Which process increases the variation in bacteria

5.    Genetics: peculiarities of bacterial genetics

5.1.           Structure of DNA

Basic knowledge of the molecular structure of nucleic acids is assumed for this chapter. At this point only a brief overview is given.

The carrier of the genetic information, the DNA (for English Deoxyribonucleicacid) is largely identical in its basic structure in all organisms and viruses. Four bases represent the genetic code, with three bases each representing the information for a triplet; a triplet codes for an amino acid. The bases are in the form of nucleotides which, linked in a specific way, result in a DNA double helix. Within this double helix there are complementary base pairs via hydrogen bonds. The biological meaning of the double helix is ​​the possibility of semiconservative reduplication, which enables both daughter cells to receive an (at least theoretically) identical genetic makeup during cell division. Mutations that change the DNA sequence at certain intervals lead to genetic variation and thus become the engine for adaptation, adaptation and evolution. Mutations can have no consequences (e.g. because a mutation affects the third position in a triplet and thus with a high degree of probability does not lead to an amino acid substitution), mutations can be detrimental to the individual (e.g. lethal mutations or those that reduce the individual's fitness) , or they can - either immediately or when the environmental conditions change - bring the individual an advantage (à see antibiotic resistance).

The main differences with regard to the genetic makeup of Pro- and Eucaryotes lie in the organization of the genetic makeup. In addition to the following information on the basic structure of DNA, the other differences are summarized in Table 6 (p. 29):

O Eucaryotes: The DNA is stabilized by histones (histone proteins). These histones are present at regular intervals. The “DNA thread” stabilized in this way is spiraled several times so that it condenses to form microscopically visible chromosomes.

O Procaryotes: No chromosomes but chromosome equivalent, i.e. the "DNA thread" is in the form of a supercoiled structure. There are neither histones nor formation of chromatin.

5.2.             Differences between eucaryotic and procaryotic genetics

Table 7: Main differences between the prokaryotes and eucaryotes in terms of genetics:

structure

Procaryotes

Eucaryotes

Chromosomes

Chromosome equivalent, circular DNA

real chromosomes

always haploid

(haploid), diploid (tetraploid or polyploid)

Occurrence of plasmids

no plasmids

Genome size
(in bp)

2-4 x 106

109 and more

DNA structure

no higher state of order

DNA coiled onto histone proteins and supercoiled

genetic regulation

Gene as a functional unit

Genes are often made up of introns and exons

spatial separation of transcription and translation

polycistronic RNA

monocistronic RNA

no splicing

splicing the RNA

In addition to the differences discussed below at the level of regulation of the DNA or genetic processes, there are also further differences with regard to the ribosomes and the enzymes involved in genetic processes. Ribosomes and RNA polymerase are indispensable, essential components of every cell, as they play a central role in protein biosynthesis. However, the basic structure of the two components is different when comparing Eu- and Procayronts. These structural differences make it possible to specifically inhibit bacterial structures using antibiotics without impairing the function of the corresponding eucaryotic structures (but do not consider mutationsàAntibiotic resistance and genetic variability).

A good treatise on the structural peculiarities of bacterial RNA polymerase and the effect of rifampicin can be found in: Campbell et al (2001): Structural Mechanism for Rifampicin Inhibition of Bacterial RNA Polymerase. Cell 104: 901-912.

Fig. 17: Differences in the structure of the ribosomes of bacteria (70S ribosomes) and eucaryotes (80S ribosomes) with regard to the ultrastructure (proteins, rRNA’s)

5.3.             Genetic regulation

5.3.1.        Structure of genes and transcription:

5.3.1.1.              Eucaryotes:

O Genes consist of introns and exons as alternating subgenic segments (Fig. 18). The exons contain the genetic information for the protein, the introns are not coding and are subsequently removed from the mRNA by splicing.

O splicing of the mRNA: removing the introns and attaching a so-called poly-atailsà mature mRNA

O monocistronic RNA, i.e. one mRNA codes for a single protein.

