Microbial Growth & Genetics
•
•
•
•
•
Growth of microbial populations
Phases of growth
Replication
Genetic recombination
Mutations
Logarithmic growth
Doubling
time
Growth of microbial populations:
Phases of growth
Bacterial Cell Division
• Bacterial genes on a circular chromosome
• Some bacteria have one to many plasmids; extra
pieces of DNA
• Plasmids carry extra genes: antibiotic resistance,
fertility factor, virulence plasmids for toxins
Genomes
• Mycoplasma is
580,000 base pairs,
smallest known
cellular genome
• E. coli is 5,000,000
base pairs
• Propionbacterium
acnes, is 2,560,000
base pairs with 2333
genes
Binary Fission
Bacterial Genetics and Evolution
• Genotype = genetic make-up
• Phenotype = physical traits
• Phenotype determined by gene
expression
Bacterial Genetic recombination
• Exchange of DNA in bacteria
• Horizontal gene transfer: same generation
• Three ways: transformation, conjugation,
transduction
Transformation: exogenous DNA
Some bacteria suck up DNA from environment
Common in soil - decomposition, lots of DNA
Implications in a hospital… what do you think?
Conjugation
Exchange of plasmid by sex pilus
Donor sex pilus attaches to recipient
Copy of plasmid moves through pilus
Antibiotic resistance on plasmids
Transduction
Bacterial genes picked up by viruses
Happens as bacteria fall apart during lysis
Carried to another bacteria during next infection
Any bacterial gene – antibiotic resistance
In transduction, the trait acquired is FROM the
previous host BACTERIUM. The virus acts
ONLY as the CARRIER.
Lysogenic Conversion
Bacteria acquire new traits from viruses (phage)
Viruses inject their genes into bacteria
Express during lysogenic phase
Example: Diphtheria toxin
In lysogenic conversion, the trait acquired is
FROM the virus.
Mutations
Naturally occur while copying DNA
Rate - one mistake per million base pairs
Mutagens enhance natural error rate
e.g. chemicals, UV, X-rays…
Mutagenicity is correlated with carcinogenicity
Ames Test: Detection of chemical
mutagens