Microbiology & Immunology - Research Publications
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ItemNo Preview AvailableGenome Sequence of the Saprophyte Leptospira biflexa Provides Insights into the Evolution of Leptospira and the Pathogenesis of Leptospirosis(PUBLIC LIBRARY SCIENCE, 2008-02-13)Leptospira biflexa is a free-living saprophytic spirochete present in aquatic environments. We determined the genome sequence of L. biflexa, making it the first saprophytic Leptospira to be sequenced. The L. biflexa genome has 3,590 protein-coding genes distributed across three circular replicons: the major 3,604 chromosome, a smaller 278-kb replicon that also carries essential genes, and a third 74-kb replicon. Comparative sequence analysis provides evidence that L. biflexa is an excellent model for the study of Leptospira evolution; we conclude that 2052 genes (61%) represent a progenitor genome that existed before divergence of pathogenic and saprophytic Leptospira species. Comparisons of the L. biflexa genome with two pathogenic Leptospira species reveal several major findings. Nearly one-third of the L. biflexa genes are absent in pathogenic Leptospira. We suggest that once incorporated into the L. biflexa genome, laterally transferred DNA undergoes minimal rearrangement due to physical restrictions imposed by high gene density and limited presence of transposable elements. In contrast, the genomes of pathogenic Leptospira species undergo frequent rearrangements, often involving recombination between insertion sequences. Identification of genes common to the two pathogenic species, L. borgpetersenii and L. interrogans, but absent in L. biflexa, is consistent with a role for these genes in pathogenesis. Differences in environmental sensing capacities of L. biflexa, L. borgpetersenii, and L. interrogans suggest a model which postulates that loss of signal transduction functions in L. borgpetersenii has impaired its survival outside a mammalian host, whereas L. interrogans has retained environmental sensory functions that facilitate disease transmission through water.
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ItemNo Preview AvailableGenomic analysis of smooth tubercle bacilli provides insights into ancestry and pathoadaptation of Mycobacterium tuberculosis(NATURE PUBLISHING GROUP, 2013-02)Global spread and limited genetic variation are hallmarks of M. tuberculosis, the agent of human tuberculosis. In contrast, Mycobacterium canettii and related tubercle bacilli that also cause human tuberculosis and exhibit unusual smooth colony morphology are restricted to East Africa. Here, we sequenced and analyzed the whole genomes of five representative strains of smooth tubercle bacilli (STB) using Sanger (4-5× coverage), 454/Roche (13-18× coverage) and/or Illumina DNA sequencing (45-105× coverage). We show that STB isolates are highly recombinogenic and evolutionarily early branching, with larger genome sizes, higher rates of genetic variation, fewer molecular scars and distinct CRISPR-Cas systems relative to M. tuberculosis. Despite the differences, all tuberculosis-causing mycobacteria share a highly conserved core genome. Mouse infection experiments showed that STB strains are less persistent and virulent than M. tuberculosis. We conclude that M. tuberculosis emerged from an ancestral STB-like pool of mycobacteria by gain of persistence and virulence mechanisms, and we provide insights into the molecular events involved.
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ItemNo Preview AvailableShiga Toxin-producing Escherichia coli Strains Negative for Locus of Enterocyte Effacement(CENTERS DISEASE CONTROL, 2009-03)Most Shiga toxin-producing Escherichia coli (STEC) infections that are associated with severe sequelae such as hemolytic uremic syndrome (HUS) are caused by attaching and effacing pathogens that carry the locus of enterocyte effacement (LEE). However, a proportion of STEC isolates that do not carry LEE have been associated with HUS. To clarify the emergence of LEE-negative STEC, we compared the genetic composition of the virulence plasmids pO113 and pO157 from LEE-negative and LEE-positive STEC, respectively. The complete nucleotide sequence of pO113 showed that several plasmid genes were shared by STEC O157:H7. In addition, allelic profiling of the ehxA gene demonstrated that pO113 belongs to a different evolutionary lineage than pO157 and that the virulence plasmids of LEE-negative STEC strains were highly related. In contrast, multilocus sequence typing of 17 LEE-negative STEC isolates showed several clonal groups, suggesting that pathogenic LEE-negative STEC has emerged several times throughout its evolution.
