Bacterial infection – morphology and structure of bacteriaTip: No need to read? Then just start our online microbiology course for free.
Picture: " Colorized scanning electron micrograph of Escherichia coli " from NIAID. License: CC BY 2.0
Definition of bacterial infection
The invasion of a host organism by bacteria, whether passive or active, is called an infection (lat. inficere "to put in") when their multiplication and the host's response to it lead to disease. Bacteria that can cause disease in humans are referred to as human pathogenic denotes.
Morphology and structure of bacteria
In a size range of 0.2 -2 μm, there are three different basic forms on which the morphology of all eubacteria is based.
Cocci: spherical or oval. Often in groups of two, four or eight (Diplococci, tetrads, sarcines) cells, in the shape of a bunch of grapes (Staphylococci) or chain form (Streptococci) stored. Rods: rod-shaped, in various appearances: slender (z.B. Mycobacterium tuberculosis), clumsy (z.B. Escherichia coli), pointed or rounded ends, etc. Helical: morphologically distinct convolutions; these include Spirillen, Borrelia, Treponemen and Leptospires
Prokaryotes are very different in their structure from our eukaryotic cells. Although bacteria differ enormously in their metabolism, structure, and virulence, there are structures that are common to all of them. The table shows the most important organelles. Their function illustrated. The cell wall will be discussed separately in the next paragraph, as it plays an important role in the classification and therapy of bacteria.
– Double-stranded ring or linear genetic code – ca. 10 6 base pairs ≅ 1000 genes (human: 6 x 10 9 bp) – Localization in cytoplasm with connection to cell membrane (There is no nucleus!)
– circular double-stranded DNA molecules – 800 – 300 000 base pairs – no genes of the bacterium essential for survival BUT – genes as Selection advantage
– Ex.: resistance, toxins, virulence properties, utilization of certain substances in a habitat
– contains no endoplasmic reticulum, no mitochondria – important enzymes of metabolism are localized in the cell membrane.
– Double layer of lipids – partially unfolds into mesosomes (membrane bodies) in which enzymes are incorporated – differs from eukaryotes in terms of lipid-protein composition
– Hypometabolous survival form of bacteria – In this state, spore-forming bacteria can survive extreme temperatures and other normally lethal environmental conditions – spore-formers are z.B. Bacilli and Clostridia
– three-part( basal body, hook, filament) constructed means of locomotion of the cell – the spiral filament is driven by sodium/proton gradient> Response to surrounding environment allows cabs – polar (single; unilateral), lophotrichous (tuft; unilateral), amphitrichous (bilateral), or peritrichous (all around) flagellation possible
– tubular structures through membrane and cell wall – sexpili for exchange of DNA (conjugation) between two bacterial cells – adhesins for attachment to structures in the host organism
The most important task of the solid cell wall is to withstand the high internal osmotic prere of the bacterium and to prevent the cell from bursting. The cell wall of all bacteria consists of peptidoglycan or also Murein. This is a lattice consisting of the polysaccharides N-acetylmuramic acid and N-acetylglucosamine, which is separated by short Peptide side chains cross-links. To make the distinction between the two subtypes – gram-positive and gram-negative – easier to understand, first the Gram stain explains.
The Gram stain
Figure: " Microscopic image of a Gram stain of mixed Gram-positive cocci (Staphylococcus aureus ATCC 25923, purple) and Gram-negative bacilli (Escherichia coli ATCC 11775, red). Magnification:1,000. " of Y tambe. License: CC BY-SA 3.0
With the Gram stain bacteria are produced with the help of Crystal violet and the Lugol's solution stained. Subsequently, the cells are washed well with alcohol and treated with Eosin (red dye) counterstained.
A thick murein layer prevents the washing out of the dye – the staining was therefore positive and the bacteria appear blue under the microscope. With a very thin layer of murein the dye is washed out of the cells again – they are Gram-negative and only visible by the red counterstain.
Gram-positive bacteria thus possess a thick cell wall consisting of several layers of peptidoglycans resp. consists of several murein layers. Anchored in this layer are further proteins, cell wall teichoic acid and a specific polysaccharide.
Gram-negative bacteria have a single layer of murein, on the outside of which there is still a second outer membrane is deposited. This membrane contains e.g. Lipopolysaccharides, which have an endotoxic effect when the cell disintegrates.
Crucial for the formation of a cell wall are the Transpeptidases, which link the individual components of the peptidoglycans with each other. They are an important target for penicillins and other antibiotics.
In this article only human pathogenic bacteria are discussed. These are always chemosynthetic ( in contrast to photosynthetic bacteria ) and organotrophic ( they obtain their energy from organic substances). For energy production, as in eukaryotes, organic matter is oxidized and electrons and H+ ions are transferred. Bacteria use two different metabolic pathways for this.
The Respiration is due to the transfer of H+ ions to oxygen characterized. There are both aerobic respiration as well as anaerobic respiration, in which the oxygen is chemically bound in a salt. The yield of respiration is ca. ten times greater than the yield of fermentation.
