3: Bacterial physiology and genetics

Chapter 3 Bacterial physiology and genetics

Bacterial physiology

Aerobic and anaerobic growth

A good supply of oxygen enhances the metabolism and growth of most bacteria. The oxygen acts as the hydrogen acceptor in the final steps of energy production and generates two molecules: hydrogen peroxide (H2O2) and the free radical superoxide (O2). Both of these are toxic and need to be destroyed. Two enzymes are used by bacteria to dispose of them: the first is superoxide dismutase, which catalyses the reaction:

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and the second is catalase, which converts hydrogen peroxide to water and oxygen:

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Bacteria can therefore be classified according to their ability to live in an oxygen-replete or an oxygen-free environment (Fig. 3.2, Table 3.1). This has important practical implications, as clinical specimens must be incubated in the laboratory under appropriate gaseous conditions for the pathogenic bacteria to grow. Thus, bacteria can be classified as follows:

Table 3.1 Effect of oxygen on the growth of bacteria

Degree of oxygenation Term Example
Oxygen essential for growth Obligate aerobe Pseudomonas aeruginosa
Grows well under low oxygen concentration (5%) Microaerophile Campylobacter fetus
Grows in the presence or absence of oxygen Facultative anaerobea Streptococcus milleri
Only grows in the absence of oxygen Obligate anaerobe Porphyromonas gingivalis

a Facultative anaerobes may be subgrouped as capnophiles or capnophilic organisms if they grow well in the presence of 8–10% carbon dioxide (e.g. Legionella pneumophila).

Bacterial genetics

Genetics is the study of inheritance and variation. All inherited characteristics are encoded in DNA, except in RNA viruses.

The bacterial chromosome

The bacterial chromosome contains the genetic information that defines all the characteristics of the organism. It is a single, continuous strand of DNA (Fig. 3.3) with a closed, circular structure attached to the cell membrane of the organism. The ‘average’ bacterial chromosome has a molecular weight of 2 × 109.

Replication

Chromosome replication is an accurate process that ensures that the progeny cells receive identical copies from the mother cell. The replication process is initiated at a specific site on the chromosome (oriC site) where the two DNA strands are locally denatured. A complex of proteins binds to this site, opens up the helix and initiates replication. Each strand then serves as a template for a complete round of DNA synthesis, which occurs in both directions (bidirectional) and on both strands, creating a replication bubble (Fig. 3.4). The two sites at which the replication occurs are called the replication forks. As replication proceeds, the replication forks move around the molecule in opposite directions opening up the DNA strands, synthesizing two new complementary strands until the two replication forks meet at a termination site. Of the four DNA strands now available, each daughter cell receives a parental strand and a newly synthesized strand. This process is called semiconservative replication. Such chromosomal replication is synchronous with cell division, so that each cell receives a full complement of DNA from the mother cell.

The main enzyme that mediates DNA replication is DNA-dependent DNA polymerase, although a number of others take part in this process. When errors occur during DNA replication, repair mechanisms excise incorrect nucleotide sequences with nucleases, replace them with the correct nucleotides and religate the sequence.

Bacteria have evolved mechanisms to delete foreign nucleotides from their genomes. Restriction enzymes are mainly used for this purpose, and they cleave double-stranded DNA at specific sequences. The DNA fragments produced by restriction enzymes vary in their molecular weight and can be demonstrated in the laboratory by gel electrophoresis. Hence, these restriction enzymes are used in many clinical analytical techniques to cleave DNA and to characterize both bacteria and viruses (see below).

Genetic variation in bacteria

Genetic variation can occur as a result of mutation or gene transfer.

Jan 4, 2015 | Posted by in General Dentistry | Comments Off on 3: Bacterial physiology and genetics
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