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Dr Leigh Monahan

Postdoctoral Fellow, The ithree Institute
Core Member, ithree - Institute of Infection, Immunity and Innovation
BSc(Adv)(Hons)(USyd), PhD (UTS)
Member, Australian Society for Microbiology
 
Phone
+61 2 9514 4066
Can supervise: Yes

Chapters

Monahan, L.G., D'Elia, M. & Harry, L. 2011, 'Mining bacterial cell division for new antibacterial drugs' in Miller, A.A. & Miller, P.F. (eds), Emerging Trends in Antibacterial Discovery: Answering the Call to Arms, Caister Academic Press, United Kingdom, pp. 35-75.
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As bacterial antibiotic resistance continues to exhaust our supply of effective antibiotics, a global public health disaster appears likely. Poor financial investment in antibiotic research has exacerbated the situation. A call to arms raised by several prestigious scientific organisations a few years ago rallied the scientific community, and now the scope of antibacterial research has broadened considerably. Multi-disciplinary approaches have yielded a wealth of new data on areas ranging from the identification of novel antibacterial targets to the use of biological agents for antibacterial therapy.
Harry, L., Monahan, L.G. & Thompson, L. 2006, 'Bacterial cell division: the mechanism and its precision' in Jeon, K.W. (ed), International Review of Cytology: A Survey of Cell biology Volume 253, Elsevier, The netherlands, pp. 27-94.
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The recent devlopment of cell biolofy technques for bacteria to allow visualisation of fundamental processes in time and space, and their use in synchronous populations of cells, has resulted in a dramatic increase in our understanding of cell division and it regulation in these tiny cells. The first stage of cell division is the formation of a Z ring, composed of apolymerised tubulin-like protein, FtZ,at the division site precisely at midcell. Several membrane-associated division proteins are then recruited to this ring to forma complex, the divisome, which causes invagination of the cell envelope layers to form a division septum. The z Ring marks the future division site, and the timing of assembly and positioning of this structure are important in determining where and when division will take place in the cell. Z ring assembly is controlled bnu many factors including negative regulatory mechanisms such as Min and nucleoid occlusion that influence Z ring positioning and FtZ accessory proteins that bind to FtZ directly and modulate its polymerisation behaviour. The replication status of the cell also influences the positionin of the Z ring,w hich may allow the tight coordination between DNA replication and cell division required toproduce two identical newborn cells.