Instead of oxygen, another organic compound serves as hydrogen acceptor. The fermentation is named after the resulting final product, e.g. the alcoholic fermentation.
The following classification of bacteria based on their metabolism results:
Facultative anaerobesBacteria can perform both respiration and fermentation obligate aerobesBacteria depend on breathing and therefore oxygen in their environment Obligate anaerobesbacteria are optimally adapted to fermentation. On contact with oxygen, metabolism is inhibited and they die off. Aerotolerant anaerobesBacteria are optimally adapted to fermentation and tolerate an oxygen-rich environment
Reproduction of bacteria – reproduction and gene transfer
Bacteria multiply by simple cell division. The two daughter cells that emerge from the original cell have two identical (apart from random mutations) copies of the genome. This vertical gene transfer from one generation to the next clonal. This means a recombination of genomes as in meiotic reproduction is not possible. Genetic diversity is achieved by bacteria through horizontal gene transfer, i.e., the exchange of information between two individuals of a generation. This exchange without meiosis is also called Parasexuality denotes.
Transformation is the incorporation of DNA into the bacterium without necessary vectors, bridges, or other, chemical means. Some bacterial species possess this natural competence for DNA uptake. With the help of receptors, they recognize DNA strands on their cell surface, fragment them into short, single-stranded pieces and thus take them up into the cell interior. If a homologous gene segment is then present, the external DNA can pair and introduce new information into the genome. This type of gene transfer only takes place within a species, because the bacteria recognize their own DNA.
Via so-called Conjugation Bridges become single Plasmids exchanged between two cells. The F-plasmid contains, among other things, the genes that allow the formation of a conjugation bridge and the transfer of a plasmid. The bacterium containing the F-plasmid is the donor cell, its partner the recipient. The donor cell forms F-Pili (pilus lat. hair), which make contact with the recipient. Subsequently, the pili shorten and fuse, so that eventually a pore or. short bridge exists between the two bacteria. Via the rolling-cycle mechanism, a single strand is synthesized at the plasmid, which is subsequently completed in the recipient to form a double-stranded F plasmid. The recipient can now also pass on the F plasmid as a donor.
Figure: " Schematic drawing of bacterial conjugation. " by Matthias M. License: CC BY-SA 3.0
This gene transfer only occurs unidirectional (in one direction). Donor and recipient are sometimes abbreviated as F+ and F- in the literature.
Growth curve of the bacterial population
The multiplication of a bacterial population follows a typical growth curve. It can be well observed when a certain number of cells of bacteria are exposed in a nutrient dish. After the cell individuals have grown and adapted to the new environmental conditions (lag phase) starts an initially slow growth of the population which rapidly turns into a exponential growth through numerous cell divisions (log phase). In the Delay phase growth slows down and culminates in a stationary phase. The bacterial number has increased to up to 10^9 cells/ml. Now, due to nutrient depletion and increase of toxic degradation products, the Death phase.
Figure: " Growth curve of a static bacterial culture " by M-Komorniczak. License: CC BY-SA 3.0
The exponential growth makes it clear that in the case of a bacterial infection, action should be taken quickly and waiting can cause a strong multiplication of the pathogens and fatal consequences. However, the rate of growth depends on nutrient supply and other environmental conditions. To estimate this for specific bacterial strains Generation time T, the duration of a division of the mother cell into two daughter cells, or. determined a doubling of the bacterial population.
During the exponential growth phase (log phase), the generation time T can be determined. experimentally determined the initial number of bacteria n0, and the number of bacteria n at the time t. Since it is an exponential growth, the equation is:
T/t indicates the number of generation times or. of the divisions to.
Logarithmized is the equation:
lg n = lg n0 + t/T x lg 2 = lg n0 + 0.301 x t/T = lg n0 + 0.301/T x t
The equation is logarithmized, since in this form it corresponds to a straight line equation. The graph of the logarithmized cell number versus time t now corresponds to a straight line with the slope 0.301/T. The generation time can thus be read from the diagram.
pathogenicity and virulence factors
The Pathogenicity of a bacterial strain denotes the ability to produce a disease. To classify how pronounced this ability is Virulence used. Virulence factors are:
– Cell wall components, which are recognized as antigens – Surface proteins or capsules – Exotoxins, which are secreted by bacteria , which are excreted – Metabolites/endotoxins, which are released after cell death
Virulence of different species can vary widely. It is indicated with the help of LD50 (lethal dose50), i.e., the dose at which 50% of an infected test group would die. With bacteria of strong virulence, there is little difference between the LD50 and the dose at which 100% of the test group dies. An example of this is Streptococcus pneumoniae. The LD50 cannot be determined because even a few cells are sufficient to kill the entire test group. In contrast, the LD50 of Salmonella enterica to be well separated from the LD100. For killing the whole population ca.