5 bacterial genetics screencasts - transformation and commentary on the extent of genetic change

how is transformation similar to transduction and conjugation?

it also brings together genetic information from two genomes and provides opportunity for recombination

what is bacterial transformation?

a process of gene transfer by which some bacteria take up genetic material (naked DNA) from the environment

what does ‘naturally competent’ mean?

it refers to bacteria that are able to take up naked DNA from the environment

why is the process of transformation thought to be advantageous in evolution?

it enhances genetic diversity

what 2 other purposes may transformation serve other than increasing genetic diversity?

• can repair damaged DNA by homologous recombination

• the DNA can be a source of nutrition

where, by who and when was bacterial transformation first reported?

in Streptococcus pneumoniae by Griffiths in 1928

what did Avery et al do in 1944?

identified DNA as the transforming principle in transformation

describe Griffith’s experiment to show that transformation took place

• used wild type S. pneumoniae (virulent, kill mice, form smooth colonies on agar)

• isolated non-virulent variants that cannot kill mice

• these variants produced rough colonies on agar

• the non virulent variants were unable to produce a polysaccharide capsule essential for immune evasion

• heat killed virulent S bacteria could also not kill mice

• but a combination of heat killed virulent S and non virulent variant could kill mice

• virulent S bacteria were recovered from the dead mice

• so the only living bacteria (the non virulent variant) had been transformed into virulent S

what did Griffiths propose from his experiment?

that a substance from the dead strain called the transforming principle was used by the living strain

what was initially proposed to be the ‘transforming principle’ from Griffith’s experiment?

the polysaccharide capsule

1 example of a further step needed to classify transformation following Griffith’s experiment

showing that cell to cell contact was unnecessary

what did Avery McCleoud and McCarthy show in 1944?

that the transforming principle was DNA

why were scientists initially reluctant to agree that the transforming principle was DNA?

they though the DNA structure was too simple to encode complex information

describe the experiment done to prove that DNA was the transforming factor

• DNA and RNA was treated with RNAase so only DNA remained

• added bacteria and it could grow on the plate (had been transformed)

• DNA and RNA then treated with DNAase so only RNA remained

• when they added bacteria it could not grow on the plate

• so DNA must be the transforming factor

in addition to being observed in labs, where is evidence of genetic exchange also being revealed?

through DNA sequencing

why are lab experiments and DNA sequencing with transformation still limiting?

they may only show a fraction of what actually happens in nature

does HGT confuse the classification of bacterial species?

no

what is bacterial classification NOT based on?

reproductive isolation

what is bacterial classification based on?

the sharing of around 20 characteristics

give 7 of the defining characteristics of E. coli

gram-negative, anaerobic, chemoautotrophs, oxidase negative, non-sporing, acid fast, straight rods

what techniques are increasingly being used in taxonomy?

molecular techniques

what can genetic exchange also occur between?

mobile genetic elements

when is genetic exchange between mobile genetic elements possible?

when the different elements reside in the same cell

can more than one phage (even the same type) infect E. coli

yes

is genetic exchange restricted to phage?

no, it can occur between all types of mobile genetic elements

other than bacteria, where else has genetic exchange between mobile genetic elements been important?

• shown to be key in the evolution of swine origin influenza virus

• now capable of human to human transmission

how did swine origin virus come to be able to infect humans?

• via number of reassortment events

• from swine, avian and human viruses contributing to the final virus

what factor is likely to facilitate the evolution of viruses to different hosts?

the proximity of animals, birds and humans in certain parts of the world