Genomic diversity of citrate fermentation in Klebsiella pneumoniae

Abstract

Background: It has long been recognized that Klebsiella pneumoniae can grow anaerobically on citrate. Genes responsible for citrate fermentation of K. pneumoniae were known to be located in a 13-kb gene cluster on the chromosome. By whole genome comparison of the available K. pneumoniae sequences (MGH 78578, 342, and NTUH-K2044), however, we discovered that the fermentation gene cluster was present in MGH 78578 and 342, but absent in NTUH-K2044. In the present study, the previously unknown genome diversity of citrate fermentation among K. pneumoniae clinical isolates was investigated.

Results: Using a genomic microarray containing probe sequences from multiple K. pneumoniae strains, we investigated genetic diversity among K. pneumoniae clinical isolates and found that a genomic region containing the citrate fermentation genes was not universally present in all strains. We confirmed by PCR analysis that the gene cluster was detectable in about half of the strains tested. To demonstrate the metabolic function of the genomic region, anaerobic growth of K. pneumoniae in artificial urine medium (AUM) was examined for ten strains with different clinical histories and genomic backgrounds, and the citrate fermentation potential was found correlated with the genomic region. PCR detection of the genomic region yielded high positive rates among a variety of clinical isolates collected from urine, blood, wound infection, and pneumonia. Conserved genetic organizations in the vicinity of the citrate fermentation gene clusters among K. pneumoniae, Salmonella enterica, and Escherichia coli suggest that the 13-kb genomic region were not independently acquired.

Conclusion: Not all, but nearly half of the K. pneumoniae clinical isolates carry the genes responsible for anaerobic growth on citrate. Genomic variation of citrate fermentation genes in K. pneumoniae may contribute to metabolic diversity and adaptation to variable nutrient conditions in different environments.

Figure 1

Comparative analysis of citrate fermentation gene locus. The 13-kb genomic region is present in K. pneumoniae MGH 78578 but absent in NTUH-K2044 (a). The location of the 13-kb genomic region for citrate fermentation, which includes two divergently transcribed operons, citS-oadGAB-citAB and citC2D2E2F2G2, are marked. The adjacent hypothetical orfs are shown in gray, among which the ltrA encodes a putative transcriptional regulator. The citYZ-citWX gene clusters downstream of the dapB in both MGH 78578 and NTUH-K2044 are also depicted. The G+C value for each orf in MGH 78578 is shown below each orf. The red bar indicates the corresponding location replaced by an apramycin resistant gene in the promoter knocked-out strain, NK8-Δcit, derived from the NK8 clinical strain. Corresponding citrate fermentation loci from S. enterica serovar Typhimurium LT2 and E. coli K12 are shown (b and c) with colours indicating homologous genes. Alternative gene names in parentheses on top of some orfs for better comparison were based on homology search. The locations of these regions in the genomes are marked below. In the LT2 genome, two clusters of citrate fermentation genes were found. The corresponding flanking genes for locus I, dcuC and rna, and locus II, rihC and dapB, are shown in black.

Figure 2

Genomic variation at the citrate fermentation gene locus. Divergence of the 13-kb genomic region in 19 K. pneumoniae strains was detected by CGH analysis using the NimbleGen chips. Hybridization signals of each probes placed in the order of the MGH 78578 genome were compared with those of the reference strain, NTUH-K2044. The probes covering the cit genes and the oad genes of the 13-kb region were shown together with that of the adjacent orfs. The normalized CGH signals for each probe are plotted as black dots. The dot position above or under the baseline represents higher or lower copy of specific genomic sequence in comparison to the reference. The scores in vertical axis are log2 values of test/reference signal intensity obtained from image scanning of hybridization results. The detection of elevated scores in the cit genes (citA-B, citS~citG2) in the last 10 strains (from NK3 to MGH 78278) is marked by solid triangles. Variations in the oad region are marked by open triangles.

Figure 3

Citrate gene cluster permits fermentation growth in AUM for the NTUH-K2044 strain. NTUH-K2044, a strain that lacks the 13-kb genomic region; NTUH-K2044-F06C06, NTUH-K2044 transformed by a fosmid (F06C06) carrying the 13-kb genomic region responsible for citrate fermentation from NK8.

Figure 4

Citrate gene cluster is necessary for fermentation growth in AUM for the NK8 strain. NK8 is a clinical strain carrying the same citrate fermentation genes as the sequenced reference strain, MGH 78578; NK8-Δcit, NK8 with the 13-kb genomic region disrupted at the promoter region. The initial OD₆₀₀ of the inoculated AUM culture is 0.0005.