How do I survive the pathology residence



Bacteria are small, unicellular organisms that differ from the cells of animal or plant organisms mainly in the lack of a cell nucleus. Therefore they are grouped under the name prokaryotes (from Greek pro = "before", "instead of" and karyon = "core"). On the one hand, bacteria are part of the healthy and necessary human colonization, but they can also trigger a number of infectious diseases. The aim here is to provide an overview of the subdivision of bacteria, their morphological characteristics and their genetics.

Basic morphological forms of bacteria

Urinary tract infection due to Escherichia coli
E. coli are gram-negative rod bacteria and belong to the physiological intestinal flora. Due to the anatomical conditions, however, especially in women, they can easily get from the intestines into the urinary tract and cause an infection there. A urinary tract infection manifests itself as pain and burning sensation when urinating. In microbiological diagnostics, the gram-negative rod bacteria appear red in the microscope.

Peculiarities of the prokaryotic cell

The morphology of the prokaryotic cell has some peculiarities, which are roughly outlined below. Further information about the individual cell compartments can be found under: "The cell". There is also a comparison of eukaryotic and prokaryotic cells.

Intracellular components and cell membrane

Prokaryotic cells do not have a nucleus - the circular DNA is located in the cytoplasm and is called a nucleoid!

Prokaryotic cells do not contain mitochondria. The respiratory chain is located on the cell membrane of the bacterial cell!

Prokaryotic cells have 70S ribosomes with a 30S and a 50S subunit!

Macrolide antibiotics bind to the 50S subunit of the ribosomes and thus inhibit bacterial protein synthesis, which inhibits the growth of the bacteria. Macrolides are used therapeutically for infections with streptococci, but also for intracellular pathogens such as chlamydia.

The tetracycline class antibiotics bind to the 30S subunit of the prokaryotic ribosomes. They prevent cell growth by inhibiting translation (protein biosynthesis). The spectrum of pathogens includes, for example, Neisseries.

The antibiotic chloramphenicol binds to the large subunit of bacterial ribosomes (50S subunit) and inhibits their peptidyl transferase activity. The bacterial metabolism is thus inhibited at the level of translation.

Characteristic of the bacterial cell wall

With a few exceptions (e.g. mycoplasma), most bacteria have a cell wall: This surrounds the cell membrane and additionally stabilizes the bacterium. The cell wall offers a target for various antibiotics.

  • function
    • Form and stabilization of the bacterium
    • Anchoring of pili and flagella
    • Protection against chemical noxa and bursting
  • Structure: layers of murein of different thicknesses, so-called "murein sacculus" → different stainability in the Gram stain
  • Dyeability
    • Gram-positive bacteria: Thick murein layers are stained more strongly in the Gram stain → blue-violet representation in the microscope
    • Gram-negative bacteria: single-layer murein sacculus → red display after Gram staining
      • Phospholipid membrane: is anchored to the monolayer murein saccharide via lipoproteins and contains lipopolysaccharides
      • Lipopolysaccharides (LPS)
        • Located on the surface of the phospholipid membrane and facing outwards
        • So-called endotoxins: trigger a toxic reaction in the host when the bacterial cell dies
      • E.g. E. coli
  • particularities
    • Cell wallless bacteria: e.g. mycoplasma
    • Atypical cell wall
      • Mycobacteria: Special wall structure made of wax and lipids → Due to the structure of the cell wall, nutrients can only get into the interior of the bacteria with difficulty → Slow growth and reproduction
  • Clinical relevance: cell wall synthesis starting point for penicillin (see below)

Grief staining
The Gram stain was developed in 1884 by the Danish pathologist Christian Gram and makes use of the different concentrations of murein in the bacterial cell walls. The first staining takes place in two steps with crystal violet and Lugol solution and rinsing with alcohol. In bacteria with a thick layer of murein, the dye cannot be washed out by the alcohol, so they appear blue under the microscope (= "gram-positive"). In the case of a thin layer of murein, the dye is washed out so that it is completely discolored again. In order to then show the gram-negative bacteria, the preparation is colored with fuchsine in a further step. Gram-positive bacteria still keep the blue color from the first steps, while the Gram-negative bacteria are now red.

Gram-negative bacterial cells are built up from the inside out as follows: 1. Inner cell membrane 2. Single-layer murein sacculus 3. Outer membrane with lipopolysaccharides!

The outer membrane of Gram-negative bacteria contains lipopolysaccharides. These so-called endotoxins, which arise when the bacterium dies, are among the triggers of severe systemic inflammatory reactions (sepsis). The endotoxins are characteristic of gram-negative bacteria!

Beta-lactam antibiotics such as penicillin have an antibiotic effect against gram-positive pathogens whose cell walls consist of thick murein layers. By irreversibly inhibiting the enzyme transpeptidase, penicillin prevents the cross-linking of the subunits of murein, which makes it bactericidal.