Microbiology Concepts: Taxis, Endobiosis, Symbiosis, and Gram Staining

Posted on Mar 23, 2025 in Biology

Key Concepts in Microbiology

Taxis

Taxis is the directed movement of a bacterium in response to a concentration gradient, either physical or chemical, allowing it to position itself where it can better perform its vital functions. There are two types:

  • Attractants (+): Substances that cause bacteria to move towards them, usually nutrients.
  • Repellents (-): Substances that cause bacteria to move away, typically toxic.

Bacteria have receptors on their surface capable of distinguishing between harmful substances and nutrients.

Endobiosis

Endobiosis describes the formation of the protoeukaryote through endosymbiosis. Aerobic bacteria associated with it, giving rise to protozoa, fungi, and animals. Later, cyanobacteria resulted in algae and plants. Peroxisomes also originated from a prokaryotic endosymbiont.

Decimal Reduction Time (Graphics)

Decimal reduction time refers to the time required to reduce the number of viable microorganisms to one-tenth of the original number under specific conditions.

  • Auxiliary Microbiostatic: Inhibits microbial growth reversibly.
  • Microbicidal: Results in cell death without cellular destruction.
  • Microbiolytic: Kills and lyses cells.

Symbiosis

Examples of symbiosis include interactions of microorganisms among themselves and with other organisms.

  • Rhizosphere: An area with humidity and a concentration of nutrients. Microorganisms provide vitamins, nutrients, and growth factors. They induce the segregation of carbon hydrates and solubilize nutrients, preventing plant pathogens from growing.
  • Mycorrhizae: Plants grow much faster when associated with mycorrhizae.
  • Lichens: Associations of fungi and algae that colonize places where other organisms cannot. They form a structure with a superior cortex, medulla, and inferior cortex. Based on the appearance of the thallus, they can be:
    • Crustose: Thallus attached to the substrate.
    • Squamulose: Flake-like.
    • Filamentary: Thalli made of thin wires.
    • Foliose: Flattened forms that bind to the substrate by hyphae.
    • Fructicose: Branch like a bush.

Benefits of Symbiosis

  • Fungus: Provides organic matter, nitrogen, and, in some cases, oxygen.
  • Algae: Provides a substrate anchorage site, protection against desiccation, water, inorganic nutrients, CO2, and growth factors.
  • The fungus provides: Fastening, protective, and inorganic nutrients.
  • The algae provides: Organic compounds.

Gram Staining

Difference Between Gram-Positive and Gram-Negative Bacteria

  • Gram-Negative (-): Synthesizes a thinner wall consisting of two layers: an inner peptidoglycan layer (2-7 nm thick) and an outer layer (7-8 nm thick) composed of lipopolysaccharide and protein. Low frequency of sugar and a few strands that originate tetrapeptides with open pores; stains pink.
  • Gram-Positive (+): Synthesizes a wall between 20 and 80 nm, uniformly thick and composed only of peptidoglycan. Many strands of sugars and high frequency of peptide bonds. The crystal violet is not dragged through the closed pores.

Gram Staining Procedure

The following steps are used:

  1. A previously fixed smear is stained with crystal violet for 1 minute. (Cells stain blue/purple).
  2. Add Lugol’s iodine, leaving it for 1 minute. This acts as a mordant, increasing the affinity of the crystal violet, causing the crystal violet-iodine complex to precipitate into the cytoplasm.
  3. Tilt the slide and drain alcohol 90° for about 20 seconds, dividing the bacteria into two groups: Gram-positive (G+) that resist the action of bleaching and remain blue/purple, and Gram-negative (G-) that become transparent.
  4. Finally, add safranin for 0.5 minutes, staining everything pink. The blue/purple color for the G+ remains, while the G- stain pink.

Pasteurization Methods

Three Methods for Pasteurizing Milk

  1. Batch Pasteurization (LTH): Low temperature holding. Disinfects at 62.8°C for 30 minutes.
  2. Flash Pasteurization (HTST): High temperature short time. Disinfects at 72°C for 15 seconds.
  3. Ultra-High Temperature (UHT): Sterilization is done for tetrabrick milk. Milk is heated by injecting steam for 2-3 seconds to 140-150°C and immediately cooled for 20-30 seconds.