Genetic variation in bacteria

Including antibiotic resistance

Adaptation

• Organisms adjust to suit the changing environment by natural selection– This is adaptation

• This increases the reproductive success of an organism by helping members survive for longer

• Adaptation and selection are important factors in the diversity of living organisms

Diversity and DNA

• DNA is the genetic information passed from one generation to the next

• Diversity in organisms arises from:– Mutations in DNA– Recombining DNA of two individuals – sexual

reproduction

Diversity in bacteria

• Namely arises via mutations and a process called conjugation

• Conjugation occurs when one bacterial cell transfers DNA to another

• Although conjugation recombines DNA from two individuals, it is not sexual reproduction

• Allows genes for antibiotic resistance and other characteristics to be exchanged by horizontal gene transfer

Conjugation

•Conjugation tube forms

• The donor cells replicates the plasmid

• The plasmid is cut and becomes linearised so it can move through conjugation tube

• The plasmid enters the recipient cell and forms a new circular plasmid

F factor = plasmid

Antibiotic resistance: two important animations!!

First one focuses on conjugationSecond is about antibiotic resistance

• http://www.hhmi.org/biointeractive/animations/conjugation/conj_frames.htm

• http://www.sumanasinc.com/scienceinfocus/sif_antibiotics.html

Antibiotics: What you need to know

• What is an antibiotic?– A substance produced by a living organism that destroys or

inhibits growth of microorganisms• How do they work?

– Many ways: e.g. Penicillin can prevent bacteria from making cell walls – inhibits synthesis and assembly of cross-linkages in bacterial cell wall.

• How do bacteria become resistant to antibiotics?– Mutations!– In penicillin-resistant bacteria, a new mutation arose causing the

bacteria to produce an enzyme that can break down penicillin– NOTE: ANTIBIOTICS DO NOT CAUSE MUTATIONS TO ARISE! They

arise randomly

Antibiotics: What you need to know

• The presence of antibiotics creates a selection pressure for bacteria

• In a normal population of bacteria, penicillin-resistance would not be an advantage

• However, in the presence of penicillin, these bacteria have an advantage – they will survive and pass on the resistance gene– Only bacteria with the resistance gene survive – they reproduce and

pass on the characteristic, increasing the numbers of resistant bacteria• Resistance can be passed vertically (from one generation to the

next) or horizontally (via conjugation and exchange of plasmids)• The more we use antibiotics, the greater the chance that mutated

bacteria will survive and pass on antibiotic resistance

Antibiotic links

• http://www.hhmi.org/biointeractive/Antibiotics_Attack/frameset.html– Click on penicillin on side menu and start form

there