Colistin: An Update on the Antibiotic of the 21st Century

Silpak Biswas; Jean-Michel Brunel; Jean-Christophe Dubus; Martine Reynaud-Gaubert; Jean-Marc Rolain


Expert Rev Anti Infect Ther. 2012;10(8):917-934. 

In This Article

Antibacterial Activity of Colistin & Mechanisms of Resistance

Susceptibility Breakpoints

Susceptibility breakpoints for colistin have been developed in France, Germany and the UK that are very conflicting to each other. The breakpoints for susceptibility are based on colistin sulfate. The French Society for Microbiology has selected ≤2 mg/l as the susceptibility breakpoint and >2 mg/l as the resistance breakpoint for Enterobacteriaceae, while the British Society for Antimicrobial Chemotherapy has selected ≤4 mg/l for the susceptibility breakpoint and ≥8 mg/l for the resistance breakpoint.[4,10,59,60]

The Clinical and Laboratory Standards Institute (CLSI) has recently revised the colistin interpretative criteria for Acinetobacter spp. as resistance breakpoints (≥ 4 mg/l), which is different from those recommended for P. aeruginosa (≥8 mg/l) and other non-Enterobacteriaceae as the cutoff to define resistance,[61,62] while the European breakpoints published by the European Committee on Antimicrobial Susceptibility Testing for Acinetobacter spp. (susceptible [S] ≤ 2 mg/l, resistant [R] > 2 mg/l) and for Pseudomonas spp. (S ≤ 4 mg/l, R > 4 mg/l). Moreover, there are no polymyxin breakpoints established for Enterobacteriaceae by the CLSI. Regarding disc diffusion testing, the CLSI recommends susceptibility breakpoints only for P. aeruginosa (10-µg colistin disc; R ≤10 mm, S ≥11 mm). The disc diffusion is not a reliable technique for determining the susceptibility of Acinetobacter spp. to polymyxins. Guidelines for colistin disc susceptibility testing have also been published by the French Society for Microbiology and the British Society for Antimicrobial Chemotherapy.[59,60,301] More clinical data will be needed to define the optimal susceptibility breakpoints, and the results must be influenced by the antimicrobial susceptibility techniques employed in the respective studies.

Antibacterial Activity

Colistin is mostly active against Gram-negative clinical isolates. Colistin is active against species of Enterobacteriaceae.[4,10,15] The nonfermentative Gram-negative bacteria P. aeruginosa and Acinetobacter species are naturally susceptible.[63–65] Colistin is also active against Haemophilus influenzae, E. coli, Salmonella spp., Shigella spp., Klebsiella spp., Legionella pneumophila, Aeromonas spp., Citrobacter spp. and Bordetella pertussis. Campylobacter species vary in susceptibility to colistin.[2,4,66–68] Table 1 describes the antimicrobial activity of colistin against pathogenic bacteria from clinical isolates in different countries.

Natural or Intrinsic Resistance

The pathogenic Neisseria spp., Moraxella catarrhalis, Helicobacter pylori, Proteus mirabilis, Serratia marcescens, Morganella morganii, Chromobacterium and Brucella species are naturally resistant to colistin.[10,69–72] Isolates of Inquilinus, Pandoraea and Burkholderia associated with CF are also intrinsically resistant to colistin.[73–76] In Proteus mirabilis, Burkholderia cepacia and Chromobacterium violaceum, polymyxin resistance has been associated with the changes in lipid A.[72,77]

Acquired Resistance

There are very limited data on acquired resistance to colistin or other polymyxins. Pitt et al. found that 3.1% of 417 CF patient isolates of P. aeruginosa from 17 hospitals in the UK had MICs higher than 4 mg/l (the British Society for Antimicrobial Chemotherapy breakpoint).[78] In a CF center in southern Germany, 15.3% of 229 non-mucoid P.aeruginosa strains and 3.2% of 156 mucoid P.aeruginosa strains had MIC values to colistin sulfate above 2 mg/l.[65] Some studies found inhaled colistin to be responsible for the resistance to colistin in P. aeruginosa from CF patients.[78–80] In Salmonella spp. and E. coli, acquired resistance to polymyxin is linked with the substitution of phosphate groups in LPS.[81,82]

Heteroresistance Against Polymyxins

In 2006, Li et al. first reported the presence of heteroresistance against polymyxins in isolates of A. baumannii.[83] They observed that resistant subpopulations existing in the inocula justified the early development of resistance. Moreover, regrowth was observed at 24 h after an early concentration-dependent killing. The recent SENTRY study reported the prevalence of colistin heteroresistance in MDR A.baumannii clinical isolates from different countries in Asia, Africa and Oceania.[84] Although the heteroresistance to colistin in A. baumannii has been described previously,[83,85] theclinical significance of colistin heteroresistance in A. baumannii has not been determined yet.[86]

Mechanisms of Resistance

There are two mechanisms of polymyxin resistance. The mechanism of colistin resistance was studied by selecting invitro colistin resistance derivatives of the MDR isolates and the drug-susceptible strain using escalating concentrations of colistin in liquid culture. Most of the reported mechanisms were described in isolates in which development of polymyxin resistance had occurred after in vitro exposure (adaptive mechanism). Regrowth of bacterial isolates with colistin monotherapy was observed in in vitro studies.[83,87–89] Earlier, Gunderson et al. reported regrowth of two MDR isolates of P. aeruginosa with colistin concentrations of up to 200 mg/l.[87] On the other hand, regrowth of A. baumannii[90] and K. pneumoniae[88] has been reported in static time-kill studies utilizing colistin concentrations up to 64 times higher than the MIC.

