Multiple mutations in lipid-A modification pathway & novel fosA variants in colistin-resistant Klebsiella pneumoniae

Aim: To investigate antimicrobial resistance mechanisms in a cluster of colistin-resistant Klebsiella pneumoniae. Methods: Antimicrobial susceptibility was tested by disk diffusion and broth microdilution. Whole-genome sequencing and genome analysis were performed. Results: The eight colistin-resistant K. pneumoniae isolates belonged to three different clones (ST11, 14 and 231). The eptA and arnT genes from lipid modification pathway had novel (R157S in arnT and Q319R in eptA) and rare mutations (V39L, R152H, S260L and A279G in eptA). Several substitutions were also identified in mgrB, pmrB, phoP and phoQ genes. The mcr genes were absent in all isolates. Isolates had variants from existing classes of fosA gene. Conclusion: Complex combination of mutations might have led to colistin resistance, which suggests that continuous surveillance of molecular mechanisms is required.

. Phenotypic susceptibility testing and polymerase chain reaction data of colistin-resistant Klebsiella pneumoniae.

Antimicrobial susceptibility
The resistance pattern for the colistin-resistant K. pneumoniae isolates (n = 8) were as given in Table 1. All eight isolates were resistant to cefpodoxime, cefotaxime, ceftazidime, cefoxitin, ciprofloxacin, gentamicin, amikacin, netilmicin and cefoperazone/sulbactam. Isolates except PM5186 were resistant to meropenem and all eight were resistant to colistin by broth microdilution. MICs for colistin ranged from 4-16 μg/ml. Isolate PM716 was resistant to fosfomycin while the remaining were either intermediate or susceptible (Table 1).

Genome analysis
Raw read assembly of the genome data presented 105-160 contigs (≥ 500 bp). The genome coverage of these isolates were about 32x-51x. The coding sequences (CDSs) of the genomes range from 5859 to 6744, rRNAs from 11 to 14, tRNAs from 64 to 73. The annotation revealed multiple antimicrobial resistance genes ranging from 26 to 38 from ARDB database, and 75-97 from CARD database (www.patricbrc.org). Similarly, for virulence genes, the virulence factor database (VFDB) and Victors database revealed the presence of 81-106 and 177-188 genes, respectively (www.patricbrc.org) ( Table 2). Whole genome sequencing of all eight isolates were deposited in Genbank/DDBJ under the accession numbers as follows: PM565 -MNPB00000000; PM1842 -MNPC00000000; PM1995 -MNPD00000000; PM138 -MNPG00000000; PM716 -MNPH00000000; PM1134 -MNPF00000000; PM5186 -MNPE00000000 and PM1168 -MNPA00000000. The sequence type of the isolates were found to be ST11 for PM565, PM1842, PM1995, PM138, ST14 for PM716, PM1134, and ST231 for PM5186 and PM1168 as analysed by MLST 1.8 tool (https://cge.cbs.dtu.dk //services/MLST/). ResFinder 2.1 (www.cbs.dtu.dk/services) returned multiple antimicrobial resistance genes for most of the antibiotic classes (Table 3). Interestingly fosfomycin, fluoroquinolone, aminoglycoside and β-lactam resistant determinants were found in all eight isolates. The fosA genes observed in these isolates were different from the existing six variants and reported for the first time in this study ( Figure 1). However, plasmid mediated colistin   T  T  T  T  T  T  T  T  T  T  T  T  T  T  T  T  T  T .   resistance determinants mcr-1 and mcr-2 were not found in any of the isolates.

Mutational analysis
Multiple mutations were observed in the genes responsible for lipid A modification and Ara-4 N pathway in K. pneumoniae isolates (Table 4). Interestingly, novel (R157S in arnT & Q319R in eptA) and rare mutations (V39L, R152H, S260L, A279G in eptA) were observed in the isolates studied. R157S in arnT was observed in all the isolates, whereas Q319R in eptA was observed in PM565, PM1995, PM138 and PM1842. It is important to note that there were deletions of three amino acids LLG at 521, 522 and 523 (Table 4). PM1168 and PM5186 had mutations in mgrB gene, V1A and L24H, respectively.

