Many medical centres in the north-eastern USA have been troubled by isolates of K. pneumoniae harbouring the carbapenemase KPC; most of these isolates are resistant to other classes of antimicrobial agents and treatment options are very limited. Analysis of the strains belonging to the two clonal groups (A and B) that represent the great majority of multidrug-resistant isolates in our region suggests that the AcrAB efflux system and the regulatory proteins MarA, SoxS and RamA are not important mediators of fluoroquinolone and aminoglycoside resistance in the clinical setting. It appears that, due to the pressures in the hospital environment, the acquisition of other mechanisms leading to fluoroquinolone and aminoglycoside (and β-lactam) resistance has offset the need for these systems and they have been down-regulated. Therefore, investigational inhibitors against the acrAB efflux system (or the purported regulatory systems) do not appear likely to be effective agents to restore antimicrobial activity. It is notable that isolates belonging to these clonal groups remained susceptible to tigecycline and had reduced expression of marA and acrB with negligible or absent expression of ramA and soxS. However, tigecycline resistance could still be induced in vitro in these isolates. Therefore, these isolates still have the potential to become resistant to this agent in the clinical setting.
It is becoming increasingly apparent that the development of resistance to tigecycline is a complex process. Reduced susceptibility to tigecycline has been linked to the AcrAB-TolC efflux system in a variety of Enterobacteriaceae.4–8
Increased expression of acrAB
has been reported in clinical isolates of E. coli
and E. cloacae
with reduced susceptibility to tigecycline and transposon mutagenesis involving the operon restored susceptibility to tigecycline.4,5
However, an association between tigecycline MICs and acrA
expression was not evident in isolates of K. pneumoniae
Whether other efflux systems are involved in resistance to this agent is unknown.
Increased expression of marA
has also been linked to AcrAB-mediated resistance to tigecycline in E. coli
However, it was apparent that genes other than marA
) can contribute to tigecycline resistance in E. coli
Also, expression of marA
has not been linked to tigecycline resistance in other Enterobacteriaceae.5
In K. pneumoniae
and E. cloacae
, increased expression of ramA
has been noted in isolates with reduced susceptibility to tigecycline,5,7
and a correlation was noted between expression of ramA
with tigecycline MICs.22
Transposon mutagenesis of the ramA
gene restored susceptibility (and also diminished acrA
expression) in one isolate of K. pneumoniae
The association of tigecycline resistance with soxS
or other regulatory genes has not been previously reported.
The results from our heterogeneous collection of isolates are in general agreement with Ruzin et al.
in that an association was observed between expression of ramA
(and not acrB
) and the MICs of tigecycline. However, we found considerably greater overlap in the expression of ramA
among the clinical isolates with different MICs of tigecycline. In addition, isolates from one clone not expressing ramA
) could still be induced in vitro
to become resistant to this agent. Taken together, these studies indicate that isolates derived in the clinical setting with moderately elevated MICs of tigecycline have increased expression of one or more of the regulatory genes, and factors other than the AcrAB efflux system may mediate resistance. When clinical isolates develop or laboratory isolates are induced to develop high-level resistance to tigecycline, marked overexpression of one (or more) of the regulatory genes and the acrAB
operon occurs and appears critical for resistance.
Given the known interplay between marA, ramA and soxS, it is not surprising that increased expression of one gene was often associated with increased expression of the other regulatory genes. It was also apparent that the absence of expression of some of these regulatory genes (ramA and soxS) does not preclude markedly increased expression of another (marA) and elevation of tigecycline MICs. Because expression of these regulatory genes can have far reaching effects (e.g. influencing porin expression and other efflux systems), it is likely that systems beyond AcrAB-TolC are involved in the development of tigecycline resistance.