A treatment option for multi-drug resistant (MDR) Enterobacteriaceae is the cationic peptide colistin. Colistin (Col) binds to the lipopolysaccharide (LPS) and phospholipids in the outer cell membrane leading to cell lysis. Addition of amino-4-deoxy-L-arabinose to lipid A renders the cell membrane charge positive, preventing colistin binding and producing resistance (ColR). LPS modifications are controlled by a 2-component regulatory system that is up-regulated by concentration of Mg+2, Al+3, Fe+3, pH, and by the presence of antimicrobial peptides. Mutations in mgrB, pmrB (basS in Enterobacter), and phoQ have been linked to colistin resistance in Klebsiella pneumoniae. While ColR is described in E. cloacae, the mechanism remains to be elucidated. Interestingly, ColR usually develops during treatment and is not sustained; suggesting that there may be a fitness cost of acquired ColR.
We obtained 9 isolates of E. cloacae from Colombia showing sustained ColR with MICs > 128 mg/µL. We designed primers to sequence the E. cloacae mgrB, basS, and phoQ analogues and compared sequences to colistin-susceptible isolates. We performed hsp60 sequencing and rep-PCR to determine genetic relatedness. We matched susceptible isolates to the same hsp60 cluster as the ColR isolates.
We identified mutations in the basS gene that resulted in amino-acid changes in 8 positions in the majority of ColR resistant isolates (Figure). Resistant isolates were mapped to hsp60 clusters VI, VIII, and XI; and were part of 4 different rep-PCR patterns. Mutations in the mgrB or phoQ genes were not found between susceptible and resistant isolates.
An association between ColR in E. cloacae and 8 amino-acid changes on the basS gene was uncovered. These single nucleotide changes are independent of the bacterial strain by rep-PCR and nomospecies by hsp60 sequencing and suggest the role of this homologue of pmrB in ColR phenotype in E. cloacae.
L. J. Rojas, None
R. Bonomo, None
M. V. Villegas, None