Abigail Clements

MrkH, a Novel c-di-GMP-Dependent Transcriptional Activator, Controls Klebsiella pneumoniae Biofilm Formation by Regulating Type 3 Fimbriae Expression

Klebsiella pneumoniae causes significant morbidity and mortality worldwide, particularly amongst hospitalized individuals. The principle mechanism for pathogenesis in hospital environments involves the formation of biofilms, primarily on implanted medical devices. In this study, we constructed a transposon mutant library in a clinical isolate, K. pneumoniae AJ218, to identify the genes and pathways implicated in biofilm formation. Three mutants severely defective in biofilm formation contained insertions within the mrkABCDF genes encoding the main structural subunit and assembly machinery for type 3 fimbriae. Two other mutants carried insertions within the yfiN and mrkJ genes, which encode GGDEF domain- and EAL domain-containing c-di-GMP turnover enzymes, respectively. The remaining two isolates contained insertions that inactivated the mrkH and mrkI genes, which encode for novel proteins with a c-di-GMP-binding PilZ domain and a LuxR-type transcriptional regulator, respectively. Biochemical and functional assays indicated that the effects of these factors on biofilm formation accompany concomitant changes in type 3 fimbriae expression. We mapped the transcriptional start site of mrkA, demonstrated that MrkH directly activates transcription of the mrkA promoter and showed that MrkH binds strongly to the mrkA regulatory region only in the presence of c-di-GMP. Furthermore, a point mutation in the putative c-di-GMP-binding domain of MrkH completely abolished its function as a transcriptional activator. In vivo analysis of the yfiN and mrkJ genes strongly indicated their c-di-GMP-specific function as diguanylate cyclase and phosphodiesterase, respectively. In addition, in vitro assays showed that purified MrkJ protein has strong c-di-GMP phosphodiesterase activity. These results demonstrate for the first time that c-di-GMP can function as an effector to stimulate the activity of a transcriptional activator, and explain how type 3 fimbriae expression is coordinated with other gene expression programs in K. pneumoniae to promote biofilm formation to implanted medical devices.

Infection strategies of enteric pathogenic Escherichia coli

Enteric Escherichia coli (E. coli) are both natural flora of humans and important pathogens causing significant morbidity and mortality worldwide. Traditionally enteric E. coli have been divided into 6 pathotypes, with further pathotypes often proposed. In this review we suggest expansion of the enteric E. coli into 8 pathotypes to include the emerging pathotypes of adherent invasive E. coli (AIEC) and Shiga-toxin producing enteroaggregative E. coli (STEAEC). The molecular mechanisms that allow enteric E. coli to colonize and cause disease in the human host are examined and for two of the pathotypes that express a type 3 secretion system (T3SS) we discuss the complex interplay between translocated effectors and manipulation of host cell signaling pathways that occurs during infection.

Subversion of trafficking, apoptosis, and innate immunity by type III secretion system effectors

Injection of effector proteins by a type III secretion system (T3SS) is a common infection strategy employed by many important human pathogens, including enteric Escherichia coli, Salmonella, Yersinia, and Shigella, to subvert cell signaling and host responses. In recent years, great advances have been made in understanding how the T3SS effectors function and execute the diverse infection strategies employed by these pathogens. In this review, we focus on effectors that subvert signaling pathways that impact on endosomal trafficking, cell survival, and innate immunity, particularly phagocytosis, nuclear factor-κB (NF-κB), and mitogen-activated protein (MAP) kinase pathways and the inflammasome.

Secondary Acylation of Klebsiella pneumoniae Lipopolysaccharide Contributes to Sensitivity to Antibacterial Peptides

Klebsiella pneumoniae is an important cause of nosocomial Gram-negative sepsis. Lipopolysaccharide (LPS) is considered to be a major virulence determinant of this encapsulated bacterium and most mutations to the lipid A anchor of LPS are conditionally lethal to the bacterium. We studied the role of LPS acylation in K. pneumoniae disease pathogenesis by using a mutation of lpxM (msbB/waaN), which encodes the enzyme responsible for late secondary acylation of immature lipid A molecules. A K. pneumoniae B5055 (K2:O1) lpxM mutant was found to be attenuated for growth in the lungs in a mouse pneumonia model leading to reduced lethality of the bacterium. B5055ΔlpxM exhibited similar sensitivity to phagocytosis or complement-mediated lysis than B5055, unlike the non-encapsulated mutant B5055nm. In vitro, B5055ΔlpxM showed increased permeability of the outer membrane and an increased susceptibility to certain antibacterial peptides suggesting that in vivo attenuation may be due in part to sensitivity to antibacterial peptides present in the lungs of BALB/c mice. These data support the view that lipopolysaccharide acylation plays a important role in providing Gram-negative bacteria some resistance to structural and innate defenses and especially the antibacterial properties of detergents (e.g. bile) and cationic defensins.

The reducible complexity of a mitochondrial molecular machine

Molecular machines drive essential biological processes, with the component parts of these machines each contributing a partial function or structural element. Mitochondria are organelles of eukaryotic cells, and depend for their biogenesis on a set of molecular machines for protein transport. How these molecular machines evolved is a fundamental question. Mitochondria were derived from an α-proteobacterial endosymbiont, and we identified in α-proteobacteria the component parts of a mitochondrial protein transport machine. In bacteria, the components are found in the inner membrane, topologically equivalent to the mitochondrial proteins. Although the bacterial proteins function in simple assemblies, relatively little mutation would be required to convert them to function as a protein transport machine. This analysis of protein transport provides a blueprint for the evolution of cellular machinery in general.

