Chlorhexidine. We love it, we just can't get enough of it, we are up to our neck in it and we cheer for it. Basically it's the greatest. But some of us have been concerned that the widespread use of CHG would lead to resistance, particularly in Acinetobacter.
There is a new report in PNAS by investigators in Australia and the UK that looked at gene expression in A. baumannii after chlorhexidine exposure. The most highly up-regulated genes were those encoding
the RND efflux system AdeAB. Importantly, the investigators also identified a gene encoding a previously uncharacterized membrane protein, AceI, which they determined to be an active CHG efflux protein capable of transporting CHG out of the cell. This novel AceI was said to be a member of the PCE family of efflux systems.
And to give you a sense of the ubiquity and potential importance of these systems in Gram-negative bacteria, I offer this quote from the authors:
"The primary features of the A. baumannii chlorhexidine resistance
response, i.e., up-regulation of genes encoding RND and
PCE family efflux systems, are conserved in other γ-proteobacteria, as well as β-proteobacteria. Given the phylogenetic distance
of these organisms, this similarity of regulatory responses
to a compound that has only been synthesized since the early
20th century is intriguing. RND efflux systems are well recognized
for their broad protective functions in Gram-negative bacteria and it is not unusual for these systems to constitute part of a
general stress response and to mediate resistance to foreign
compounds, particularly amphipathic antimicrobials such as chlorhexidine.
However, the involvement of the PCE family proteins
in seemingly specific resistance to chlorhexidine is unexpected. It is
likely that the physiological function(s) of this transporter family
were originally unrelated to chlorhexidine resistance and that
these proteins provide a fortuitous intrinsic resistance capacity."
h/t Ayush Kumar and Dan Ricciuto
image source: wikipedia