Hydrogen oxidizing bacteria, or sometimes Knallgas-bacteria, are bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 which oxidize hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

. See microbial metabolism

Microbial metabolism

Microbial metabolism is the means by which a microbe obtains the energy and nutrients it needs to live and reproduce. Microbes use many different types of metabolic strategies and species can often be differentiated from each other based on metabolic characteristics...

 (hydrogen oxidation). These bacteria include Hydrogenobacter thermophilus

Hydrogenobacter thermophilus

Hydrogenobacter thermophilus is an obligately chemolithoautotrophic, extremely thermophilic hydrogen-oxidizing bacterium whose optimal growth temperature is around 70 to 75°C and was isolated from hot springs...

, Hydrogenovibrio marinus, and Helicobacter pylori

Helicobacter pylori

Helicobacter pylori , previously named Campylobacter pyloridis, is a Gram-negative, microaerophilic bacterium found in the stomach. It was identified in 1982 by Barry Marshall and Robin Warren, who found that it was present in patients with chronic gastritis and gastric ulcers, conditions that were...

. There are both Gram positive and Gram negative knallgas bacteria.

Most grow best under microaerophilic conditions. They do this because the hydrogenase

Hydrogenase

A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen . Hydrogenases play a vital role in anaerobic metabolism....

 enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 used in hydrogen oxidation is inhibited by the presence of oxygen, but oxygen is still needed as a terminal electron acceptor

Electron acceptor

An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....

.

The word "Knallgas" is Swedish

Swedish language

Swedish is a North Germanic language, spoken by approximately 10 million people, predominantly in Sweden and parts of Finland, especially along its coast and on the Åland islands. It is largely mutually intelligible with Norwegian and Danish...

 and German

German language

German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language in the European Union....

 for "bang-gas" and means a mixture of hydrogen and oxygen: see oxyhydrogen

Oxyhydrogen

Oxyhydrogen is a mixture of hydrogen and oxygen gases, typically in a 2:1 molar ratio, the same proportion as water. This gaseous mixture is used for torches for the processing of refractory materials and was the first gaseous mixture used for welding...

.