Discussion
Currently, there are increasing reports of CRE which results in less choice of antimicrobials for therapy. Fosfomycin is gaining interest for the treatment of carbapenem-resistant K. pneumoniae [2]. In this scenario, resistance to fosfomycin is an alarming threat to those treating infections by Enterobacteriaceae, especially nosocomial pathogens. The eight selected colistin-resistant K. pneumoniae isolates were observed to be of three different clonal types (ST11, ST14 and ST231) as observed by eBURST analysis and UPGMA dendogram (Figures 2 & 3). These were the commonly reported sequence types previously reported from India [11]. Also, among the seven meropenemresistant isolates, bla OXA producers; bla OXA-232 (n = 5) and bla OXA181 (n = 1) were commonly seen followed by bla NDM-1 (n = 2).
Polymyxins are known to serve as drug of choice for carbapenem-resistant K. pneumoniae either alone or in combination. Polymyxin acts like cationic detergents and disrupts the cytoplasmic membrane by attacking phosphate groups of membrane phospholipid; this ultimately leads to leakage of cytoplasmic contents and death of cell [12]. In this regard, lipid A modification genes were largely known to be involved in chromosomal-mediated colistin resistance. Among the seven genes which are known to be involved in the lipid A modifications (paqP, pmrA, pmrB, phoP, phoQ, eptA and eptB), only four genes (pmrA, pmrB, phoP and phoQ) were extensively discussed in the literature. In K. pneumoniae, mutations including G53C, E35A in pmrA [13]; S85R, T140P, T157P, S205P [13], T157P [14], T157P and S208N with deletion of three nucleotide at 14 and 209 in pmrB [15]; L26Q in phoP [13]; S174N and L384Q in phoQ [15] were previously reported.
In this study, mutations were observed in ten genes (paqP, pmrB, phoP, phoQ and eptA of lipid A modifications and arnA DH/FT, arnB, arnC, arnT and pmrJ of lipid A-Ara4N pathway) which includes novel (R157S in arnT & Q319R in eptA) and rare mutations (V39L, R152H, S260L and A279G in eptA) which might be conferring for colistin resistance. Recently, in one of our studies, we have observed novel mutations in eptA gene of lipid A modification pathway and arnT gene of lipid A-Ara4N pathway among cluster of isolates from South India [4]. Also, two study isolates (PM1168 and PM5186) exhibited mutations in mgrB gene, the most commonly reported genetic determinant for colistin resistance. L24H observed in PM5186 was previously reported by Cannatelli et al. [16], whereas V1A (GTG -GCC) was not previously reported. However, no major change was observed in colistin MIC levels for the isolates with and without mutation in mgrB. The role of observed mutations in colistin resistance development should be further analyzed with confirmatory tests.
It is also worth noting that there were deletions of LLG aminoacids in eptA gene in one isolate (PM1842). The arnT gene belonging to L-Ara4N moiety and eptA were known to be responsible for attachment of modified  arabinose to lipid A 4 -phosphate group. This reduces bacterial susceptibility towards polymyxin and cationic antimicrobial peptides [17].
Interestingly, plasmid-mediated colistin resistance genes mcr-1 and mcr-2 were not seen in these isolates. In spite of their absence, the isolates were resistant to colistin (with MICs 8 and 16 μg/ml) indicating the clinical importance of chromosomal mutations in the lipid A modification pathway.
In addition, intravenous fosfomycin had been proposed as a treatment option for systemic infections by resistant K. pneumoniae [18]. However, resistance may develop to fosfomycin during treatment. Resistance to fosfomycin involves various mechanisms, majorly chromosomal-mediated and plasmid-mediated. Transferable plasmids with fosfomycin-resistant determinants result in accelerated dissemination of fosfomycin resistance. Also, fosA and fosB were reported to be responsible for plasmid-mediated resistance, whereas fosX was cited to be responsible for chromosomal-mediated resistance [18]. The fosA gene encodes a glutathione S-transferase and fosB encodes an Lcysteine thiol transferase, while fosX encodes an epoxide hydrolase [19]. Among these fosA seems to be widely reported in K. pneumoniae isolates [20][21][22]. To date, six variants of fosA have been reported worldwide which includes fosA (NC 011617.1), fosA2 (ACC85616.1), fosA3 (NC 019073.1), fosA4 (WP 034169466.1), fosA5 (NC 022374.1) and fosA6 (AMQ12811.1). However, to the best of our knowledge, reports are lacking at variant level identification of fosA from India. In this study we observed clusters of K. pneumoniae isolates with novel fosA variants. The variant numbers (PM1168 -fosA7; PM716 -fosA8; PM138, PM1995, PM1842 -fosA9; PM5186 -fosA10; PM565 -fosA11 and PM1134 -fosA12) were assigned based on the phylogenetic variation of fosA genes (Figure 4). The isolate with a fosA8 gene had a high MIC of 1024 μg/ml for fosfomycin. However, all other variants of fosA genes reported in this study were noted to be either susceptible or moderately susceptible to fosfomycin. In addition, eight more K. pneumoniae isolates were screened for fosA genes, where all eight were positive for the gene but phenotypically susceptible to fosfomycin. Further studies are required to understand the mechanisms behind fosfomycin resistance and the non-functional variants.

Conclusion
Overall the study reports novel and rare mutations in the arnT gene of the Ara-4 N pathway and the eptA gene of lipid A modifications. The complex combination of such mutations leads to high MIC levels for colistin. The result of the study provide an argument for continuous surveillance of the molecular mechanism behind the colistin resistance.

Future perspective
Most recently, colistin resistance is rapidly increasing among K. pneumoniae. The major mechanism reported for colistin resistance is mutations in lipid A modification genes, in which several novel mutations are being reported.
Functional validation of such mutations might reveal the level of resistance with each mutation. Plasmid-mediated colistin resistance is seen predominantly in animals, while chromosomal-mediated resistance is higher in humans. It is important to better understand resistance mechanisms -either chromosomal-or plasmid-mediated -and the trend of plasmid-mediated resistance will help us to delineate transmission dynamics of animal to human spread. This information will facilitate the appropriate containment of colistin-resistant pathogen infections.

Summary points
• To the best of our knowledge, variants of fos A from India have not yet been characterized. The study reports novel variants of fos A genes at amino acid level from colistin-resistant Klebsiella pneumoniae. The variant numbers (PM1168 -fosA7; PM716 -fosA8; PM138, PM1995, PM1842 -fosA9; PM5186 -fosA10 and PM565 -fosA11) were assigned based on the phylogenetic variation of fosA genes.
• The study also reports novel mutations in arnT gene of Ara-4 N pathway and rare mutations in eptA gene of lipid A modification pathway involved in contributing to colistin resistance.
• The plasmid-mediated colistin resistance genes mcr-1 and mcr-2 were absent in all eight K. pneumoniae isolates.
• The bla OXA-232 was seen in most of the isolates, conferring resistance to carbapenem.
• The common sequence types observed from this study were ST-11 followed by ST-14 and ST-231. Financial & competing interests disclosure