10intro2-bw - Bacterial Genetics The bacterial genome...

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10/10/2010 1 Bacterial Genetics Impact on medicine Antibiotic resistance through mutation, plasmids, genetic exchange • Genetics of virulence (bacteriophages encoding virulence traits) Plasmids encoding virulence traits and antibiotic resistance The bacterial genome: chromosome plasmids bacteriophage insertion sequences transposons haploid - one copy of chromosome, mutations are homozygous (exception – Neisseria ) Mutation change in the DNA sequence •1 0 -9 /base or 10 -6 /gene per replication • happen all of the time, regardless of growth conditions - are they selected for to become enriched in the bacterial population? point mutations - single bases (insertion, deletion, missense, nonsense) macro-mutation - affect >1 base (insertion, deletion, inversion, duplication) can change a gene so that protein is no longer affected by an antibiotic, yet still retains its function effects of genotype on phenotype • silent • loss of function • altered function • completely new genes are not constructed by a single mutation Genetic exchange Importance • moving antibiotic resistance genes among bacteria • moving virulence gene among bacteria • changing the antigenic make-up to avoid immunity Mechanisms transformation - uptake of naked DNA transduction - bacteriophage as vectors conjugation - plasmids moved by cell-cell contact Transformation • recipient cell must be competent for uptake of DNA natural competence versus artificial competence • only certain bacteria are naturally transformable - Streptococcus pneumoniae, Haemophilus
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10/10/2010 2 Transduction bacteriophage (phage) are viruses of bacteria lytic - always lyse (kill) host bacterial cell temperate - can stably infect and coexist within bacterial cell ( lysogeny ) until a lytic phase is induced lysogeny • phage genome = prophage • bacterial cell = lysogen lysogenic conversion phage encodes observable function (e.g., diphtheria toxin in Corynebacterium diphtheriae ) Specialized transduction • Some prophages integrate into the bacterial genome at a specific location. • When a prophage is induced to lytic phase , it may drag along a piece of the bacterial genome next to the integration site and move that bacterial sequence into the new recipient host cell, changing the recipient's genome. • Not very important medically since only selected genes can be transferred. Generalized transduction • When a phage lyses the host bacterial cell, it normally packages phage genome into the capsid. • Sometimes the capsid is
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10intro2-bw - Bacterial Genetics The bacterial genome...

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