O Separation of transcription, splicing (in the nucleus) and translation (in the cytoplasm)

Fig. 18: Organization, transcription and maturation of eucaryotic genes or mRNAs. Possible post-translational events at the protein level are not shown.

5.3.1.2.              Procaryotes

O no division of the genes into introns and exons (colinearity of genes and proteins, Fig. 19).

O no further maturation of the mRNA, this is immediately sent to translation (protein synthesis).

O Procaryotic RNAs can be polycistronic, i.e. several genes, often belonging to a structural unit (àOperons), are combined in one mRNA.

O no separation of transcription and translation: even during transcription, ribosomes begin to translate the "unfinished" RNA.

Fig. 19: Organization and transcription of procaryotic genes or mRNAs. Possible post-translational events at the protein level are not shown.

5.3.2.        Operons

(intelligently controlled genetic units)

With the help of operons, bacteria are able to quickly, flexibly and energetically adapt their metabolism to new environmental conditions. Operons are often substrate-controlled, the gene products are only expressed when the substrate that is to be degraded reaches a certain concentration in the cell. Example:

lac-Operon

The lac-Operon or lactose operon is the example of a cassette of genes whose gene products form a functional unit (interaction of different proteins in the metabolism of lactose: specific transport protein = permease, transacetylase and ß-galactosidase) and are subject to intelligent regulation by the target substrate. The lac-Repressor, which negatively regulates the operon in the absence of lactose, is sterically changed in its conformation when the intracellular lactose level rises by binding the lactose and releases the operon until the lactose in the milieu becomes limiting again. By clicking on the following link, a PowerPoint animation on the function of the lac operon can be called: PowerPoint2003 animation / PowerPoint version95 and higher animation).

Fig. 20: Structure of known operons: era-Operon (arabinose metabolism) his-Operon (histidine biosynthesis) trp-Operon (tryptophan biosynthesis)

What all of the operons mentioned have in common is that they are subject to regulation by the substrate. The genetic mechanism varies, but the basic principle is comparable.

In the routine laboratory, the different abilities of bacteria are used to metabolize certain carbohydrates or other substances (àMetabolism), in order to differentiate (identify) bacteria grown from the test material. Bacterial species usually have a specific metabolic profile for their respective species, which can be checked with identification systems. In connection with the external shape of the bacteria and the staining behavior (à cell wall) a type of bacteria can be identified (see also Section 1.2).

5.4.             Genetic recombination ...

... or: "Bacteria have sexual intercourse"!

Sexuality is basically nothing more than the exchange and recombination of genetic material. These processes also take place in bacteria and, in addition to the mutations, are responsible for them for constant change and mixing (recombination) of the genetic make-up and thus for constant adaptation to changing habitats. The genetic material of the population changes (so-calledgenespool) steadily and independently of actual environmental conditions. Rather, the resulting genetic variability becomes significant under actually changing environmental conditions (genetic variability).

5.4.1.        conjugation

Exchange of plasmids (short, circular DNAs with additional genetic features such as resistance genes). Plasmids can be passed within a population of a particular bacterial species, or even between different gram-negative bacteria. The so-called F-Pilus (also Sex-Pilus) has a model character E. coli. F + bacteria transfer the plasmid to F recipient bacteria via this F pilus.

5.4.2.        Transduction

Modification of the bacteria by bacteriophages, these either transfer bacterial DNA between the bacteria (classic transduction) or modify bacteria by incorporating additional phage-coded genes (lysogenization e.g. of Corynebacteriumdiphtheria (then becomes a toxin generator) or Streptococcuspyogenes (also becomes a toxin generator)).

5.4.3.        transformation

Undirected uptake of free DNA from the environment (DNA of lysed microorganisms can be taken up by some bacteria such as Neisseria species [including pathogens causing meningitis and gonorrhoeae]). Under certain circumstances, recombination events occur here, which increase the genetic variability of the bacteria.

The processes described serve the exchange and the associated recombination of genetic material. Recombination leads to an increase in genetic variability. Genetic variability is the basis of adaptation to changing environmental conditions and thus the engine of evolution.

