Whereas essentially all eukaryotic organisms require oxygen to thrive, many species of bacteria can grow under anaerobic conditions. Bacteria that require oxygen to grow are called obligate aerobic bacteria. Bacteria and many microorganisms are very sensitive to oxygen concentrations. Some will only grow in its presence and are called obligate aerobes. Facultative aerobes will grow either aerobically or in the absence of oxygen (anaerobic conditions), but they generally do better with oxygen. Obligate anaerobes, which live only in the absence of oxygen, do not possess the defenses that make aerobic life possible and therefore cannot survive in air. Like all other organisms, bacteria need water to survive, but the surfaces of leaves experience daily changes in moisture, tending to be much wetter at night than during the day.

Bacteria that grow only in the absence of oxygen, such as Clostridium, Bacteroides, and the methane-producing archaea, are called obligate anaerobes because their energy-generating metabolic processes are not coupled with the consumption of oxygen. Obligate anaerobes, which live only in the absence of oxygen, do not possess the defenses that make aerobic life possible and therefore cannot survive in air. Like all other organisms, bacteria need water to survive, but the surfaces of leaves experience daily changes in moisture, tending to be much wetter at night than during the day.The pathogen Acinetobacter baumannii can survive on hospital surfaces without water for months, an ability that has helped it become a leading cause of hospital-acquired infections. Single-celled bacteria lack that ability, so they must rely on finding enough available water in their environment to through their cell membranes. Many bacteria can survive for extended periods without moisture, but without it they can't grow and reproduce. Water is a prerequisite for microorganisms to grow and proliferate; however, most microorganisms do not possess mechanisms to actively uptake water. Instead, they rely on osmotically active substances in the cytoplasm to maintain a positive turgor. Bacteria and many microorganisms are very sensitive to oxygen concentrations. Some will only grow in its presence and are called obligate aerobes. Facultative aerobes will grow either aerobically or in the absence of oxygen but they generally do better with oxygen. Bacteria that require oxygen to grow are called obligate aerobic bacteria. In most cases, these bacteria require oxygen to grow because their methods of energy production and respiration depend on the transfer of electrons to oxygen, which is the final electron acceptor in the electron transport reaction.

Obligate anaerobes, which live only in the absence of oxygen, do not possess the defenses that make aerobic life possible and therefore cannot survive in air. The excited singlet oxygen molecule is very reactive. Therefore, superoxide must be removed for the cells to survive in the presence of oxygen.Bacteria and many microorganisms are very sensitive to oxygen concentrations. Some will only grow in its presence and are called obligate aerobes. Facultative aerobes will grow either aerobically or in the absence of oxygen (anaerobic conditions), but they generally do better with oxygen. Whereas essentially all eukaryotic organisms require oxygen to thrive, many species of bacteria can grow under anaerobic conditions. Bacteria that require oxygen to grow are called obligate aerobic bacteria. Whereas essentially all eukaryotic organisms require oxygen to thrive, many species of bacteria can grow under anaerobic conditions. Bacteria that require oxygen to grow are called obligate aerobic bacteria. Aerobic bacteria are bacteria that thrive and grow in an aerobic environment. A single bacterium that needs oxygen for survival is referred to as an aerobic bacterium and anaerobes cannot survive in the presence of molecular oxygen. In fact, 50% of cells survive, on the average, after 4–5 min of atmospheric O2; this percentage decreases to 3–5% after only 20 min, and after 40 min only one cell in a thousand survives; all strains reached 100% mortality in a time range of 100–120 min.