Mechanisms of polymyxin resistance: acquired and intrinsic resistance in bacteria

Abstract

Polymyxins are polycationic antimicrobial peptides that are currently the last-resort antibiotics for the treatment of multidrug-resistant, Gram-negative bacterial infections. The reintroduction of polymyxins for antimicrobial therapy has been followed by an increase in reports of resistance among Gram-negative bacteria. Some bacteria, such as Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii, develop resistance to polymyxins in a process referred to as acquired resistance, whereas other bacteria, such as Proteus spp., Serratia spp., and Burkholderia spp., are naturally resistant to these drugs. Reports of polymyxin resistance in clinical isolates have recently increased, including acquired and intrinsically resistant pathogens. This increase is considered a serious issue, prompting concern due to the low number of currently available effective antibiotics. This review summarizes current knowledge concerning the different strategies bacteria employ to resist the activities of polymyxins. Gram-negative bacteria employ several strategies to protect themselves from polymyxin antibiotics (polymyxin B and colistin), including a variety of lipopolysaccharide (LPS) modifications, such as modifications of lipid A with phosphoethanolamine and 4-amino-4-deoxy-L-arabinose, in addition to the use of efflux pumps, the formation of capsules and overexpression of the outer membrane protein OprH, which are all effectively regulated at the molecular level. The increased understanding of these mechanisms is extremely vital and timely to facilitate studies of antimicrobial peptides and find new potential drugs targeting clinically relevant Gram-negative bacteria.

Keywords: Enterobacteriaceae; antibiotic resistance; lipid A; lipopolysaccharides; mutation; non-fermentative bacilli; two-component systems.

Figure 1

Activation of lipopolysaccharide-modifying genes involved in polymyxin resistance in Gram-negative bacteria. Both MgrB and MicA (in Escherichia coli) exert negative feedback on the phoP/phoQ regulatory system, while mutations (denoted by red-colored star symbols) in mgrB or phoP/phoQ typically lead to the constitutive induction of the phoP/phoQ two-component system. The activation of this two-component system (phoP/phoQ) activates pagL (which deacylates lipid A in Salmonella) and pmrD (which in turn activates pmrA) and represses eptB via the activation of MgrR, with the resultant lipopolysaccharide (LPS) modifications all participating in the mediation of polymyxin resistance. Additionally, the phoP/phoQ regulatory system can directly activates the arnBCADTEFoperon in some bacteria such as Klebsiella pneumoniae. The repression of eptB prevents the modification of 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo) with phosphoethanolamine (PEtN). The pmrA/pmrB two-component system is activated via pmrD (which is activated by phoP) or through mutations in the pmrA/pmrB genes. Once induced, the phosphorylated pmrA activates the arnBCADTEF and pmrE genes, which collectively modify LPSs with 4-amino-4-deoxy-L-arabinose (L-Ara4N). The lipid A and the LPS core are further modified with PEtN by the pmrCAB operon and cptA, respectively. Additional pmrA-activated genes include pmrR, which represses lpxT (that phosphorylates lipid A) upon activation and lpxR gene (which deacylates lipid A). Lastly, etK can additionally phosphorylate the pmrE gene. The findings illustrated here are limited to modifications that have been shown to affect sensitivity to polymyxins. ^*pagL has only been found in Salmonella.

