Panel B, Fold-change in adeI,

adeJ and adeK HDAC inhibitor expression in DB versus DBΔadeIJK, and R2 versus R2ΔadeIJK; Black bars, DB; grey bars, R2; horizontal stripes, DBΔadeIJK; white bars, R2ΔadeIJK. Panel C, Fold-change in adeL, adeF, adeG, adeH, adeI, adeJ and adeK expression in DB versus DBΔadeFGHΔadeIJK, and R2 versus R2ΔadeFGHΔadeIJK. Black bars, DB; grey bars, R2; horizontal stripes, DBΔadeFGHΔadeIJK; white bars, R2ΔadeFGHΔadeIJK. All differences in fold-change in gene expression between the parental strains and deletion mutants were significant (*, p < 0.05; **, p < 0.01). Successful inactivation of adeJ was also similarly confirmed by the absence of adeJ transcripts in the DBΔadeIJK and R2ΔadeIJK mutants (Figure  4B). A small quantity of adeI transcripts was udetectable in DBΔadeIJK and R2ΔadeIJK mutants, albeit at 56% and 31% of wild-type levels, respectively. This was due to the location of the adeI qRT-PCR primers within the UP fragment, i.e. within the 5’ undeleted portion of the adeI

gene (Figure  1C). Next, we tested the feasibility of our marker-less deletion strategy for creating isogenic mutants carrying a combination of pump gene deletions. We applied this strategy to delete adeIJK in the DBΔadeFGH and R2ΔadeFGH mutants to create DBΔadeFGHΔadeIJK and R2ΔadeFGHΔadeIJK mutants, respectively. As expected, the DBΔadeFGHΔadeIJK and R2ΔadeFGHΔadeIJK mutants showed significantly reduced expression of adeL, adeF, adeG, adeH, LDE225 nmr adeJ and adeK (Figure  4C). Expression of adeI in DBΔadeFGHΔadeIJK and R2ΔadeFGHΔadeIJK mutants was

reduced to 38% and 58% of DB and R2 levels, respectively. Antimicrobial susceptibility profiles of pump deletion mutants The parental isolates, DB and R2, were MDR including to quinolones (nalidixic acid), fluoroquinolones (ciprofloxacin), chloramphenicol, tetracycline, carbapenems (meropenem Phosphoribosylglycinamide formyltransferase and imipenem), β-lactams (piperacillin, oxacillin), cephalosporins (ceftazidime), macrolides (erythromycin), lincosamides (clindamycin), trimethoprim and aminoglycosides (gentamicin and kanamycin) (Table  1). Inactivation of the adeIJK in isolates DB and R2 resulted in at least a 4-fold increased susceptibility to nalidixic acid, chloramphenicol, clindamycin, tetracycline, minocycline and tigecycline, but had no effect on antimicrobial susceptibility to β-lactams (oxacillin and piperacillin), cephalosporins (ceftazidime), fluoroquinolones (ciprofloxacin), carbapenems (meropenem and imipenem), erythromycin and aminoglycosides (gentamicin and kanamycin). DBΔadeIJK and R2ΔadeIJK mutants were also 8-fold more susceptible to trimethoprim when compared to the parental isolates. Table 1 Antimicrobial susceptibility of MDR A.