Bacterial Structure, Types, and Functions

Posted on Dec 13, 2024 in Biology

Bacteria are very simple organisms, classified as prokaryotes. Based on their shape, they can be categorized as:

  • Rod-shaped bacilli
  • Spherical cocci
  • Spiral spirilla
  • Comma-shaped vibrios

Bacterial Structure

Capsule

The capsule is a mucus layer that surrounds the bacterial wall, present only in some bacteria.

Function:

  • Makes it difficult for antibodies and phagocytic cells of the host to recognize and destroy the bacteria.
  • Facilitates the formation of colonies.
  • Participates in exchange processes involving water, ions, and nutrients.

Bacterial Wall

The bacterial wall gives shape to the bacterial cell. The different compositions of the bacterial wall cause bacteria to react differently when treated with compounds of the Gram stain. Gram-positive bacteria stain blue, and Gram-negative bacteria stain red.

In both types of bacteria, the wall has a layer of murein, which is composed of peptidoglycan chains that are joined together.

  • Gram-positive: The wall is mono-layered, formed by a thick layer of murein associated with teichoic acids.
  • Gram-negative: The wall is bi-layered, with a thin layer of murein.

Function:

  • Maintains the shape of bacteria.
  • Is permeable to certain molecules.
  • Lysozyme, an enzyme, breaks the glycosidic links in murein.
  • Penicillin acts by preventing the peptide links between the long chains of peptidoglycan during its synthesis.

Plasma Membrane

The plasma membrane surrounds the cytoplasm. This membrane differs from that of eukaryotes in that it contains no cholesterol.

Function:

  • Directs DNA replication by DNA polymerase.
  • Facilitates respiration through the presence of the bacterial enzyme ATP synthase.

Bacterial Chromosome

The bacterial chromosome consists of a double-stranded circular DNA molecule, usually condensed in a region of the cytoplasm called the nucleoid. The bacterial chromosome is associated with proteins and RNA. Bacteria can also contain small circular DNA molecules called plasmids, which have the capability of autonomous replication. Some plasmids can reversibly integrate into the bacterial chromosome and are then called episomes.

Pili

Pili are hollow, elongated structures that bacteria use to adhere to different surfaces. They are only found in Gram-negative bacteria. Pili are composed of molecules of a protein called pilin. There are two types:

  • Sex pili are long and are used for bacterial conjugation, where two bacteria bind and exchange genetic material.
  • Fimbriae are short and are used to adhere to different surfaces.

Role of Nutrition

Bacteria can perform all types of metabolism. They can be:

  • Photoautotrophs: Utilize sunlight to fix carbon dioxide (CO2), e.g., green sulfur bacteria, purple bacteria, and cyanobacteria.
  • Photoheterotrophs: Require light energy but also use organic molecules such as alcohols, fatty acids, and carbohydrates as a carbon source, e.g., green and purple non-sulfur bacteria.
  • Chemoautotrophs: Such as nitrifying bacteria.
  • Chemoheterotrophs: Feed on dead organic matter (the majority) or living bacteria that are pathogenic.

Reproduction

Bacterial reproduction is asexual and takes place through binary fission, which is preceded by a doubling of the bacterial DNA and separation of the two molecules obtained. The offspring are genetically identical. Bacteria also have mechanisms of parasexual reproduction, by which they exchange genetic information with others, whether of the same species or not. There are three types of genetic exchange mechanisms:

Conjugation

In conjugation, a donor bacterium passes DNA through a sex pilus to a recipient bacterium. Donor bacteria have, in addition to their DNA, a special type of plasmid called the F plasmid or F factor (fertility), containing genes that code for the production of sex pili. Bacteria with F plasmids are called F+, and recipient bacteria are called F-. In the transmission of plasmids from one bacterium to another, only one of the two plasmid strands is transferred. The F- recipient becomes F+. Sometimes the F plasmid integrates (recombines) into the bacterial DNA. In this case, the plasmid is called an episome, and the bacterium is called Hfr.

Transduction

Transduction is a phenomenon of genetic exchange through accidental contamination, usually by a virus, which carries DNA from the last bacterial cell it parasitized. This occurs because the virus accidentally incorporated a fraction of the bacterial DNA during its assembly.

Transformation

Transformation is a process by which bacteria incorporate into their DNA fragments that were free in the environment and came from the lysis of other bacteria.