Figure 2

(A) Domains of the PmrA/PmrB two-component system and positions of all mutations conferring polymyxin resistance to Salmonella enterica serovar Typhimurium. PmrA domains, cheY-homologous receiver domain [REC]; aa 1–112. Transcriptional regulatory protein, C-terminal domain [Trans_reg_C]; aa 145–216. PmrB domains, First transmembrane domain [TM1]; aa 13–35. ^dPeriplasmic domain [PD]; aa 35–66. Second transmembrane domain [TM2]; aa 66–88. Histidine kinases, adenylyl cyclases, methyl-binding proteins, and phosphatases [HAMP domain]; aa 89–141. Histidine kinase A (phosphoacceptor) domain [HisKA]; aa 142–202. Histidine kinase-like ATPases [HATPase_c]; aa 249-356. ^*HisKA, with the active site at H148 in PmrB of Salmonella enterica subsp. enterica serovar Typhimurium str. LT2 (GenBank accession no. AE006468). ^dPeriplasmic domain was not predicted in SMART but was assumed to be between the TM1 and TM2; aa, amino acid. (B) Domains of the PmrA/PmrB two-component system and positions of all mutations conferring polymyxin resistance to Klebsiella pneumoniae. PmrA domains, cheY-homologous receiver domain [REC]; aa 1–112. Transcriptional regulatory protein, C-terminal domain [Trans_reg_C]; aa 145–216.PmrB domains, First transmembrane domain [TM1]; aa 13–35. ^dPeriplasmic domain [PD]; aa 35–67. Second transmembrane domain [TM2]; aa 67–89. Histidine kinases, adenylyl cyclases, methyl-binding proteins, and phosphatases [HAMP domain]; aa 90–142. Histidine kinase A (phosphoacceptor) domain [HisKA]; aa 143–203. Histidine kinase-like ATPases [HATPase_c]; aa 250–358. ^*HisKA, with the active site at H153 in PmrB of K. pneumoniae subsp. pneumoniae MGH 78578 (GenBank accession no. CP000647). ^#Same mutation as that reported for colistin-resistant Enterobacter aerogenes. ^dPeriplasmic domain was not predicted in SMART but was assumed to be between TM1 and TM2; aa, amino acid; Δ, deletion.

Figure 3

(A) Genetic representation of the phoP/phoQ negative regulator, mgrB. (i) K. pneumoniae with intact mgrB (colistin-susceptible), and (ii) K. pneumoniae with mgrB truncated by an insertion sequence (colistin-resistant). (B) Alignment of unmutated MgrB from colistin-susceptible K. pneumoniae and mutated MgrB from a colistin-resistant strain with a missense mutation and premature termination of MgrB. Premature termination (-).

Figure 4

(A) Domains of the PmrA/PmrB two-component system and positions of all mutations conferring polymyxin resistance to Acinetobacter baumannii. PmrA domains, cheY-homologous receiver domain [REC]; aa 2–112. Transcriptional regulatory protein, C-terminal domain [Trans_reg_C]; aa 150–221. PmrB domains, First transmembrane domain [TM1]; aa 10–29. ^dPeriplasmic domain [PD]; aa 29–142. ^†Second transmembrane domain [TM2]; aa 142–164. ^†Histidine kinases, adenylyl cyclases, methyl-binding proteins, and phosphatases [HAMP domain]; aa 145–214. Histidine kinase A (phosphoacceptor) domain [HisKA]; aa 218–280. Histidine kinase-like ATPases [HATPase_c]; aa 326–437. ^*HisKA, with the active site at H228 in PmrB of Acinetobacter baumannii ATCC17978 (GenBank accession no. CP000521). ^dPeriplasmic domain was not predicted in SMART but was assumed to be between TM1 and TM2. ^†TM2 and HAMP overlapped based on SMART prediction. ^††Fr denotes frameshift mutation; aa, amino acid. (B) Domains of the PmrA/PmrB two-component system and positions of all mutations conferring polymyxin resistance to Pseudomonas aeruginosa. PmrA domains, cheY-homologous receiver domain [REC]; aa 1–112. Transcriptional regulatory protein, C-terminal domain [Trans_reg_C]; aa 145–216. PmrB domains, First transmembrane domain [TM1]; aa 15–37. ^dPeriplasmic domain [PD]; aa 38–160. Second transmembrane domain [TM2]; aa 161–183. Histidine kinases, adenylyl cyclases, methyl-binding proteins, and phosphatases [HAMP domain]; aa 186–238. Histidine kinase A (phosphoacceptor) domain [HisKA]; aa 239–304. Histidine kinase-like ATPases [HATPase_c]; aa 348–459. ^*HisKA, with the active site at H249 in PmrB of Pseudomonas aeruginosa PAO1 (GenBank accession no. AE004091). ^dPeriplasmic domain was not predicted in SMART but was assumed to be between TM1 and TM2; aa, amino acid.