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ItemComparative analysis of the complete genome of an epidemic hospital sequence type 203 clone of vancomycin-resistant Enterococcus faecium(BMC, 2013-09-01)BACKGROUND: In this report we have explored the genomic and microbiological basis for a sustained increase in bloodstream infections at a major Australian hospital caused by Enterococcus faecium multi-locus sequence type (ST) 203, an outbreak strain that has largely replaced a predecessor ST17 sequence type. RESULTS: To establish a ST203 reference sequence we fully assembled and annotated the genome of Aus0085, a 2009 vancomycin-resistant Enterococcus faecium (VREfm) bloodstream isolate, and the first example of a completed ST203 genome. Aus0085 has a 3.2 Mb genome, comprising a 2.9 Mb circular chromosome and six circular plasmids (2 kb-130 kb). Twelve percent of the 3222 coding sequences (CDS) in Aus0085 are not present in ST17 E. faecium Aus0004 and ST18 E. faecium TX16. Extending this comparison to an additional 12 ST17 and 14 ST203 E. faecium hospital isolate genomes revealed only six genomic regions spanning 41 kb that were present in all ST203 and absent from all ST17 genomes. The 40 CDS have predicted functions that include ion transport, riboflavin metabolism and two phosphotransferase systems. Comparison of the vancomycin resistance-conferring Tn1549 transposon between Aus0004 and Aus0085 revealed differences in transposon length and insertion site, and van locus sequence variation that correlated with a higher vancomycin MIC in Aus0085. Additional phenotype comparisons between ST17 and ST203 isolates showed that while there were no differences in biofilm-formation and killing of Galleria mellonella, ST203 isolates grew significantly faster and out-competed ST17 isolates in growth assays. CONCLUSIONS: Here we have fully assembled and annotated the first ST203 genome, and then characterized the genomic differences between ST17 and ST203 E. faecium. We also show that ST203 E. faecium are faster growing and can out-compete ST17 E. faecium. While a causal genetic basis for these phenotype differences is not provided here, this study revealed conserved genetic differences between the two clones, differences that can now be tested to explain the molecular basis for the success and emergence of ST203 E. faecium.
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ItemExploring the Zoonotic Potential of Mycobacterium avium Subspecies paratuberculosis through Comparative Genomics(PUBLIC LIBRARY SCIENCE, 2011-07-22)A comparative genomics approach was utilised to compare the genomes of Mycobacterium avium subspecies paratuberculosis (MAP) isolated from early onset paediatric Crohn's disease (CD) patients as well as Johne's diseased animals. Draft genome sequences were produced for MAP isolates derived from four CD patients, one ulcerative colitis (UC) patient, and two non-inflammatory bowel disease (IBD) control individuals using Illumina sequencing, complemented by comparative genome hybridisation (CGH). MAP isolates derived from two bovine and one ovine host were also subjected to whole genome sequencing and CGH. All seven human derived MAP isolates were highly genetically similar and clustered together with one bovine type isolate following phylogenetic analysis. Three other sequenced isolates (including the reference bovine derived isolate K10) were genetically distinct. The human isolates contained two large tandem duplications, the organisations of which were confirmed by PCR. Designated vGI-17 and vGI-18 these duplications spanned 63 and 109 open reading frames, respectively. PCR screening of over 30 additional MAP isolates (3 human derived, 27 animal derived and one environmental isolate) confirmed that vGI-17 and vGI-18 are common across many isolates. Quantitative real-time PCR of vGI-17 demonstrated that the proportion of cells containing the vGI-17 duplication varied between 0.01 to 15% amongst isolates with human isolates containing a higher proportion of vGI-17 compared to most animal isolates. These findings suggest these duplications are transient genomic rearrangements. We hypothesise that the over-representation of vGI-17 in human derived MAP strains may enhance their ability to infect or persist within a human host by increasing genome redundancy and conferring crude regulation of protein expression across biologically important regions.
