Reproduciton in Bacteria
Growth and Production in bacteria
Unlike in multicellular organisms, increases in cell size (cell growth and reproduction by cell division) are tightly linked in unicellular organisms. Bacteria grow to a fixed size and then reproduce through binary fission, a form of asexual reproduction.[92]
Under optimal conditions, bacteria can grow and divide extremely
rapidly, and bacterial populations can double as quickly as every
9.8 minutes.[93] In cell division, two identical clone
daughter cells are produced. Some bacteria, while still reproducing
asexually, form more complex reproductive structures that help disperse
the newly formed daughter cells. Examples include fruiting body
formation by Myxobacteria and aerial hyphae formation by Streptomyces, or budding. Budding involves a cell forming a protrusion that breaks away and produces a daughter cell.
In the laboratory, bacteria are usually grown using solid or liquid media. Solid growth media such as agar plates
are used to isolate pure cultures of a bacterial strain. However,
liquid growth media are used when measurement of growth or large volumes
of cells are required. Growth in stirred liquid media occurs as an even
cell suspension, making the cultures easy to divide and transfer,
although isolating single bacteria from liquid media is difficult. The
use of selective media (media with specific nutrients added or
deficient, or with antibiotics added) can help identify specific
organisms.[95]
Most laboratory techniques for growing bacteria use high levels of
nutrients to produce large amounts of cells cheaply and quickly.
However, in natural environments nutrients are limited, meaning that
bacteria cannot continue to reproduce indefinitely. This nutrient
limitation has led the evolution of different growth strategies (see r/K selection theory). Some organisms can grow extremely rapidly when nutrients become available, such as the formation of algal (and cyanobacterial) blooms that often occur in lakes during the summer.[96] Other organisms have adaptations to harsh environments, such as the production of multiple antibiotics by Streptomyces that inhibit the growth of competing microorganisms.[97] In nature, many organisms live in communities (e.g., biofilms) that may allow for increased supply of nutrients and protection from environmental stresses.[37] These relationships can be essential for growth of a particular organism or group of organisms (syntrophy).[98]
Bacterial growth
follows four phases. When a population of bacteria first enter a
high-nutrient environment that allows growth, the cells need to adapt to
their new environment. The first phase of growth is the lag phase,
a period of slow growth when the cells are adapting to the
high-nutrient environment and preparing for fast growth. The lag phase
has high biosynthesis rates, as proteins necessary for rapid growth are
produced.[99] The second phase of growth is the log phase, also known as the logarithmic or exponential phase. The log phase is marked by rapid exponential growth. The rate at which cells grow during this phase is known as the growth rate (k), and the time it takes the cells to double is known as the generation time (g).
During log phase, nutrients are metabolised at maximum speed until one
of the nutrients is depleted and starts limiting growth. The third phase
of growth is the stationary phase
and is caused by depleted nutrients. The cells reduce their metabolic
activity and consume non-essential cellular proteins. The stationary
phase is a transition from rapid growth to a stress response state and
there is increased expression of genes involved in DNA repair, antioxidant metabolism and nutrient transport.[100] The final phase is the death phase where the bacteria runs out of nutrients and dies.
Genetics
Most bacteria have a single circular chromosome that can range in size from only 160,000 base pairs in the endosymbiotic bacteria Candidatus Carsonella ruddii,[101] to 12,200,000 base pairs in the soil-dwelling bacteria Sorangium cellulosum.[102] Spirochaetes of the genus Borrelia are a notable exception to this arrangement, with bacteria such as Borrelia burgdorferi, the cause of Lyme disease, containing a single linear chromosome.[103] The genes in bacterial genomes are usually a single continuous stretch of DNA and although several different types of introns do exist in bacteria, these are much more rare than in eukaryotes.[104]Bacteria may also contain plasmids, which are small extra-chromosomal DNAs that may contain genes for antibiotic resistance or virulence factors.
Bacteria, as asexual organisms, inherit identical copies of their parent's genes (i.e., they are clonal). However, all bacteria can evolve by selection on changes to their genetic material DNA caused by genetic recombination or mutations. Mutations come from errors made during the replication of DNA or from exposure to mutagens. Mutation rates vary widely among different species of bacteria and even among different clones of a single species of bacteria.[105] Genetic changes in bacterial genomes come from either random mutation during replication or "stress-directed mutation", where genes involved in a particular growth-limiting process have an increased mutation rate.[106]
DNA transfer
Some bacteria also transfer genetic material between cells. This can occur in three main ways. First, bacteria can take up exogenous DNA from their environment, in a process called transformation. Genes can also be transferred by the process of transduction, when the integration of a bacteriophage introduces foreign DNA into the chromosome. The third method of gene transfer is conjugation, where DNA is transferred through direct cell contact.Transduction of bacterial genes by bacteriophage appears to be a consequence of infrequent errors during intracellular assembly of virus particles, rather than a bacterial adaptation. Conjugation, in the well-studied E. coli system is determined by plasmid genes, and is an adaptation for transferring copies of the plasmid from one bacterial host to another. Infrequently, a conjugative plasmid may integrate into the host bacterial chromosome, and subsequently transfer part of the host bacterial DNA to another bacterium. Plasmid mediated transfer of host bacterial DNA also appears to be an accidental process rather than a bacterial adaptation.
Transformation, unlike transduction or conjugation, depends on numerous bacterial gene products that specifically interact to perform this complex process,[107] and thus transformation is clearly a bacterial adaptation for DNA transfer. In order for a bacterium to bind, take up and recombine donor DNA into its own chromosome, it must first enter a special physiological state termed competence (see Natural competence). In Bacillus subtilis about 40 genes are required for the development of competence.[108] The length of DNA transferred during B. subtilis transformation can be between a third of a chromosome up to the whole chromosome.[109][110] Transformation appears to be common among bacterial species, and thus far at least 60 species are known to have the natural ability to become competent for transformation.[111] The development of competence in nature is usually associated with stressful environmental conditions, and seems to be an adaptation for facilitating repair of DNA damage in recipient cells.[112]
Ordinarily transduction, conjugation and transformation involve transfer of DNA between individual bacteria of the same species, but occasionally transfer may occur between individuals of different bacterial species and this may have significant consequences, such as the transfer of antibiotic resistance.[113] In such cases, gene acquisition from other bacteria or the environment is called horizontal gene transfer and may be common under natural conditions.[114] Gene transfer is particularly important in antibiotic resistance as it allows the rapid transfer of resistance genes between different pathogens.[115]
Bacteriophages
Main article: Bacteriophage
Bacteriophages are viruses that infect bacteria. Many types of bacteriophage exist, some simply infect and lyse their host bacteria, while others insert into the bacterial chromosome. A bacteriophage can contain genes that contribute to its host's phenotype: for example, in the evolution of Escherichia coli O157:H7 and Clostridium botulinum, the toxin genes in an integrated phage converted a harmless ancestral bacterium into a lethal pathogen.[116] Bacteria resist phage infection through restriction modification systems that degrade foreign DNA,[117] and a system that uses CRISPR
sequences to retain fragments of the genomes of phage that the bacteria
have come into contact with in the past, which allows them to block
virus replication through a form of RNA interference.[118][119] This CRISPR system provides bacteria with acquired immunity to infection.
http://bugs.bio.usyd.edu.au/learning/resources/CAL/Microconcepts/moviePages/Ecoli.html
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