Journal articles

Monahan, L.G. & Harry, E.J. 2016, 'You Are What You Eat: Metabolic Control of Bacterial Division.', Trends in microbiology, vol. 24, no. 3, pp. 181-189.
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Fluctuations in nutrient availability are a fact of life for bacterial cells in the 'wild'. To survive and compete, bacteria must rapidly modulate cell-cycle processes to accommodate changing nutritional conditions and concomitant changes in cell growth. Our understanding of how this is achieved has been transformed in recent years, with cellular metabolism emerging as a central player. Several metabolic enzymes, in addition to their normal catalytic functions, have been shown to directly modulate cell-cycle processes in response to changing nutrient levels. Here we focus on cell division, the final event in the bacterial cell cycle, and discuss recent compelling evidence connecting division regulation to nutritional status and metabolic activity.
Turnbull, L., Toyofuku, M., Hynen, A.L., Kurosawa, M., Pessi, G., Petty, N.K., Osvath, S.R., Carcamo-Oyarce, G., Gloag, E.S., Shimoni, R., Omasits, U., Ito, S., Yap, X., Monahan, L.G., Cavaliere, R., Ahrens, C.H., Charles, I.G., Nomura, N., Eberl, L. & Whitchurch, C.B. 2016, 'Explosive cell lysis as a mechanism for the biogenesis of bacterial membrane vesicles and biofilms', NATURE COMMUNICATIONS, vol. 7.
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Labbate, M., Islam, A., Monahan, L.G., Charles, I.G. & Stokes, H.W. 2015, 'A genomic island integrated into recA of Vibrio cholerae contains a divergent recA and provides multi-pathway protection from DNA damage', Environmental Microbiology.
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Monahan, L.G., Turnbull, L., Osvath, S.R., Birch, D., Charles, I.G. & Whitchurch, C.B. 2014, 'Rapid conversion of Pseudomonas aeruginosa to a spherical cell morphotype facilitates tolerance to carbapenems and penicillins but increases susceptibility to antimicrobial peptides', Antimicrobial Agents and Chemotherapy, vol. 58, no. 4, pp. 1956-1962.
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The Gram negative human pathogen Pseudomonas aeruginosa is able to tolerate high concentrations of -lactam antibiotics. Despite inhibiting the growth of the organism, these cell wall-targeting drugs exhibit remarkably little bactericidal activity. However, the mechanisms underlying -lactam tolerance are currently unclear. Here we show that P. aeruginosa undergoes a rapid en masse transition from normal rod shaped cells to viable, cell wall defective spherical cells when treated with -lactams from the widely used carbapenem and penicillin classes. When the antibiotic is removed, the entire population of spherical cells quickly converts back to the normal bacillary form. Our results demonstrate that these rapid population-wide cell morphotype transitions function as a strategy to survive antibiotic exposure. Taking advantage of these findings, we have developed a novel approach to efficiently kill P. aeruginosa by using carbapenem treatment to induce en masse transition to the spherical cell morphotype and then exploiting the relative fragility and sensitivity of these cells to killing by antimicrobial peptides (AMPs) that are relatively inactive against P. aeruginosa bacillary cells. This approach could broaden the repertoire of antimicrobial compounds used to treat P. aeruginosa and serve as a basis for developing new therapeutics to combat bacterial infections.
Monahan, L.G., Liew, A.T., Bottomley, A.L. & Harry, L. 2014, 'Division site positioning in bacteria: one size does not fit all', Frontiers in Microbiology, vol. 5, no. 19.
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Spatial regulation of cell division in bacteria has been a focus of research for decades. It has been well studied in two model rod-shaped organisms, Escherichia coli and Bacillus subtilis, with the general belief that division site positioning occurs as a result of the combination of two negative regulatory systems, Min and nucleoid occlusion. These systems influence division by preventing the cytokinetic Z ring from forming anywhere other than midcell. However, evidence is accumulating for the existence of additional mechanisms that are involved in controlling Z ring positioning both in these organisms and in several other bacteria. In some cases the decision of where to divide is solved by variations on a common evolutionary theme, and in others completely different proteins and mechanisms are involved. Here we review the different ways bacteria solve the problem of finding the right place to divide. It appears that a one-size-fits-all model does not apply, and that individual species have adapted a division-site positioning mechanism that best suits their lifestyle, environmental niche and mode of growth to ensure equal partitioning of DNA for survival of the next generation.
Turnbull, L., Strauss, M.P., Liew, A.T.F., Monahan, L.G., Whitchurch, C.B. & Harry, E.J. 2014, 'Super-resolution Imaging of the Cytokinetic Z Ring in Live Bacteria Using Fast 3D-Structured Illumination Microscopy (f3D-SIM)', JOVE-JOURNAL OF VISUALIZED EXPERIMENTS, no. 91.
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Monahan, L.G., Hajduk, I.V., Blaber, S.P., Charles, I.G. & Harry, E.J. 2014, 'Coordinating Bacterial Cell Division with Nutrient Availability: a Role for Glycolysis', MBIO, vol. 5, no. 3.
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Gloag, E.S., Turnbull, L., Huang, A., Vallotton, P., Wang, H., Nolan, L.M., Mililli, L., Hunt, C., Lu, J., Osvath, S.R., Monahan, L.G., Cavaliere, R., Charles, I.G., Wand, M., Gee, M., Ranganathan, P. & Whitchurch, C.B. 2013, 'Self-organization of bacterial biofilms is facilitated by extracellular DNA', Proceedings of the National Academy of Sciences of the United States of America, vol. 110, no. 28, pp. 11541-11546.
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Twitching motility-mediated biofilm expansion is a complex, multicellular behavior that enables the active colonization of surfaces by many species of bacteria. In this study we have explored the emergence of intricate network patterns of interconnected trails that form in actively expanding biofilms of Pseudomonas aeruginosa. We have used high-resolution, phase-contrast time-lapse microscopy and developed sophisticated computer vision algorithms to track and analyze individual cell movements during expansion of P. aeruginosa biofilms. We have also used atomic force microscopy to examine the topography of the substrate underneath the expanding biofilm. Our analyses reveal that at the leading edge of the biofilm, highly coherent groups of bacteria migrate across the surface of the semisolid media and in doing so create furrows along which following cells preferentially migrate. This leads to the emergence of a network of trails that guide mass transit toward the leading edges of the biofilm. We have also determined that extracellular DNA (eDNA) facilitates efficient traffic flow throughout the furrow network by maintaining coherent cell alignments, thereby avoiding traffic jams and ensuring an efficient supply of cells to the migrating front. Our analyses reveal that eDNA also coordinates the movements of cells in the leading edge vanguard rafts and is required for the assembly of cells into the bulldozer aggregates that forge the interconnecting furrows. Our observations have revealed that large-scale self-organization of cells in actively expanding biofilms of P. aeruginosa occurs through construction of an intricate network of furrows that is facilitated by eDNA