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ItemThe Dominant Australian Community-Acquired Methicillin-Resistant Staphylococcus aureus Clone ST93-IV [2B] Is Highly Virulent and Genetically Distinct(PUBLIC LIBRARY SCIENCE, 2011-10-03)Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) USA300 has spread rapidly across North America, and CA-MRSA is also increasing in Australia. However, the dominant Australian CA-MRSA strain, ST93-IV [2B] appears distantly related to USA300 despite strikingly similar clinical and epidemiological profiles. Here, we compared the virulence of a recent Australian ST93 isolate (JKD6159) to other MRSA, including USA300, and found that JKD6159 was the most virulent in a mouse skin infection model. We fully sequenced the genome of JKD6159 and confirmed that JKD6159 is a distinct clone with 7616 single nucleotide polymorphisms (SNPs) distinguishing this strain from all other S. aureus genomes. Despite its high virulence there were surprisingly few virulence determinants. However, genes encoding α-hemolysin, Panton-Valentine leukocidin (PVL) and α-type phenol soluble modulins were present. Genome comparisons revealed 32 additional CDS in JKD6159 but none appeared to encode new virulence factors, suggesting that this clone's enhanced pathogenicity could lie within subtler genome changes, such as SNPs within regulatory genes. To investigate the role of accessory genome elements in CA-MRSA epidemiology, we next sequenced three additional Australian non-ST93 CA-MRSA strains and compared them with JKD6159, 19 completed S. aureus genomes and 59 additional S. aureus genomes for which unassembled genome sequence data was publicly available (82 genomes in total). These comparisons showed that despite its distinctive genotype, JKD6159 and other CA-MRSA clones (including USA300) share a conserved repertoire of three notable accessory elements (SSCmecIV, PVL prophage, and pMW2). This study demonstrates that the genetically distinct ST93 CA-MRSA from Australia is highly virulent. Our comparisons of geographically and genetically diverse CA-MRSA genomes suggest that apparent convergent evolution in CA-MRSA may be better explained by the rapid dissemination of a highly conserved accessory genome from a common source.
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ItemEvolution of Multidrug Resistance during Staphylococcus aureus Infection Involves Mutation of the Essential Two Component Regulator WalKR(PUBLIC LIBRARY SCIENCE, 2011-11)Antimicrobial resistance in Staphylococcus aureus is a major public health threat, compounded by emergence of strains with resistance to vancomycin and daptomycin, both last line antimicrobials. Here we have performed high throughput DNA sequencing and comparative genomics for five clinical pairs of vancomycin-susceptible (VSSA) and vancomycin-intermediate ST239 S. aureus (VISA); each pair isolated before and after vancomycin treatment failure. These comparisons revealed a frequent pattern of mutation among the VISA strains within the essential walKR two-component regulatory locus involved in control of cell wall metabolism. We then conducted bi-directional allelic exchange experiments in our clinical VSSA and VISA strains and showed that single nucleotide substitutions within either walK or walR lead to co-resistance to vancomycin and daptomycin, and caused the typical cell wall thickening observed in resistant clinical isolates. Ion Torrent genome sequencing confirmed no additional regulatory mutations had been introduced into either the walR or walK VISA mutants during the allelic exchange process. However, two potential compensatory mutations were detected within putative transport genes for the walK mutant. The minimal genetic changes in either walK or walR also attenuated virulence, reduced biofilm formation, and led to consistent transcriptional changes that suggest an important role for this regulator in control of central metabolism. This study highlights the dramatic impacts of single mutations that arise during persistent S. aureus infections and demonstrates the role played by walKR to increase drug resistance, control metabolism and alter the virulence potential of this pathogen.
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ItemTwo Novel Point Mutations in Clinical Staphylococcus aureus Reduce Linezolid Susceptibility and Switch on the Stringent Response to Promote Persistent Infection(PUBLIC LIBRARY SCIENCE, 2010-06)Staphylococcus aureus frequently invades the human bloodstream, leading to life threatening bacteremia and often secondary foci of infection. Failure of antibiotic therapy to eradicate infection is frequently described; in some cases associated with altered S. aureus antimicrobial resistance or the small colony variant (SCV) phenotype. Newer antimicrobials, such as linezolid, remain the last available therapy for some patients with multi-resistant S. aureus infections. Using comparative and functional genomics we investigated the molecular determinants of resistance and SCV formation in sequential S. aureus isolates from a patient who had a persistent and recurrent S. aureus infection, after failed therapy with multiple antimicrobials, including linezolid. Two point mutations in key staphylococcal genes dramatically affected clinical behaviour of the bacterium, altering virulence and antimicrobial resistance. Most strikingly, a single nucleotide substitution in relA (SACOL1689) reduced RelA hydrolase activity and caused accumulation of the intracellular signalling molecule guanosine 3', 5'-bis(diphosphate) (ppGpp) and permanent activation of the stringent response, which has not previously been reported in S. aureus. Using the clinical isolate and a defined mutant with an identical relA mutation, we demonstrate for the first time the impact of an active stringent response in S. aureus, which was associated with reduced growth, and attenuated virulence in the Galleria mellonella model. In addition, a mutation in rlmN (SACOL1230), encoding a ribosomal methyltransferase that methylates 23S rRNA at position A2503, caused a reduction in linezolid susceptibility. These results reinforce the exquisite adaptability of S. aureus and show how subtle molecular changes cause major alterations in bacterial behaviour, as well as highlighting potential weaknesses of current antibiotic treatment regimens.
