Spores, Capsules and Flagella
Bacteria use special structures to survive, hide and move. Three of the most important are spores, capsules and flagella. Each structure serves a different job. Together they explain why some microbes last in soil, resist immune attack or spread through liquid.
A spore is a resting form. Certain Gram-positive genera, especially Bacillus and Clostridium, can form endospores. The cell packs its DNA and a little cytoplasm inside a tough coat. Water leaves the core. Metabolism almost stops. Heat, drying, radiation and many chemicals then fail to kill it. When conditions improve, the spore germinates and a vegetative cell returns. That is why autoclaves must reach high temperature and pressure.
A capsule is a sticky layer outside the cell wall. It is usually made of polysaccharide. Some species use polypeptide instead. The coat helps the cell cling to surfaces and form biofilms. It also hides surface antigens from phagocytes. Encapsulated pneumococci and meningococci therefore cause more severe disease than rough strains. A capsule can be seen as a clear halo after negative staining.
Flagella are thin protein filaments used for swimming. The filament is built from flagellin. A rotary motor sits in the membrane and wall. Energy from the proton gradient turns the hook. One flagellum or many can be arranged at one pole or all around the cell. Chemotaxis then steers the cell toward food or away from harm. Some pathogens use flagella to cross mucus and reach tissue.
These structures are not present in every species. Many harmless gut bacteria lack spores. Some aquatic forms rely on flagella. Capsules appear when the cell needs protection in the host or in dry air. Staining, motility tests and electron microscopy still identify them in the teaching laboratory.
Spores explain survival. Capsules explain evasion. Flagella explain movement. A short look at all three therefore links bacterial anatomy to infection, food safety and sterilisation practice.