Les systèmes toxine-antitoxine (TA) bactériens ont été découverts il y a une vingtaine d’année sur les plasmides à bas nombre de copie. Ils sont composés de deux gènes organisés en opéron, l’un codant pour une toxine stable et l’autre pour une antitoxine instable capable de neutraliser l’effet de la toxine. Les systèmes TA sont fortement représentés au sein de l’ensemble des génomes bactériens. Ils se localisent aussi bien sur des éléments génétiques mobiles (plasmides, phages, transposons,…) que dans les chromosomes, ce qui suggère que le transfert horizontal de gènes participe à leur dissémination. Le système TA ccd du plasmide F d’Escherichia coli (ccdF) est composé de l’antitoxine CcdA et de la toxine CcdB. Le système ccdF contribue à la stabilité du plasmide F en tuant les bactéries-filles n’ayant pas reçu de copies plasmidiques lors de la division bactérienne (tuerie post-ségrégationelle). Au cours de ce travail, nous avons caractérisé un homologue du système toxine-antitoxine ccd du plasmide F (ccdF) qui se situe dans le chromosome de la souche pathogène E. coli O157:H7 EDL933 entre les gènes folA et apaH (ccdO157). Les systèmes ccdF et ccdO157 coexistent naturellement dans les souches d’E. coli O157:H7, le système ccdF se trouvant sur le plasmide pO157 qui dérive du plasmide F. Nos résultats montrent que l’antitoxine plasmidique CcdAF neutralise l’effet de la toxine chromosomique CcdBO157, tandis que l’antitoxine chromosomique CcdAO157 ne contrecarre pas la toxicité de la toxine plasmidique CcdBF. Nous avons également montré que le système ccdF cause une tuerie post-ségrégationelle, lorsqu’il est cloné dans un plasmide instable, dans une souche possédant le système chromosomique ccdO157. Le système ccdF est donc fonctionnel en présence de son homologue chromosomique. Le système ccdO157 est absent du chromosome de la souche de laboratoire E. coli K-12 MG1655, où une région intergénique de 77 pb sépare les gènes folA et apaH. Celle-ci contient une séquence cible pour la transposition. Nous avons étudié la distribution du système ccdO157 au sein de 523 souches d’E. coli représentatives de l’ensemble des sérogroupes décrits. Nos résultats montrent que le système ccdO157 est présent au sein de souches appartenant à 47 sérogroupes différents. Nos résultats mettent en évidence la diversité de la région intergénique folA-apaH d’E. coli. Celle-ci peut contenir gènes codant pour des protéines présentant de l’homologie avec des protéines d’espèce bactériennes éloignées d’E. coli ou d’organismes eucaryotes, ainsi qu’un élément génétique mobile, l’IS621, ce qui montre que le système ccdO157 a intégré le chromosome d’E. coli via le transfert horizontal de gènes.

Thesis (PH. D.) - University of Michigan, 1943. / Reprinted from the Journal of immunology, vol. 47, no. 5, November, 1943. References: p. 423-424.

No description available.

Thesis (M.S.P.H.)--University of Michigan, 1937.

Thesis (M.S.P.H.)--University of Michigan, 1937.

Almost all bacteria contain genes that may lead to their growth stasis and death.Normally, these toxins are believed to be neutralized with their cognate antitoxinsfrom a toxin-antitoxin (TA) operon. These modules are also abundant in pathogenic bacteria suggesting a role for them both in normal bacterial physiology and pathogenicity. Their functions have been subject to intense debates. Due to the cell killing capability of the toxin and the gene silencing capability of the antitoxin, they have been utilized for basic research, biotechnology and medical applications. However, further advancements of these applications have been impeded by our limited knowledge of the biology of TAs. Among these TA systems is the Ibs/Sib (A-E) family. Here, we discuss our efforts in characterizing these systems, with a focus on the IbsC/SibC member. Studying them has shown to not be straightforward due to the complexity of their underlying mechanisms and the current approaches being laborious and lacking sensitivity to be applied to these low abundant molecules. We have developed fluorescence-based platforms to take advantage of sensitive and high throughput and resolution techniques such as Fluorescence Assisted Cell Sorting (FACS) to study these molecules instead of relying on traditional culturing methods. While developing these platforms, we gained insights about the biology and regulation of these molecules. To expand this knowledge, we actively pursued investigating the regulation of these molecules at the transcriptional and post-transcriptional levels, both in their native context and in artificial systems. The rest of this thesis summarizes our efforts in solving one of the biggest pieces of the Ibs/Sib puzzle, namely their physiological expressions. With the strategies we have optimized for specific detection of these low abundance molecules, and the knowledge of their biology and regulation presented, we are now at an exciting phase to interrupt the long pause in the study of functions by these molecules and advancement of TA-based applications. / Thesis / Doctor of Philosophy (PhD) / Almost all bacteria contain genes that may lead to their growth stasis or death. Normally, these toxins are believed to be neutralized with their cognate antitoxins. In spite of the efforts to understand these toxin-antitoxin (TA) systems, their physiological roles are subject to intense debate. These systems are hard to study mainly because 1) they are only activated under specific conditions and 2) they are low in abundance. Current approaches are not high throughput and sensitive enough. In this thesis, we developed DNA constructs, bacterial strains and methodologies to facilitate the study of these molecules, particularly the Ibs-Sib family. We next employed these tools to gain a fundamental knowledge of their expression under different conditions, which revealed surprising information about the function of these molecules. We believe that future studies can greatly benefit from the tools offered here to tremendously enhance our understanding of these systems and lead to useful applications.

