1. Topic 6 Growth & Reproduction of Bacteria
Biology 1001
October 5, 2005

2. Growth of Bacterial Populations
Growth of bacteria refers to number rather than size of cells
Under optimal conditions, a single prokaryote cell divides to produce two daughter cells every ~ 1-3 hours
Each round of division is a generation
Bacterial population growth is therefore rapid and exponential
1 cell  2 cells  4 cells  8 cells  16 cells etc.
A colony from a single cell in 12 hours

3. Bacteria Divide by Binary Fission
Figure
12.11!
Prokaryotes reproduce asexually by cell division called binary fission
First, the bacterial chromosome begins to replicate, starting at the origin of replication
Replication continues, one origin moves to the opposite side of the cell, and the cell elongates
Replication finishes, the plasma membrane grows inward, and a new cell wall is laid down
Produces two genetically identical daughter cells – clones
Binary fission is the evolutionary precursor to mitosis

4. Mechanisms That Produce Variation
If binary fission produces clonal offspring, why are bacteria so genetically diverse???
Two factors contribute to genetic diversity among and within bacterial species
Mutation
Recombination

5. Mutation
A mutation is a change in the DNA of a gene, ultimately leading to genetic diversity
Mutations can be spontaneous or caused by mutagens
Spontaneous - Errors during DNA replication
Mutagens - Chemical or physical factors that damage DNA
Spontaneous mutations are extremely rare, occurring on average only once in 10 million cell divisions, per gene
Because bacteria divide rapidly & exponentially, mutation is a relevant factor generating genetic diversity

6. Why Mutation Is Important For Bacterial Populations Example
That’s 2 X 1010 / 10 million = 2000 E. coli with a mutation in a single gene per day.
Multiplied by the number of genes in the E. coli genome, 4300, that’s ~ 9 million mutated bacteria per day in a single human host.
2 X 1010 new E. coli are produced per day in the human intestine
Say the human population of ~ 6 billion replaces itself about once every 25 years. Because humans have about 30,000 genes per genome that’s about 18 million mutations in 25 years or only ~ 2000 per day, in the entire human population.

7. Genetic Recombination
The combining of DNA from two sources
In sexually reproducing organisms this is the main way genetic variation is produced
In eukaryotes, it involves the sexual processes of meiosis and fertilization
In prokaryotes three other processes are used – transformation, transduction, and conjugation (= bacterial “sex”)
Results in horizontal gene transfer – the transfer of genetic material within a generation, instead of from one generation to the next – a major force in the long-term evolution of bacteria

8. Conjugation
The direct transfer of genetic material between two bacteria cells that are temporarily joined
DNA transfer is one-way, from “male” to “female”
The donor (“male”) uses an appendage called the sex pilus that forms a cytoplasmic mating bridge
DNA gets transferred via this bridge in the form of a plasmid
The plasmid encodes the ability to mate as well as other traits such as antibiotic resistance

9. Transformation
The alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment
Many bacteria possess cell surface proteins that facilitate transformation in natural populations
E. coli is used in biotechnology applications of genetic recombination (genetic engineering)
Cells are cultured in high CaCl2 to become “competent”
Cells are then transformed with human genes that code for proteins such as insulin or growth hormone that are needed in large amounts

10. Transduction
Phages (viruses that infect bacteria) carry bacterial genes from one host cell to another as a result of mistakes in the phage reproductive cycle
In the process called generalized transduction, this transfer is random
Figure 18.16!

11. Bacterial Populations Evolve Rapidly
Natural selection operates on genetic (heritable) variation, such as is generated readily by mutation in bacteria
A mutation that confers a reproductive advantage increases in frequency in subsequent generations, and eventually becomes fixed in the population
Bacteria reproduce quickly and therefore have a short generation time relative to most other organisms
The rapid evolution of antibiotic resistance in bacteria is a medically important example of natural selection at work