Microorganisms are tiny organisms that require a microscope to be seen, including fungi, bacteria, algae, protozoa, and viruses. Microbiology is the study of these microbes. While most microorganisms need magnification, some eukaryotic microorganisms are visible without it. Organisms are classified into three domains based on evolutionary lines: Bacteria, Archaea, and Eukaryotes.Eubacteria, or “true bacteria,” are simple, unicellular prokaryotes that reproduce asexually. They lack a nuclear membrane, mitochondria, Golgi bodies, and endoplasmic reticulum. Found in soil, air, and water, they can be autotrophic or heterotrophic, sometimes acting as parasites, saprophytes, or in symbiotic relationships. Their cells have a thick, rigid wall made of peptidoglycan and contain both DNA and RNA. Omnipresent in various environments, they lack true chlorophyll, although some possess bacteriochlorophyll. With no true nucleus or complex organelles, their minute size requires an electron microscope for study, revealing structures both external and internal to the cell wall. In contrast, eukaryotic microbes are diverse, responsible for many diseases, and equipped with complex organelles and a cytoskeleton for shape, movement, and reproduction, with genetic material distributed through mitosis and meiosis.

Bacteria are morphologically classified based on features such as cell shape, multicellular aggregates, motility, spore formation, and reaction to Gram stain. The primary shapes are cocci (spherical), bacilli (rod-shaped), and spirochaetes (spiral or corkscrew-shaped). Bacteria are further categorized based on cell wall composition, which affects their Gram staining properties. Gram-positive bacteria retain a purple stain due to a thick peptidoglycan layer, while Gram-negative bacteria appear red due to a thinner peptidoglycan layer. The Gram staining process involves steps like heat fixing, applying crystal violet, adding iodine, decolorizing, and counterstaining with safranin.

Bacteria also differ in their nutritional types. Autotrophic bacteria, which produce their own food from inorganic substances, include photoautotrophs (using sunlight) and chemoautotrophs (using inorganic chemicals). Heterotrophic bacteria, which derive carbon from organic molecules, include photoheterotrophs (using sunlight) and chemoheterotrophs (using organic molecules). They are classified into nutritional classes based on energy sources and electron donors, including photolithoautotrophs, chemoorganoheterotrophs, and chemolithoautotrophs. Each class plays a role in various ecological processes and nutrient cycles

Bacteria can reproduce through two primary mechanisms: asexual and sexual reproduction. Asexual reproduction primarily occurs via binary fission, where a bacterial cell grows, replicates its DNA, and divides into two genetically identical daughter cells. This process does not involve mitotic spindles like in eukaryotes, and although the exact mechanisms of chromosome segregation are not fully understood, specialized proteins aid in this process. Sexual reproduction in bacteria is called conjugation, where two cells of different mating types exchange genetic material. In this process, one cell, known as the donor (F cell), transfers DNA to a recipient cell through a specialized structure called a sex pilus, forming a conjugation bridge. This transfer involves the F factor, a DNA sequence that can be present as a plasmid or integrated into the bacterial chromosome and is crucial for the transfer of genetic material, including genes for antibiotic resistance.