Growth and reproduction of bacteria

The growth and multiplication of bacteria can be examined in terms of individual cells or populations
of cells.

During the cell division cycle a bacterium assimilates nutrients from the surrounding medium and increases in size.

When a predetermined size has been reached, the DNA duplicates itself and a cross-wall will be produced, dividing the large cell into two daughter cells, each containing a copy of the parent chromosome.

The daughter cells part, and the process is known as binary fission. In a closed
environment, such as a culture in a test tube, the rate at which cell division occurs varies according to the conditions, and this manifests itself in characteristic changes in the population concentration.

When fresh medium is inoculated with a small number of bacterial cells, the number remains static for a short
time while the cells undergo a period of metabolic adjustment. This period is called the lag phase and its length depends on the degree of readjustment necessary.

Once the cells have adapted to the
environment, they begin to divide in the manner described previously, and this division occurs at regular intervals.

The numbers of bacteria during this period increase in an exponential fashion (i.e. 2, 4, 8, 16, 32, 64, 128, etc.), and this is therefore termed the exponential or logarithmic phase. When cell numbers are plotted on a log scale against time, a straight line results for this phase.

During exponential growth the
medium undergoes continuous change, as nutrients are consumed and metabolic waste products excreted.

The fact that the cells continue to divide exponentially during this period is a tribute to their physiological
adaptability.

Eventually, the medium becomes so changed, due to either substrate exhaustion or excessive concentrations of toxic products, that it is unable
to support further growth.

At this stage cell division slows and eventually stops, leading to the stationary phase.

During this period some cells lyse and die, whereas others sporadically divide, but the cellbnumbers remain more or less constant.

Gradually all the cells lyse and the culture enters the phase of decline. It should be appreciated that this sequence of events is not a characteristic of the cell but a consequence of the interaction of the organisms with the nutrients in a closed environment.

It does not necessarily reflect the way in which the organism would behave in vivo.

Genetic exchange

In addition to mutations, bacteria can alter their genetic make-up by transferring information from
one cell to another, either as fragments of DNA or in the form of small extrachromosomal elements
(plasmids).

Transfer can be achieved in three ways:
by transformation, transduction or conjugation.

  • Transformation. When bacteria die, they lyse and release cell fragments, including DNA, into the
    environment. Several bacterial genera (e.g. Bacillus, Haemophilus, Streptococcus) are able to take up these DNA fragments and incorporate them into their own
    chromosome, thereby inheriting the characteristics carried on that fragment. Cells able to participate in transformation are called competent. The development of competence has been shown in some cases to occur synchronously in a culture under the action
    of specific inducing proteins.
  • Transduction. Some bacteriophages can infect a
    bacterial cell and incorporate their nucleic acid into the host cell chromosome, with the result that the viral genes are replicated along with the bacterial DNA. In many instances this is a dormant lysogenic state for the phage but sometimes it is triggered into
    action and lysis of the cell occurs with liberation of phage particles. These new phage particles may havebbacterial DNA incorporated into the viral genome,Vand this will infect any new host cell. On entering a new lysogenic state, the new host cell will replicate
    the viral nucleic acid in addition to that portion received from the previous host. Bacteria in which
    this has been shown to occur include members of the genera Mycobacterium, Salmonella, Shigella and Staphylococcus.
  • Conjugation. Gram-negative bacteria such as Salmonella species, Shigella species and Escherichia
    coli have been shown to transfer genetic material conferring antibiotic resistance by cellular contact. This process is called conjugation and is controlled
    by an R-factor plasmid, which is a small circular strand of duplex DNA replicating independently from the
    bacterial chromosome. R factor comprises a region containing resistance transfer genes that control the formation of sex pili, together with a variety of genes
    that code for the resistance to drugs. Conjugation is initiated when the resistance transfer genes stimulate the production of a sex pilus and random motion
    brings about contact with a recipient cell. One strand
    of the replicating R factor is nicked and passes through the sex pilus into the recipient cell. On receipt of
    this single strand of plasmid DNA, the complementary strand is produced and the free ends are joined. For a short time afterwards this cell has the ability tobform a sex pilus itself and so transfer the R factor further. This is by no means an exhaustive discussion of genetic exchange in bacteria,

Bacterial nutrition

Bacteria require certain elements in fairly large quantities for growth and metabolism, including carbon, hydrogen, oxygen and nitrogen.

Sulphur and phosphorus are also required but not in such large amounts.
Only low concentrations of iron, calcium, potassium, sodium, magnesium and manganese are needed.

Some elements, such as cobalt, zinc and copper, are required only in trace amounts, and an actual requirement
may be difficult to demonstrate.

The metabolic capabilities of bacteria differ considerably, and this is reflected in the form in which nutrients may be assimilated.

