Both these non-fermentors have few porins,small in the case of A. baumannii, which have given them apredisposition to survive in an antibiotic-rich environment.They are responsible for most hospital-acquiredpneumonia in patients previously treated with antibiotics[18] as well as causing many other life-threatening infections.Their ability to rapidly acquire resistance to cephalosporinsand fluoroquinolones has left the carbapenemsas the only remaining viable treatment option. Bothspecies can become resistant to carbapenems by geneticmutation, to hyperproduction of the class C chromosomalb-lactamase in P. aeruginosa increasing the number ofactive sites able to bind carbapenems even if they areinefficient at hydrolysing them or by loss of porins inA. baumannii. However, it is their capability to accepttransferable b-lactamase genes that confer carbapenemresistance at a level that will result in clinical failure thatcauses most concern. In the mid 1990s, P. aeruginosastrains isolated in Japan were found to harbour a transferableclass B metallo-b-lactamase called IMP-1 [19]. Thisenzyme could increase the MIC to greater than 128mg/Ldepending on the permeability characteristics of thestrain. This gene was thought originally to be confinedto southeast Asia but it and a myriad of similar genes(blaIMP-2 – blaIMP-12) have now been isolated in P. aeruginosaand other Gram-negative bacteria from around theworld. A similar class of b-lactamases (VIM 1–4) havebeen found in Asia and Europe, almost exclusively inP. aeruginosa [20]. The extent and diversity of theseenzymes suggests that they will be very difficult toovercome.
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