Eubacteria, like Archaea, is single-celled and a prokaryote with no nucleus. They linger in less extreme enviornments than Archaebacteria. Depending on the species, some of these organisms can die from being expose to oxygen, or thrive. For a few types, Oxygen is necessary to live. Some use photosynthesis or absorb their food. Their shapes include rods and spheres, only measuring at a few micrometers. They are crucial in the nutrient cycle, though few species are harmful. Most bacteria in the human body have a positive effect and are protective parts of the immune system, while other species may be beneficial. A few types of harmful Eubateria include Salmonella enteritidis, Clostridium tetani, and Escherichia coli. Bacteria can be used to make yogurt, cheese, and many antibiotics. Bacteria reproduce asexually through binary fission. This process can be rapid and intense, or slow. Bacteria tends to excrete chemicals to change their enviornment, breaking down food or digesting it. There are trillions of species of Bacteria.