ABSTRACT
Microorganisms are those organisms that are too small to be visible without the aid of a microscope. Their size is about from 1 µm to 70 µm. All living organisms are composed of cells. Prokaryotic cells are relatively simple in structure; they lack a true membrane-covered nucleus. The most common shapes are a spherical cell and rod-shaped cell. Eukaryotic cells contain organelles that are compartments for special metabolic functions. The cytoplasmic membrane is a selective barrier separating the cytoplasm from the external environment. The cell wall protects a cell from changes in osmotic pressure. Bacteria with a thick cell wall stain as Gram-positive cells. Cells that stain as Gram-negative have a thin cell wall. The glycocalyx (capsule) is an extracellular polysaccharide covering the bacterial cell. It contributes to pathogenicity, helps bacteria to attach to surfaces, and protects the cell against desiccation, heavy metals, antibiotics, and disinfectants. Lipopolysaccharides of the outer membrane of Gram-negative bacteria are often toxic or allergenic for humans. Intracellular inclusions include granules of glycogen or poly-β-hydroxybutyrate in the cytoplasm, elemental sulfur granules, and polyphosphate granules. The vegetative cell can form endospores. Fungi and algae often have a cell wall made up of polysaccharides such as cellulose or chitin. Some algae have inorganic compounds such as calcium carbonate or silica in their cell walls. The three important organelles of eukaryotic cells are mitochondria, chloroplasts, and the nucleus. The nucleus contains the genetic material of the eukaryotic cell. Mitochondria are the organelles responsible for energy generation. Chloroplasts are cell organelles that assimilate light energy. A microbial biofilm is a layer or aggregates consisting of microbial cells and organic matter attached and growing on the surface. The concentration of dissolved oxygen in biofilms drops to zero at a depth of 100–200 μm. Therefore, deeper layers of the biofilm consist of facultative anaerobic and obligate anaerobic microorganisms. A tutorial with solutions and a quiz bank are added to this chapter.
