Bacterial Immunity: a brief view
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1
Université Montpellier 2, France
As all the living organisms, bacteria have to manage with alien deleterious entities, notably Mobile Genetic Elements (bacteriophages, plasmids, transposons…) but also bacteria or even protozoa. The mechanisms allowing bacterial resistance represent an immune response against nonself. Due to their unicellular nature, bacteria possess an immune system based only on molecular components.
As expected, an innate immunity, genetically encoded and transmitted to daughter cells (i.e. inherited), is involved in bacterial defense.
As a first line of defense, bacteria can disrupt phage adsorption to the cell surface by eliminating or masking the corresponding receptors; translocation of foreign DNA in the cell can also be blocked by immunity proteins.
Within the bacterial cell, foreign DNA can be recognized as nonself and cleaved by the restriction/modification systems (I-IV) while nonself small RNA are recognized by Ago proteins, leading to DNA cleavage and RNA interference as in eukaryotic cells. Abortive infection systems (Abi) interfere with various aspects of phage replication and packaging, leading to death of the bacterial cell; this mechanism looks like the eukaryotic programmed cell death.
Finally, some bacterial species are able to kill aggressive micro-organisms (bacteria, yeasts, amoebas) by injecting them lethal effectors through a Type VI Secretion System (T6SS), but only when they are under attack
In a more amazing way, bacteria display also an adaptive immune system targeting foreign DNA, the CRISPR/Cas immune system. This adaptive immunity evolves according to the intruding nonself DNA identity and gives rise to a memory which allows a more efficiency response during a later challenge with an already met DNA; these characteristics are comparable to those of the adaptive immunity of the vertebrates.
This system consists of an array of short conserved DNA-repeat sequences that are interspaced (clustered regularly interspaced short palindromic repeats, CRISPR) by stretches of variable sequence called spacers, which originate from foreign DNA. A set of cas (CRISPR-associated) genes is typically located in the vicinity to repeat-spacer array.
The CRISPR/Cas mechanism takes place in 3 steps:
i) the immunization or adaptation step: during a foreign DNA intrusion, Cas proteins cleave it and form a complex with a small sequence (26-72 bp) that they insert in a preexisting CRISPR locus.
ii) the expression step: transcription of the CRISPR locus and of the cas genes ; the CRISPR-RNA is matured by Cas proteins in small sequences, leading to Cas/crRNA complexes. This step doesn’t depend on the presence of foreign DNA
iii) the interference step: the Cas/crRNA target the crRNA complementary sequence of a intruding foreign DNA previously met, leading to DNA cleavage
Keywords:
mobile genetic element,
restriction/modification system,
abortive infection,
RNA Interference,
CRiSPR/Cas
Conference:
The First International Congress of Immunology and Molecular Immunopathology (CIMIP2014), Tlemcen, Algeria, 17 Oct - 20 Oct, 2014.
Presentation Type:
Oral Presentation
Topic:
Immunomicrobiology
Citation:
Aymeric
J
(2014). Bacterial Immunity: a brief view.
Front. Immunol.
Conference Abstract:
The First International Congress of Immunology and Molecular Immunopathology (CIMIP2014).
doi: 10.3389/conf.fimmu.2014.04.00024
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Received:
01 Oct 2014;
Published Online:
01 Dec 2014.
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Correspondence:
Dr. Jean-Luc Aymeric, Université Montpellier 2, Montpellier, 34090, France, jean-luc.aymeric@univ-montp2.fr