The most common mechanisms of resistance to colistin are modifications to LPS, the initial site of action of colistin.[10,15] LPS is an immunogenic glycolipid that constitutes most of the outer leaflet of the OM of Gram-negative bacteria. LPS consists of three domains: the O antigen, the core oligosaccharide and the lipid A moiety.[91–93] In E. coli, S. enterica, K. pneumoniae and P. aeruginosa, the net LPS charge is reduced owing to the modification of the lipid A with 4-amino-4-deoxy-L-arabinose (L-Ara4N) and/or phosphoethanolamine via activation of Ugd, the PmrF (or PbgP) operon and PmrC, which encode uridine diphosphate-glucose dehydrogenase, Ara4N biosynthetic enzymes and lipid A phosphoethanolamine transferase, respectively.[92–97] In K. pneumoniae and Neisseria meningitidis, the presence of capsules may also be important for polymyxin resistance.[98–100] A study by Antoniadou et al. has demonstrated that extensive use of colistin leads to the development of resistance among K.pneumoniae isolates with increasing morbidity and mortality of critically ill patients.[101]

A recent study indicated that resistance to colistin in A. baumannii is associated with mutations in the PmrAB two-component system.[86] The mechanism of colistin resistance in A. baumannii was studied by selecting in vitro colistin resistance derivatives of the MDR A. baumannii isolates. In this study, mutation in the genes encoding the two-component system proteins PmrA and/or PmrB was obtained using the DNA sequencing method.[86]

Recently, Moffatt et al. showed that the complete loss of all LPS was responsible for the polymyxin resistance in A. baumannii.[102] They found mutations in the three genes lpxA, lpxC and lpxD for 13 colistin-resistant A. baumannii strains. These mutations in lpxA, lpxC and lpxD were responsible for the complete loss of LPS production. They also reported that the complete loss of LPS production was triggered by the ISAba11 movement that causes high-level colistin resistance in A. baumannii.[103] Very recently, Beceiro et al. reported that one point mutation in PmrB leads to addition of phosphoethanolamine to lipid A, which can cause colistin resistance in A. baumannii.[104] Park et al. described the role of increased expression of the PmrAB system for colistin resistance in A. baumannii.[105]

Studies on P. aeruginosa suggest a role for OprH, an OM protein that is overexpressed in low-Mg2+ environments, resulting in resistance to PMB.[106–108] OprH is a small (21 kDa), slightly basic protein that has been shown to be structurally related to the porin family. Under low-Mg2+ starvation conditions, OprH becomes the major protein in the OM. It has been proposed that OprH occupies Mg2+ ion binding sites in the OM when these cations are limited, and thus contributes to membrane stability and resistance to PMB.[109,110]

Multidrug efflux systems can also be responsible for the polymyxin resistance in P. aeruginosa.[111,112] Resistance to polymyxin has been observed by production of colistinase in B. polymyxa.[113] This is a 'self-protection' mechanism, as it is produced only by B. polymyxavar. colistinus. While working with 24 MDR clinical isolates of P.aeruginosa and susceptibility testing for polymyxins, Schurek et al. reported that polymyxin resistance can be induced by growing bacterial isolates in crescent concentrations of PMB or polymyxin E.[114] The induced polymyxin resistance is consequent to the ability of the bacterial cell to sense specific polymyxin concentrations and induces the PhoP–PhoQ and PmrA–PmrB systems. Fernández et al. recently described the identification and characterization of a novel P. aeruginosa two-component regulator affecting polymyxin adaptive resistance, ParR–ParS.[115] This system was required for activation of the arnBCADTEF LPS modification operon in the presence of subinhibitory concentrations of polymyxin.

In Enterobacteriaceae, changes in the regulatory loci pmrA and phoP are responsible for polymyxin resistance.[54] Two regulatory loci, pmrA and phoP, control polymyxin resistance in Salmonella typhimurium. The pmrA locus encodes a two-component system, PmrA–PmrB, and a putative membrane protein PmrC.[113,116] The phoP locus encodes a distinct two-component system, PhoP–PhoQ, that governs resistance to several amphipathic antimicrobial peptides and was recently shown to also be required for resistance to polymyxin.[117,118]

In their study, Sun et al. isolated 44 independent mutants of S.enterica serovar Typhimurium with reduced susceptibility to colistin and identified 27 different missense mutations located in the pmrA and pmrB genes that conferred the increased resistance.[119] Recently, Lopez-Rojas et al. demonstrated that an in vitro mutant of A. baumannii resistant to colistin had reduced in vivo fitness and decreased virulence, in terms of both mortality and survival, in a mouse model of peritoneal sepsis.[120,121] The authors have recently reported a patient from Marseille (France) colonized with a colistin-resistant A. baumannii strain after colistin therapy without clinical signs of infection, pointing out the low virulence of A. baumannii with acquired resistance to colistin.[122]

Recently, Miller et al. reported that loss of fuction of phoQ alleles was responsible for the polymyxin resistance in P. aeruginosa strains from CF patients treated with inhaled colistin.[123]

A summary of the resistance mechnisms of polymyxin is presented in Table 2 .