Bacteria can be classified according to their requirements for carbon and energy.

Lithotrophs (synonym: autotrophs).

These utilize carbon dioxide as their main source of carbon. Energy
is derived from different sources within this group:

  • chemolithotrophs (chemosynthetic autotrophs) obtain their energy from the oxidation of
    inorganic compounds; and
  • photolithotrophs (photosynthetic autotrophs) obtain their energy from sunlight.

Organotrophs (synonym: heterotrophs).

Organotrophs utilize organic carbon sources and can similarly be divided into:

  • chemoorganotrophs, which obtain their energy from oxidation or fermentation of organic compounds; and
  • photoorganotrophs, which utilize light energy.

Oxygen requirements

As mentioned already, all bacteria require elemental oxygen in order to build up the complex materials necessary for growth and metabolism, but many organisms also require free oxygen as the final electron acceptor in the breakdown of carbon and energy
sources.

These organisms are called aerobes. If the organism will only grow in the presence of air, it is called a strict aerobe, but most organisms can either grow in its presence or its absence and are called facultative anaerobes.

A strict anaerobe cannot grow
and may even be killed in the presence of oxygen, because some other compound replaces oxygen as the final electron acceptor in these organisms. A fourth group of microaerophilic organisms has also been recognized which grow best in only trace amounts
of free oxygen and usually prefer an increased carbon dioxide concentration.

Influence of environmental factors on
the growth of bacteria

The rate of growth and metabolic activity of bacteria is the sum of a multitude of enzyme reactions.

It follows that those environmental factors that influence enzyme activity will also affect growth rate.

Such factors include temperature, pH and osmolarity.

  • Temperature. Bacteria can survive wide limits of temperature but each organism will exhibit minimum,
    optimum and maximum growth temperatures and on this basis bacteria fall into three broad groups:

    • Psychrophiles. These grow best below 20°C but have a minimum growth temperature of approximately 0°C and a maximum growth temperature of 30°C. These organisms are responsible for low-temperature spoilage.
    • Mesophiles. These exhibit a minimum growth temperature of 5°C to 10°C and a maximum
      growth temperature of 45°C to 50°C. Within this group, two populations can be identified:
      saprophytic mesophiles, with an optimum temperature of 20°C to 30°C, and parasitic
      mesophiles, with an optimum temperature of 37°C. The vast majority of pathogenic organisms are in this latter group.
    • Thermophiles. These can grow at temperatures up to 70°C to 90°C but have an optimum of 50°C to 55°C and a minimum of 25°C to
      40°C. Organisms kept below their minimum growth
      temperature will not divide but can remain viable. As a result, very low temperatures (–70°C) are used to preserve cultures of organisms for many years. Temperatures in excess of the maximum growth temperature have a much more injurious effect,
  • pH. Most bacteria grow best at around neutral pH, in the pH range from 6.8 to 7.6. There are, however,
    exceptions, such as the acidophilic organism lactobacillus, a contaminant of milk products, which grows best at pHs between 5.4 and 6.6. Helicobacter species
    have been associated with gastric ulcers and are found in the stomach growing at pHs of 1–3. At the other
    extreme, Vibrio cholera is capable of growing at pHs between 8 and 9. Yeasts and moulds prefer acid
    conditions with an optimum pH range of 4–6. The difference in pH optima between fungi and bacteria
    is used as a basis for the design of media permitting the growth of one group of organisms at the expense
    of others. Sabouraud medium, for example, has a pH of 5.6 and is a fungal medium, whereas nutrient
    broth, which is used routinely to cultivate bacteria, has a pH of 7.4. The adverse effect of extremes of
    pH has for many years been used as a means of preserving foods against microbial attack, e.g. by pickling in acidic vinegar.
  • Osmotic pressure. Bacteria tend to be more resistant to extremes of osmotic pressure than other cells
    owing to the presence of a very rigid cell wall. The concentration of intracellular solutes gives rise to an
    osmotic pressure equivalent to between 5 bar and 20 bar, and most bacteria will thrive in a medium
    containing approximately 0.75% w/v sodium chloride. Staphylococci have the ability to survive higher than normal salt concentrations. This has enabled the
    formulation of selective media, such as mannitol salt agar containing 7.5% w/v sodium chloride, which will support the growth of staphylococci but restrict
    the growth of other bacteria. Halophilic organisms can grow at much higher osmotic pressures but these are all saprophytic and are not pathogenic to humans.
    High osmotic pressures generated by either sodium chloride or sucrose have for a long time been used as preservatives. Syrup BP contains 66.7% w/w sucrose
    and is of sufficient osmotic pressure to resist microbial attack. This is used as a basis for many oral pharmaceutical preparations

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