Bacterial Ultrastructure - Cytoplasmic Membrane, Cytoplasm, Nucleoid, etc.
Bacteria are prokaryotic organisms. Prokaryotic cells are simpler than eukaryotic cells. Unlike eukaryotes, they lack a true nucleus and membrane-bound organelles such as mitochondria, Golgi apparatus, endoplasmic reticulum, and lysosomes.
- Nucleoid (Nucleus Equivalent)
- The bacterial genetic material is located in a region called the nucleoid.
- It is a discrete area or a tangle of double-stranded DNA and is not surrounded by a nuclear membrane.
- The bacterial chromosome is typically a single, circular, double-stranded DNA molecule. It is highly compacted or supercoiled.
- There are exceptions, such as Borrelia, which has linear chromosomes.
- Unlike eukaryotic DNA, bacterial DNA is not associated with histones.
- Bacteria are generally haploid.
- The nucleoid region has a high concentration of RNA, including mRNA.
- The lack of a nuclear membrane allows transcription and translation to occur simultaneously.
- Cytoplasm
- The cytoplasm is the jelly space or amorphous matrix within the cell membrane.
- It is simpler than eukaryotic cytoplasm and lacks membrane-bound organelles like ER, Golgi, mitochondria, and lysosomes.
- It contains a large number of solute low- and high-molecular-weight substances.
- Key components include ribosomes, inclusions, and plasmids.
- Ribosomes
- Bacterial ribosomes are 70S, composed of a 30S small subunit and a 50S large subunit.
- They are the organelles for protein synthesis.
- Bacterial ribosomes are major targets for antibacterial drugs. Eukaryotic ribosomes are 80S.
- Cytoplasmic Inclusions
- These are granules of organic and inorganic substances found in the cytoplasm.
- They serve as reserve nutrient substances or deposits of metabolic waste.
- Examples include Metachromatic granules (Volutin), Glycogen granules, Polyphosphate granules, Polyhydroxybutyrate (PHB), starch, or sulfur.
- Volutin granules can be diagnostically important, for example, in Corynebacterium diphtheriae.
- Plasmids
- Plasmids are smaller genetic structures than the chromosome.
- They are extrachromosomal and typically circular, double-stranded DNA molecules. Rare linear plasmids exist.
- They replicate independently of the bacterial chromosome.
- They are non-essential for basic cell survival.
- Plasmids often carry genes that provide a selective advantage, such as antibiotic resistance, virulence factors, or metabolic capabilities.
- They can be transferred between bacteria.
- Cytoplasmic Membrane (Cell Membrane)
- The bacterial cytoplasmic membrane is a phospholipid bilayer.
- It is similar to eukaryotic membranes but contains no cholesterol, except for mycoplasmas. Mycoplasmas are an exception as they incorporate sterols from the host. Some bacteria may contain hopanoids.
- It contains embedded proteins.
- It acts as a selective barrier (selective permeability), controlling substance transport. Transport proteins are involved.