Summary
Highlights
Bacteria are prokaryotic cells with important roles in human health and disease. They are present in various environments, including the human body. Their structure is less complex than eukaryotic cells, lacking a nucleus and membrane-bound organelles.
Bacterial cells are enclosed by a cell envelope, which includes the cell wall. The cell wall, made of peptidoglycan, protects the bacteria and helps classify them as gram-positive (thick peptidoglycan, retains dye) or gram-negative (thin peptidoglycan, does not retain dye). It also maintains cell shape (cocci, bacilli, spirilla, pleomorphic). Some bacteria have an additional protective capsule, which prevents phagocytosis and aids in adhesion, acting as a virulence factor.
Beneath the cell wall lies the plasma membrane, a crucial layer that encloses the cytoplasm. It regulates nutrient intake and waste elimination, maintaining the cell's internal environment. The plasma membrane is selectively permeable and in bacteria, it also plays roles in respiration, photosynthesis, and the synthesis of lipids and cell wall components.
The bacterial cytoplasm contains ribosomes, which are essential for protein synthesis. Proteins destined for intracellular use are made by free ribosomes, while those for the cell envelope or export are made by plasma membrane-associated ribosomes. The nucleoid, an irregular region, holds the cell's chromosome(s), typically a circular double-stranded DNA, containing genes for protein synthesis. Additionally, many bacteria have plasmids, extra-chromosomal DNA molecules that, while not essential for basic survival, often confer selective advantages like antibiotic resistance.
Bacteria use external structures for movement and attachment. Pili are hair-like structures for attachment to other cells, with shorter versions called fimbriae specifically for surface adhesion. Flagella are long, whip-like protrusions that enable bacterial motility.