Bacteria are under constant threat by their viral parasites, the bacteriophages (phages) and have evolved a range of anti-phage systems to defend themselves. One of these systems is termed abortive infection (Abi) where, upon phage infection, an Abi system may be activated which initiate a bacteriostatic or bactericidal response. While the infected bacteria do not obviously benefit from the activation of these systems, the cessation of bacterial growth or premature cellular suicide prevents the release of phage progeny. Thus Abi can be viewed as an altruistic process as only the remaining clonal bacterial population benefits. The Type III toxin-antitoxin systems have previously been shown to be involved in Abi, however the mechanisms through which these systems are activated are still poorly understood. A common approach to reveal the phage product involved in triggering these systems is to first determine the mutations that a previously sensitive phage evolves to escape after exposure to an Abi system. Analysis of viral "escape" mutants has been used in this study to try to elucidate the activation mechanism(s) of two Type III systems (ToxIN$_P$$_a$ and TenpIN$_P$$_l$) of several environmental phages. Several new phage products were identified in escape mutants as candidate factors involved in circumventing Abi - and possible roles in phage metabolism predicted. Furthermore, the genomes of several phages that could not evolve escapes, or were insensitive to Abi, are sequenced and these data exposed interesting curiosities regarding Abi (as well as the discovery of several novel and rare phages). Previously, no coliphage was identified that was capable of escape of the ToxIN$_P$$_a$ or TenpIN$_P$$_l$ systems. However, this study defined and characterised the first ToxIN$_P$$_a$ and TenpIN$_P$$_l$ coliphage escapes as well as a new method for isolating host-dependent coliphage escapes. Finally, multiple phages that infect the insect pathogen $\textit{Photorhabdus luminescens}$ TT01 (the bacterial strain from which the TenpIN$_P$$_l$ system originated) were isolated, genomically sequenced and characterised in terms of host range. The results revealed a large superfamily of flagellum-dependent phages that exhibit remarkable host promiscuity, possibly defining the most promiscuous phages thus far identified.

Type II toxin-antitoxin (TA) systems are composed of a stable toxin and its cognate unstable antitoxin that impedes the toxin through direct interaction. The human oral pathogen Streptococcus mutans uses a quorum-sensing peptide (CSP) as a stress-inducible pheromone to synchronize gene expression in response to specific stressors. The objectives of this study were to investigate the role of S. mutans MazEF TA in cell survival and characterize the functionality of CSP-inducible chromosomal type II TAs. Our results suggest that MazEF represents a stress-response element. Interestingly, S. mutans negatively regulates its MazEF system under high-cell-density environment that is characteristic of oral biofilms. S. mutans also encodes a novel chromosomal type II TA involved in biofilm formation and development of dormant persister cells. The results from this study suggest a complex interplay between quorum-sensing (signal), type II TA activation (response), and persister formation (phenotype) as a reaction to environmental perturbations.

Type II toxin-antitoxin (TA) systems are composed of a stable toxin and its cognate unstable antitoxin that impedes the toxin through direct interaction. The human oral pathogen Streptococcus mutans uses a quorum-sensing peptide (CSP) as a stress-inducible pheromone to synchronize gene expression in response to specific stressors. The objectives of this study were to investigate the role of S. mutans MazEF TA in cell survival and characterize the functionality of CSP-inducible chromosomal type II TAs. Our results suggest that MazEF represents a stress-response element. Interestingly, S. mutans negatively regulates its MazEF system under high-cell-density environment that is characteristic of oral biofilms. S. mutans also encodes a novel chromosomal type II TA involved in biofilm formation and development of dormant persister cells. The results from this study suggest a complex interplay between quorum-sensing (signal), type II TA activation (response), and persister formation (phenotype) as a reaction to environmental perturbations.

Thesis (M.Sc. (Clinical Pathology))--Mahidol University, 1978. / Supported by the National Research Council.