Topic 6 Growth & Reproduction of Bacteria

Biology 1001

October 5, 2005

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

Bacteria Divide by Binary Fission

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

Figure 12.11!

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

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

2 X 1010 new E. coli are produced per day in the human intestine

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.

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.

Why Mutation Is Important For Bacterial PopulationsExample

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

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

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

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!

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