Monahan, L.G. & Harry, L. 2013, 'Identifying How Bacterial Cells Find Their Middle: A New Perspective', Molecular Microbiology, vol. 87, no. 2, pp. 231-234.
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Bacterial cell division begins with the polymerization of the FtsZ protein to form a Z ring at the division site. This ring subsequently recruits the division machinery to allow cytokinesis. How the Z ring is positioned correctly remains a challenging qu
Strauss, M., Liew, A.T., Turnbull, L., Whitchurch, C.B., Monahan, L.G. & Harry, L. 2012, '3D-SIM super resolution microscopy reveals a bead-like arrangement for FtsZ and the division machinery: implications for triggering cytokinesis', Plos Biology, vol. 10, no. 9, p. e1001389.
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FtsZ is a tubulin-like GTPase that is the major cytoskeletal protein in bacterial cell division. It polymerizes into a ring, called the Z ring, at the division site and acts as a scaffold to recruit other division proteins to this site as well as providing a contractile force for cytokinesis. To understand how FtsZ performs these functions, the in vivo architecture of the Z ring needs to be established, as well as how this structure constricts to enable cytokinesis. Conventional wide-field fluorescence microscopy depicts the Z ring as a continuous structure of uniform density. Here we use a form of super resolution microscopy, known as 3D-structured illumination microscopy (3D-SIM), to examine the architecture of the Z ring in cells of two Gram-positive organisms that have different cell shapes: the rod-shaped Bacillus subtilis and the coccoid Staphylococcus aureus. We show that in both organisms the Z ring is composed of a heterogeneous distribution of FtsZ. In addition, gaps of fluorescence were evident, which suggest that it is a discontinuous structure. Time-lapse studies using an advanced form of fast live 3D-SIM (Blaze) support a model of FtsZ localization within the Z ring that is dynamic and remains distributed in a heterogeneous manner. However, FtsZ dynamics alone do not trigger the constriction of the Z ring to allow cytokinesis. Lastly, we visualize other components of the divisome and show that they also adopt a bead-like localization pattern at the future division site. Our data lead us to propose that FtsZ guides the divisome to adopt a similar localization pattern to ensure Z ring constriction only proceeds following the assembly of a mature divisome.
Peters, P.C., Cox, G., Monahan, L.G. & Harry, L. 2011, 'Super-resolution imaging of the bacterial cytokinetic protein FtsZ', Micron, vol. 42, no. 4 Special Issue, pp. 336-341.
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The idea of a bacterial cytoskeleton arose just 10 years ago with the identification of the cell division protein, FtsZ, as a tubulin homolog. Fts,Z plays a pivotal role in bacterial division, and is present in virtually all prokaryotes and in some eukaryotic organelles. The earliest stage of bacterial cell division is the assembly of FtsZ into a Z ring at the -division site, which subsequently constricts during cytokinesis. FtsZ also assembles into dynamic helical structures along the bacterial ceil, which are thought to act as precursors to the Z fing via a cell-cycle-mediated FtsZ polymer remodelling. The fine structures of the FtsZ helix and ring are unknown but crucial for identifying the molecular details of Z ring assembly and its regulation. We now reveal, using STED microscopy that the FtsZ helical structure in cells of the gram positive bacterium, Bacillus subtilis, is a highly irregular and discontinuous helix of FtsZ; very different to the smooth cable-like appearance observed by conventional fluorescence optics. STED also identifies a novel FtsZ helical structure of smaJ!er pitch that is invisible to standard optical methods, identifying a possible third intermediate in the pathway to Z ring assembly, which commits bacterial cells to divide.
Monahan, L.G. & Harry, L. 2010, 'The bacterial cytoskeleton', Australian Biochemist, vol. 40, no. 2, pp. 4-8.
Review in token-refereed journal on the bacterial cytoskeketon
Monahan, L.G., Robinson, A. & Harry, L. 2009, 'Lateral Ftsz Association And The Assembly Of The Cytokinetic Z Ring In Bacteria', Molecular Microbiology, vol. 74, no. 4, pp. 1004-1017.
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Cell division in bacteria is facilitated by a polymeric ring structure, the Z ring, composed of tubulin-like FtsZ protofilaments. Recently it has been shown that in Bacillus subtilis, the Z ring forms through the cell cycle-mediated remodelling of a helical FtsZ polymer. To investigate how this occurs in vivo, we have exploited a unique temperature-sensitive strain of B. subtilis expressing the mutant protein FtsZ(Ts1). FtsZ(Ts1) is unable to complete Z ring assembly at 49°C, becoming trapped at an intermediate stage in the helix-to-ring progression. To determine why this is the case, we used a combination of methods to identify the specific defect of the FtsZ(Ts1) protein in vivo. Our results indicate that while FtsZ(Ts1) is able to polymerize normally into protofilaments, it is defective in the ability to support lateral associations between these filaments at high temperatures. This strongly suggests that lateral FtsZ association plays a crucial role in the polymer transitions that lead to the formation of the Z ring in the cell. In addition, we show that the FtsZ-binding protein ZapA, when overproduced, can rescue the FtsZ(Ts1) defect in vivo. This suggests that ZapA functions to promote the helix-to-ring transition of FtsZ by stimulating lateral FtsZ association.
Michie, K.A., Monahan, L.G., Beech, P.L. & Harry, L. 2006, 'Trapping of a spiral-like intermediate of the bacterial cytokinetic protein FtsZ', Journal Of Bacteriology, vol. 188, no. 5, pp. 1680-1690.
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The earliest stage in bacterial cell division is the formation of a ring, composed of the tubulin-like protein FtsZ, at the division site. Tight spatial and temporal regulation of Z-ring formation is required to ensure that division occurs precisely at m