Questions about your own examination of the material:

Think about the practical relevance that the transmission of resistance factors via plasmids has in the environment.

5.5.             Advantages of genetic variability - adaptation to the milieu

Genetic uniformity is a dead end. Only populations that can show a certain genetic variability are able to adapt to changing environmental conditions (genetic variability is a property of populations, not individuals, Fig. 21). The one associated with genetic variability adaptability can be a property of the population (under optimal environmental conditions the genespool the population has sufficient variability for possibly changing environmental conditions). In the case of rapid and extreme changes in the environment or in the case of colonization of new habitats, adaptability is an individual characteristic. The section from the whole that is suitable under changed environmental conditions genespool ensures the continued survival of the species and enables the constancy of the population affected by the changed environmental conditions. The science discipline that deals with these questions is population genetics. Population genetic processes have far more than just theoretical significance. Every colonization and every infection of humans as well as every control intervention by antibiotics represents an intervention in the population genetics and dynamics of the bacterial population concerned.

Fig. 21: Example of a bacterial population under optimal environmental conditions. Due to genetic recombination and mutation, a total of 8 (A-H) different variants are represented in the population. All variants have a comparable probability of survival under normal conditions.

A change in environmental conditions leads to the extinction of individuals who have not adapted to the changed conditions. Variants whose individual properties match the new environmental conditions ensure the conservation of the species and the continued existence of the population. New genetic variability will emerge over time.

The consequences of this consideration become clear using the example of antibiotic resistance. In a human population of bacteria in which one of the bacteria happens to acquire the property through a point mutation to be resistant to rifampicin (firstline Antibiotic against the causative agent of tuberculosis, M. tuberculosis) (e.g. variant D in Fig. 21), first of all all bacteria are more or less comparably well adapted to the current habitat. If the milieu changes in the sense of an increasing concentration due to treatment with rifampicin, the daughter bacteria of the bacterium that has become resistant have a considerable location advantage and are able to ensure the continued existence of the population. So while the majority of the population is eradicated due to inadequacy to the changed environmental conditions (variants A-C and E-H in Fig. 21), a minority secures a certain section of the genespool represents the survival of the species. One speaks of selection of the best adapted variant. Similar processes can be observed in many cases when antibiotics, for example, are not given in sufficient concentration or for a sufficiently long period of time.

It is of considerable importance that, at least in some cases, the acquisition of the property antibiotic resistance can also create a locational disadvantage for the person who has this property under normal environmental conditions. For example, the causative agents of purulent meningitis, N. meningitidis, under therapy with rifampicin a clear advantage if the mutation to rifampicin resistance is present. However, under normal environmental conditions, this advantage turns into a disadvantage. The mutation that induces resistance takes place in the gene for RNA polymerase. Resistance is mediated by this mutation, but apparently the polymerase is slower in its activity. This increases the generation time of the resistant pathogens. Although this change is only of a minor nature, it is sufficient to be inferior to the wild type, i.e. sensitive strain, in a rifampicin-free environment. If you bring a resistant and a sensitive strain of N. meningitidis at the same time in a culture, one can measure the displacement of the resistant strain by the sensitive strain with appropriate methods (Fig. 22). This displacement mechanism can have the effect that in the absence of selection pressure (i.e. the antibiotic) in the environment, the sensitive variants are promoted to the detriment of the resistant ones. Corresponding changes have been described after changing the preferred antibiotic in hospitals.

Fig. 22: Disadvantage of rifampicin more resistant N. meningitidis in an environment without selection pressure (without rifampicin). A.) Resistant (open rectangles) and sensitive (black diamonds) N. meningitidis are grown separately in liquid cultures. After the growth density has been determined and normalized, the curves show the increase in cell density over 24 hrs with the same initial situation. B.) Conversion of the data from A. to the number of generations per 24 hrs. C.) Results of an experiment in which sensitive and resistant N. meningitidis were inoculated into a liquid culture at the same density at time 0. The course of the curve shows the decline in the resistant strain, which became visible from the ninth day of culture (with daily repositioning). After the 18th day there were no resistant ones N. meningitidis more verifiable.