bacterial genetics Flashcards
what are mutation?
- mutations are permanent alterations in DNA
- account for evolutionary changes in microbes & for alterations that produce different strains in species
Spontaneous mutations vs induced mutations
spontaneous mutations
- arise during the replication of DNA
- due to error in the base pairing of nucleotides in the old & new strands of DNA
induced mutations
- produced by mutagens which increase mutation rate
point mutations vs frame shift mutations
point mutations
- effect a single based
frameshift mutations
- nucleotide is either deleted or inserted into the DNA
- can affect more than one base
what does recombination involve?
- combining genes from 2 different cells to greatly increase the genetic diversity of organisms
what are the 4 mechanisms of recombination?
- transformation
- transduction
- conjunction
- insertion sequences & transposable elements
describe the transformation mechanism of recombination
- transformation is the ability of a bacteria to bind & translocate DNA fragments or plasmids & incorporate them into their chromosome
- does not occur very often naturally
describe the transduction mechanism of recombination
transduction involves gene transfer from a donor to a recipient via a bacteriophage (a virus that infects bacteria)
what are the 2 types of bacteriophages
- lytic phage - when virus particles replicate within cell, it results in lysis
- lysogenic phage - viral genome is inserted into chromosome & replicates along with the host
what is a prophage?
- is phage DNA in the quiescent/dormant state
describe the conjunction mechanism of recombination
- conjunction involves gene transfer from a donor to a recipient by direct physical contact between cells
- done cell has a F factor (sex pilus)
- recipient cell lacks an F factor
what are plasmids?
- plasmids are small extrachromosomal circular pieces of DNA that replicate independently of the chromosome
- not essential for growth but carry useful genes for virulence factors, resistance, replication or conjunction
- plasmid transfer occurs within & between species
- plasmids can integrate occasionally with chromosomal DNA
describe conjugative plasmids (F-plasmids & R-plasmids)
- F-plasmids have the capacity for horizontal gene transfer by containing a sex pilus to transfer genetic material to another bacteria
- R-plasmids - refresh to plasmids encoding antimicrobial resistance
what are transposable genetic elements in regards to as a mechanism for recombination
- transposable genetic elements are segments of DNA that have the capacity to move from one location to another (jumping genes)
- includes: insertion sequences, transposons, integrative conjugative elements and genetic cassettes
what are insertion sequences?
- sequences that carry no known genes except those that are required for transposition (transfer of a segment of DNA from one site to another in genome)
what are transposons?
- special DNA segments that can move from one location in the genome to another
- cause rearrangement of the genome
what are integrative conjugative elements (ICE)?
- are self transmissible mobile genetic elements
- transfer via conjugation
- integrate & replicate within host chromosome
- contain accessory genes for gene cassettes with antimicrobial resistance & virulence factors
what are integrons?
- integrons are DNA elements which capture & expression circular DNA elements (gene cassettes)
- widely distributed in environment
what is antimicrobial resistance?
- is the relative or complete lack of effect of an antimicrobial agent against a previously susceptible microbe
- an increase in MIC occurs (minimum inhibitory concentration)
how can antimicrobial resistance develop?
- a new mutation in a gene that determines sensitivity or resistance
- gene transfer
cross resistance vs multiple resistance?
cross resistance
- is when a microbe uses a single mechanism for resistance
- so closely related antibiotics are rendered ineffective
multiple resistance
- is when a microbe uses multiple mechanisms for resistance
- unrelated antibiotics are rendered ineffective
- when microbes acquire multiple plasmids
how can antibiotics create selection pressures that favour antimicrobial resistance?
- antibiotics do not induce mutations but they can create environments that favour survival of mutant resistant microbes
- kill of bacteria that are not resistance
- resistant bacteria that survive multiply & become dominant
what are the different mechanisms bacteria can use to be resistant to certain antibiotics?
- using an alternative metabolic pathway
- producing an enzyme that destroys the antibiotic
- altering permeability to later influx or efflux of antibiotic
- modifying the antibiotic target site on bacteria
what are the 3 types of resistance? and how do they occur?
- natural or intrinsic resistance
- occurs naturally to all members of a specific bacterial taxonomic group
- due to structural or biochemical characteristic that naturally occurs in the wild-type of microbe species - mutation resistance
- occurs by a spontaneous chromosomal mutation - extra-chromosomal or acquired resistance
- occurs by transformation, transduction & conjunction
what is co-selection?
- occurs when a single antimicrobial provides selection pressures for the persistence & transfer of multi-drug resistance in bacteria that have a single plasmid with multiple antibiotic resistant genes
what is the mutant preventing concentration (MPC)?
- is the minimum concentration of antibiotic required to prevent the expansion of first step spontaneously generated antibiotic resistant mutants in a large population of bacteria
- is used for determining the optimal dosing regime to minima the formation of resistant mutants
what factors promote antimicrobial resistance?
- exposure to sub-optimum levels of antimicrobial agent
- exposure to microbes carrying resistant genes
- inappropriate use of antimicrobials
what are 4 strategies for the control of antimicrobial resistance?
- Prescribing guidelines - AVA code of practice
- better infection control (eg biosecurity measures)
- better vaccines
- alternatives to antibiotics (probiotics)
