microbial genetics genes for the germs 10. deadly diarrhea 1968 guatemala –bloody dysentery hit...
TRANSCRIPT
Microbial Genetics
Genes for the Germs
10
Deadly Diarrhea
• 1968 Guatemala– Bloody dysentery hit 100,000 people, with 12,000 deaths– Caused by Shigella– Standard antibiotics had no effect– Arose through genetic selection of antibiotic-resistant strains
Bacterial DNA
• The bacterial chromosome– DNA– Double helix– Closed, circular loop– Free in cytoplasm– If extended into line, 1.5 mm long– Shrinks to fit inside 1 m cell by looping
and supercoiling– E. coli
• Over 4,000 genes
Figure 10.1: A) An electron micrograph of an E. coli cell immediately after disruption. The tangled mass is the organism’s DNA. B) The loops in the structure chromosome, viewed head-on
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Bacterial DNA
• The bacterial chromosome– Replication
• Unwinding by helicase and/or gyrase
• Single strands held apart by single-stranded DNA binding protein
• New synthesis initiated by primase
• All new DNA synthesized by DNA polymerase
• Synthesis is semi-conservative
Bacterial DNA: Replication
Figure 10.2: Replication of the E. coli chromosome
Bacterial DNA
• Plasmids– Extrachromosomal
independent units– Closed, circular,
double-stranded DNA– May confer selective
advantage to microbe– “R” factors
Figure 10.3: A TEM of bacterial plasmids.
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Gene Mutations
• Mutation is permanent change in an organism’s DNA• Change is passed from originator to all its progeny• Method by which some drug resistance occurs
Gene Mutations
• Causes of mutation– Spontaneous errors by DNA polymerase
• Estimated at 1 observable change in every billion replications
• Over 1 billion bacteria in an observable colony
• Hence, at least 1 mutant, perhaps drug resistant, in the population
• That resistant survivor can now successfully replicate to occupy the niche where all the other susceptible bacteria have died
– Mutagens• Increase spontaneous error rate of DNA polymerase
• Therefore, increase presence of mutants
• Examples
– Ultraviolet radiation
– Chemicals
Gene Mutations: Mutagens
Figure 10.4: How nitrous acid causes bacterial mutations
Gene Mutations
• Causes of mutation– Changes in the way proteins are encoded may alter the way in
which antibiotics bind• Antibiotics no longer effective against the mutated target
• Transposons– Small segments of DNA that can move from one position to
another in the chromosome– Jumping genes in corn; Barbara McClintock’s Nobel prize– Insertions of DNA into new spots in chromosome may also alter
protein function
Gene Recombinations
• Transfer of genetic information between bacteria• Conjugation
– Two live bacteria– F+ donor cell
• F(ertility) factor
• F plasmid
• Sex pili
– F- recipient cell– Conjugation bridge– Replication and passage of DNA through bridge– Frequently F plasmids also encode genes for drug resistance– Hfr bacteria– Many genera: Escherichia, Salmonella, Shigella
Gene Recombinations: Conjugation
Figure 10.5: Bacterial conjugation
Gene Recombinations: Conjugation
Figure 10.5: Bacterial conjugation, cont’d.
Gene Recombinations
• Transduction– Gene transfer with the assistance of bacterial viruses– Bacteriophages
• Also known as phages ()
• Insert DNA into cytoplasm of bacteria
• Turn bacteria into phage factories, executing phage DNA program
• Phage production is usually lytic
• Sometimes random segment of host DNA packaged in progeny
• That segment of host DNA is delivered to new host
• Result is transfer of DNA from one bacterium to another
• DNA may encode drug resistance
Gene Recombinations: Generalized Transduction
Figure 10.7: Generalized transduction
Gene Recombinations: Generalized Transduction
Figure 10.7: Generalized transduction, cont’d.
Gene Recombinations
• Transformation– Acquisition of
genes from surrounding environment
• Requires competent cells
• Requires only naked DNA
– Some of this DNA may encode drug resistance genes
Fig. 10.9: Bacterial transformation
Gene Recombinations
• In today’s world– Gene transfers have resulted in proliferation of drug resistant bacteria– Staphylococcus aureus
• Part of normal flora• Harmful if they penetrate skin
– Open wounds– Piercings– Damaged hair follicles– Cuts, scratches
• Many S. aureus are acquiring multiple drug resistance genes: MRSA• Diseases
– Boils– Abscesses– Pneumonia– Septicemia– Endocarditis– Toxic shock
• Now appearing in clinics: VRSA (vancomycin-resistant S. aureus)
Genetic Engineering
• Manipulation of DNA sequences in vitro• Cutting and splicing DNA segments together in new
combinations that never existed before in nature• Not possible until the 1970s
Genetic Engineering
• The beginning of genetic engineering– Endonucleases
• Restriction enzymes
– Expressed by bacteria to restrict infection by phages
– Cut DNA at specific sequences
– Create sticky ends
– Sticky ends can be put back together in new combinations
Genetic Engineering: Restriction Enzymes
Fig. 10.10: A) A restriction enzyme cuts through two strands of a DNA molecule to produce two fragments. B) The recognition sites of several restriction enzymes
Genetic Engineering
• The first recombinant DNA molecule• Cut gene from SV40 with restriction enzyme
• Cut E. coli plasmid with same enzyme
• Pasted to sticky DNAs together with ligase
• Created new plasmid
Figure 10.11: Construction of a recombinant DNA molecule
Genetic Engineering
• The first recombinant DNA molecule
Figure 10.11: Construction of a recombinant DNA molecule, cont’d.
Genetic Engineering
• The implications– US government guidelines on recombinant DNA technology
– New field of biotechnology• Food production• New medicines• Pollution control• New vaccines
– Threat of new bioterrorism agents
– Recombinant human insulin produced from bacteria
– Recombinant Factor VIII , produced in bacteria, for hemophiliacs
– Recombinant human growth factor
– Advances in agriculture, medical diagnostics, forensic science