Like the pediococci, Tetragenococcus are ho-mofermentative, tetrad-forming, facultative anaerobes. They are mesophilic and neu-traphilic, with temperature optima generally between 25 °C and 30°C and pH optima between 6.5 and 8.0. The genus contains only three species, Tetragenococcus halophilus, Tetragenococcus muriaticus, and Tetrageno-coccus solitarius. Based on 16S rRNA sequences, these bacteria are phylogenetically more closely related to Lactobacillus and Ente-rococcus, than to Pediococcus.
The genus was first recognized in 1990, when Pediococcus halophilus was re-classified as T. halophilus. A second species, T. muriati-cus, isolated from salty fish sauce, was proposed in 1997, and in 2005, Enterococcus soli-tarius was reclassified as T. solitarius. All of these species are characterized by their remarkable tolerance to salt. Not only is growth possible in media containing up to 25% salt, but maximum growth rates require the presence of 3% to 10% NaCl, and no growth occurs when salt is absent.They also grow well when the water activity is low, presumably due to their ability to maintain osmotic balance by accumulating betaine, carnitine, and other compatible solutes.
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