The Major Surface-Associated Saccharides of Klebsiella pneumoniae Contribute to Host Cell Association

Analysing the pathogenic mechanisms of a bacterium requires an understanding of the composition of the bacterial cell surface. The bacterial surface provides the first barrier against innate immune mechanisms as well as mediating attachment to cells/surfaces to resist clearance. We utilised a series of Klebsiella pneumoniae mutants in which the two major polysaccharide layers, capsule and lipopolysaccharide (LPS), were absent or truncated, to investigate the ability of these layers to protect against innate immune mechanisms and to associate with eukaryotic cells. The capsule alone was found to be essential for resistance to complement mediated killing while both capsule and LPS were involved in cell-association, albeit through different mechanisms. The capsule impeded cell-association while the LPS saccharides increased cell-association in a non-specific manner. The electrohydrodynamic characteristics of the strains suggested the differing interaction of each bacterial strain with eukaryotic cells could be partly explained by the charge density displayed by the outermost polysaccharide layer. This highlights the importance of considering not only specific adhesin:ligand interactions commonly studied in adherence assays but also the initial non-specific interactions governed largely by the electrostatic interaction forces.

Interactions between TonB from Escherichia coli and the Periplasmic Protein FhuD

For uptake of ferrichrome into bacterial cells, FhuA, a TonB-dependent outer membrane receptor of Escherichia coli, is required. The periplasmic protein FhuD binds and transfers ferrichrome to the cytoplasmic membrane-associated permease FhuB/C. We exploited phage display to map protein-protein interactions in the E. coli cell envelope that contribute to ferrichrome transport. By panning random phage libraries against TonB and against FhuD, we identified interaction surfaces on each of these two proteins. Their interactions were detected in vitro by dynamic light scattering and indicated a 1:1 TonB-FhuD complex. FhuD residue Thr-181, located within the siderophorebinding site and mapping to a predicted TonB-interaction surface, was mutated to cysteine. FhuD T181C was reacted with two thiol-specific fluorescent probes; addition of the siderophore ferricrocin quenched fluorescence emissions of these conjugates. Similarly, quenching of fluorescence from both probes confirmed binding of TonB and established an apparent KD of ∼300 nm. Prior saturation of the siderophorebinding site of FhuD with ferricrocin did not alter affinity of TonB for FhuD. Binding, further characterized with surface plasmon resonance, indicated a higher affinity complex with KD values in the low nanomolar range. Addition of FhuD to a preformed TonB-FhuA complex resulted in formation of a ternary complex. These observations led us to propose a novel mechanism in which TonB acts as a scaffold, directing FhuD to regions within the periplasm where it is poised to accept and deliver siderophore.

Bax Inhibitor 1 in apoptosis and disease

Bax inhibitor 1 (BI-1) was originally discovered as an inhibitor of Bax-induced apoptosis; this review highlights the fundamental importance of BI-1 in a wider context, including in tissue homeostasis and as a regulator of cellular stress. BI-1 has been shown to interact with a broad range of partners to inhibit many facets of apoptosis, such as reactive oxygen species production, cytosolic acidification and calcium levels as well as endoplasmic reticulum stress signalling pathways. BI-1s anti-apoptotic action initially enables the cell to adapt to stress, although if the stress is prolonged or severe the actions of BI-1 may promote apoptosis. This almost universal anti-apoptotic capacity has been shown to be manipulated during infection with enteropathogenic and enterohaemorrhagic Escherichia coli inhibiting host cell death through direct interaction between their effector NleH and BI-1. In addition, BI-1 activity is important in a large number of cancers, promoting metastasis by modulating actin dynamics, a process dependent upon the BI-1 C-terminus and BI-1:actin interaction. Manipulation of BI-1 therefore has the potential for significant therapeutic benefit in a wide range of human diseases.

Seroepidemiology of Klebsiella pneumoniae in an Australian Tertiary Hospital and Its Implications for Vaccine Development

ABSTRACT The aim of this study was to determine the diversity of Klebsiella pneumoniae capsular serotypes in an Australian setting. Consecutive (n = 293) nonrepetitive isolates of K. pneumoniae from a large teaching hospital laboratory were analyzed. The majority of isolates were from urinary specimens (60.8%); the next most common source was sputum (14.3%), followed by blood (14%). Serotyping revealed a wide range of capsule types. K54 (17.1%), K28 (4.1%), and K17 (3.1%) were the most common, and K54 isolates displayed a high degree of clonality, suggesting a common, nosocomial source. In vitro, one K54 isolate was more adherent to urinary catheters and HEp-2 cells than four other tested isolates; it was slightly more resistant to chlorhexidine but was more susceptible to drying than heavily encapsulated strains. This is the first seroprevalence survey of K. pneumoniae to be performed on Australian isolates, and the high level of diversity of serotypes suggests that capsule-based immunoprophylaxis might not be useful for Australia. In addition there are significant differences in the predominance of specific serotypes compared to the results of surveys performed overseas, which has important implications for capsule-based immunoprophylaxis aimed at a global market.

Tinkering Inside the Organelle

Debate about eukaryote evolution includes alternate views on the processes that gave rise to mitochondria. Among the questions about the evolution of eukaryotes is the debate over how they acquired the membrane-bound organelle, mitochondria. Mitochondria produce energy in nearly all eukaryotic cells (1) and regulate cell metabolism by controlling the flow of factors such as ions, amino acids, and carbohydrates between themselves and the cytoplasm. Mitochondria evolved from a bacterial endosymbiont (an α-proteobacterium), and this process depended on the establishment of new pathways that facilitated the import of proteins into and across the double membrane (inner and outer) of the ancestral endosymbiont. Herein lies a debate: How did the process of protein import in mitochondria—which facilitated the evolution of this organelle, and thus, eukaryotic cell evolution—arise? Was the process driven by the ancestral host cell or by the prokaryotic endosymbiont, or by both?