Bacteria are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep biosphere of Earth's crust. Bacteria play a vital role in many stages of the nutrient cycle by recycling nutrients and the fixation of nitrogen from the atmosphere. The nutrient cycle includes the decomposition of dead bodies; bacteria are responsible for the putrefaction stage in this process. In the biological communities surrounding hydrothermal vents and cold seeps, extremophile bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy. Bacteria also live in mutualistic, commensal and parasitic relationships with plants and animals. Most bacteria have not been characterised and there are many species that cannot be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.

जीवाणवः लघुजन्तवः सन्ति। वयं तान् स्वनेत्राभ्यां द्रष्टुं न शक्नुमः। परं वयं सूक्ष्मदर्शन्या जीवाणून् द्रष्टुम् समर्थाः स्म। केचन जीवाणवः अपकारिणः। ते व्याधीहेतुकाः सन्ति अथवा भोजनम् मलिनीकुर्वन्ति। अन्ये उपकारकाः सन्ति। ते दधिदाधिके सुराम् औषधानि वा उत्पादयन्ति। तेषां विविधानि रूपाणि सन्ति - वर्तुलाकारं(coccus), कम्बु-रूपं (spirillium), पदविच्छेदचिह्नरूपं (vibrio) अण्डाकारकम् (bacillus) च। नीलजीवाणवः आहारं स्वयम् उत्पादयन्ति ।