Prokaryotes Immunity In

Vincent Geli and Yves Corda, Laboratoire d'Ingéniérie et de Dynamique des Systèmes Membranaires, Institut de Biologie Structurale et Microbiologie, CNRS Marseille, France

In this article, we review a type of immunity which confers a highly specific insensitivity upon bacteria to the lethal action of a substance. The immunity is a genetic character carried by a bacterial chromosome, a bacteriophage genome or a transmissible plasmid.

Many antimicrobial agents are produced and released extracellularly by bacteria. These compounds have a specific action on the targeted bacteria. They can be classified as 1) enzymatically synthesized antibiotics, 2) post-translationally modified peptide antibiotics, 3) protein antibiotics as bacterio-cins, protein exotoxins and bacteriolytic enzymes, and 4) intemperate or temperate bacterial viruses. These agents penetrate into the bacterial cell and, once inside, they have to remain unmodified to exert their action.

Many levels of insensitivity to these lethal sub stances are reported for bacteria. For example, resistance to the antibiotic streptomycin can result either from a mutation in the bacterial genome which alters the 30S ribosomal subunit or from a plasmid encoding enzyme that chemically modifies and inactivates streptomycin. In such a case, the resistance can be overcome by adding a high external concentration of the antibiotic in the culture medium. Similarly, insensitivity to bacteriocins or phages may result from mutations of bacterial components involved in the uptake of the toxic tnacromolecules. Since many bacteriophages and bacteriocins share similar uptake pathways, a single mutation may confer insensitivity to various bacteriocins or phages. Bacteria carrying such mutations are termed resistant or tolerant (but not immune) to the particular toxic agent.

The system of restriction-modification directed against foreign genetic material is another example of a bacterial defense mechanism. When the DNA of a phage is transferred from one bacterial strain to another bacterial strain with a different restriction-modification system, the phage DNA is immediately degraded after its injection by the host restriction endonuclease. The host DNA itself is protected from the action of its own restriction endonuclease by a strain-specific modification system. Bacterial strains able to restrict DNA are protected against the invasion of foreign DNA, but the specificity of the protection is not strictly restricted to a particular DNA. These protective mechanisms arc not highly-specific to the action of a particular killing agent and thus will not be considered as immunity. This entry will mainly focus on immunity against bacteriocins and bacteriophages.

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