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ItemPseudomonas aeruginosa AES-1 Exhibits Increased Virulence Gene Expression during Chronic Infection of Cystic Fibrosis Lung(PUBLIC LIBRARY SCIENCE, 2011-09-15)Pseudomonas aeruginosa, the leading cause of morbidity and mortality in people with cystic fibrosis (CF), adapts for survival in the CF lung through both mutation and gene expression changes. Frequent clonal strains such as the Australian Epidemic Strain-1 (AES-1), have increased ability to establish infection in the CF lung and to superimpose and replace infrequent clonal strains. Little is known about the factors underpinning these properties. Analysis has been hampered by lack of expression array templates containing CF-strain specific genes. We sequenced the genome of an acute infection AES-1 isolate from a CF infant (AES-1R) and constructed a non-redundant micro-array (PANarray) comprising AES-1R and seven other sequenced P. aeruginosa genomes. The unclosed AES-1R genome comprised 6.254Mbp and contained 6957 putative genes, including 338 not found in the other seven genomes. The PANarray contained 12,543 gene probe spots; comprising 12,147 P. aeruginosa gene probes, 326 quality-control probes and 70 probes for non-P. aeruginosa genes, including phage and plant genes. We grew AES-1R and its isogenic pair AES-1M, taken from the same patient 10.5 years later and not eradicated in the intervening period, in our validated artificial sputum medium (ASMDM) and used the PANarray to compare gene expression of both in duplicate. 675 genes were differentially expressed between the isogenic pairs, including upregulation of alginate, biofilm, persistence genes and virulence-related genes such as dihydroorotase, uridylate kinase and cardiolipin synthase, in AES-1M. Non-PAO1 genes upregulated in AES-1M included pathogenesis-related (PAGI-5) genes present in strains PACS2 and PA7, and numerous phage genes. Elucidation of these genes' roles could lead to targeted treatment strategies for chronically infected CF patients.
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ItemFis Is Essential for Capsule Production in Pasteurella multocida and Regulates Expression of Other Important Virulence Factors(PUBLIC LIBRARY SCIENCE, 2010-02)P. multocida is the causative agent of a wide range of diseases of animals, including fowl cholera in poultry and wild birds. Fowl cholera isolates of P. multocida generally express a capsular polysaccharide composed of hyaluronic acid. There have been reports of spontaneous capsule loss in P. multocida, but the mechanism by which this occurs has not been determined. In this study, we identified three independent strains that had spontaneously lost the ability to produce capsular polysaccharide. Quantitative RT-PCR showed that these strains had significantly reduced transcription of the capsule biosynthetic genes, but DNA sequence analysis identified no mutations within the capsule biosynthetic locus. However, whole-genome sequencing of paired capsulated and acapsular strains identified a single point mutation within the fis gene in the acapsular strain. Sequencing of fis from two independently derived spontaneous acapsular strains showed that each contained a mutation within fis. Complementation of these strains with an intact copy of fis, predicted to encode a transcriptional regulator, returned capsule expression to all strains. Therefore, expression of a functional Fis protein is essential for capsule expression in P. multocida. DNA microarray analysis of one of the spontaneous fis mutants identified approximately 30 genes as down-regulated in the mutant, including pfhB_2, which encodes a filamentous hemagglutinin, a known P. multocida virulence factor, and plpE, which encodes the cross protective surface antigen PlpE. Therefore these experiments define for the first time a mechanism for spontaneous capsule loss in P. multocida and identify Fis as a critical regulator of capsule expression. Furthermore, Fis is involved in the regulation of a range of other P. multocida genes including important virulence factors.