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Factors Mediating Environmental Biofilm Formation by Legionella pneumophila

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      Abstract

      Legionella pneumophila ( L. pneumophila) is an opportunistic waterborne pathogen and the causative agent for Legionnaires' disease, which is transmitted to humans via inhalation of contaminated water droplets. The bacterium is able to colonize a variety of man-made water systems such as cooling towers, spas, and dental lines and is widely distributed in multiple niches, including several species of protozoa In addition to survival in planktonic phase, L. pneumophila is able to survive and persist within multi-species biofilms that cover surfaces within water systems. Biofilm formation by L. pneumophila is advantageous for the pathogen as it leads to persistence, spread, resistance to treatments and an increase in virulence of this bacterium. Furthermore, Legionellosis outbreaks have been associated with the presence of L. pneumophila in biofilms, even after the extensive chemical and physical treatments. In the microbial consortium-containing L. pneumophila among other organisms, several factors either positively or negatively regulate the presence and persistence of L. pneumophila in this bacterial community. Biofilm-forming L. pneumophila is of a major importance to public health and have impact on the medical and industrial sectors. Indeed, prevention and removal protocols of L. pneumophila as well as diagnosis and hospitalization of patients infected with this bacteria cost governments billions of dollars. Therefore, understanding the biological and environmental factors that contribute to persistence and physiological adaptation in biofilms can be detrimental to eradicate and prevent the transmission of L. pneumophila. In this review, we focus on various factors that contribute to persistence of L. pneumophila within the biofilm consortium, the advantages that the bacteria gain from surviving in biofilms, genes and gene regulation during biofilm formation and finally challenges related to biofilm resistance to biocides and anti-Legionella treatments.

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      Most cited references 127

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      Biofilm formation as microbial development.

      Biofilms can be defined as communities of microorganisms attached to a surface. It is clear that microorganisms undergo profound changes during their transition from planktonic (free-swimming) organisms to cells that are part of a complex, surface-attached community. These changes are reflected in the new phenotypic characteristics developed by biofilm bacteria and occur in response to a variety of environmental signals. Recent genetic and molecular approaches used to study bacterial and fungal biofilms have identified genes and regulatory circuits important for initial cell-surface interactions, biofilm maturation, and the return of biofilm microorganisms to a planktonic mode of growth. Studies to date suggest that the planktonic-biofilm transition is a complex and highly regulated process. The results reviewed in this article indicate that the formation of biofilms serves as a new model system for the study of microbial development.
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        Bacterial iron homeostasis.

        Iron is essential to virtually all organisms, but poses problems of toxicity and poor solubility. Bacteria have evolved various mechanisms to counter the problems imposed by their iron dependence, allowing them to achieve effective iron homeostasis under a range of iron regimes. Highly efficient iron acquisition systems are used to scavenge iron from the environment under iron-restricted conditions. In many cases, this involves the secretion and internalisation of extracellular ferric chelators called siderophores. Ferrous iron can also be directly imported by the G protein-like transporter, FeoB. For pathogens, host-iron complexes (transferrin, lactoferrin, haem, haemoglobin) are directly used as iron sources. Bacterial iron storage proteins (ferritin, bacterioferritin) provide intracellular iron reserves for use when external supplies are restricted, and iron detoxification proteins (Dps) are employed to protect the chromosome from iron-induced free radical damage. There is evidence that bacteria control their iron requirements in response to iron availability by down-regulating the expression of iron proteins during iron-restricted growth. And finally, the expression of the iron homeostatic machinery is subject to iron-dependent global control ensuring that iron acquisition, storage and consumption are geared to iron availability and that intracellular levels of free iron do not reach toxic levels.
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          Cyclic di-GMP: the first 25 years of a universal bacterial second messenger.

          Twenty-five years have passed since the discovery of cyclic dimeric (3'→5') GMP (cyclic di-GMP or c-di-GMP). From the relative obscurity of an allosteric activator of a bacterial cellulose synthase, c-di-GMP has emerged as one of the most common and important bacterial second messengers. Cyclic di-GMP has been shown to regulate biofilm formation, motility, virulence, the cell cycle, differentiation, and other processes. Most c-di-GMP-dependent signaling pathways control the ability of bacteria to interact with abiotic surfaces or with other bacterial and eukaryotic cells. Cyclic di-GMP plays key roles in lifestyle changes of many bacteria, including transition from the motile to the sessile state, which aids in the establishment of multicellular biofilm communities, and from the virulent state in acute infections to the less virulent but more resilient state characteristic of chronic infectious diseases. From a practical standpoint, modulating c-di-GMP signaling pathways in bacteria could represent a new way of controlling formation and dispersal of biofilms in medical and industrial settings. Cyclic di-GMP participates in interkingdom signaling. It is recognized by mammalian immune systems as a uniquely bacterial molecule and therefore is considered a promising vaccine adjuvant. The purpose of this review is not to overview the whole body of data in the burgeoning field of c-di-GMP-dependent signaling. Instead, we provide a historic perspective on the development of the field, emphasize common trends, and illustrate them with the best available examples. We also identify unresolved questions and highlight new directions in c-di-GMP research that will give us a deeper understanding of this truly universal bacterial second messenger.
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            Author and article information

            Affiliations
            1Department of Biology and Biochemistry, Birzeit University , West Bank, Palestine
            2Department of Microbial Infection and Immunity, Center for Microbial Interface Biology, College of Medicine, Ohio State University , Columbus, OH, United States
            Author notes

            Edited by: Justin Merritt, Oregon Health & Science University, United States

            Reviewed by: Haijian Zhou, National Institute for Communicable Disease Control and Prevention (China CDC), China; Marta Palusinska-Szysz, Marie Curie-Sklodowska University, Poland; Eric D. Cambronne, Oregon Health & Science University, United States

            *Correspondence: Arwa Abu Khweek arwakhweek@ 123456gmail.com
            Contributors
            Journal
            Front Cell Infect Microbiol
            Front Cell Infect Microbiol
            Front. Cell. Infect. Microbiol.
            Frontiers in Cellular and Infection Microbiology
            Frontiers Media S.A.
            2235-2988
            27 February 2018
            2018
            : 8
            5835138 10.3389/fcimb.2018.00038
            Copyright © 2018 Abu Khweek and Amer.

            This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

            Counts
            Figures: 2, Tables: 0, Equations: 0, References: 127, Pages: 10, Words: 8809
            Funding
            Funded by: Center for Clinical and Translational Science, Ohio State University 10.13039/100008747
            Award ID: R21 AI113477
            Award ID: R01 AI24121
            Award ID: R01 HL127651
            Categories
            Microbiology
            Review

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