Leuconostoc

Leuconostoc is a genus of gram-positive bacteria in the family of Leuconostocaceae which belongs to the order Lactobacillales ( lactic acid bacteria). Leuconostoc species are widespread in the natural environment, they play an important role in various industrial and food fermentations.

  • 4.1 Outer systematics
  • 4.2 Internal systematics
  • 5.1 Literature
  • 5.2 Notes and references

Features

Appearance

Most species occur in liquid culture as cocci in appearance: the cell shape is round to oval. The cells occur singly, in as diplococci and short chains of cocci as. In the Gram stain they behave Gram- positive, do not have flagella for active movement and do not form Überdauerungsformen as endospores. Their appearance in the light microscope is similar to members of the genus Lactococcus, Leuconostoc, however, in not the typical cocci spherical cells predominate, but there are more oval or ovoid ( " egg " ) to observe cell lines.

The genus name can be traced back to the appearance of colonies ( leukos from ancient Greek meaning " bright" or "clear" ).

Growth and metabolism

As a typical representative of the lactic acid bacteria grow anaerobically, but aerotolerant, that is, they grow in the presence of atmospheric oxygen, but do not require oxygen for their metabolism. They are catalase - negative and oxidase - negative. However, they are able to form cytochromes when cultured on nutrient media containing hemins or blood. In this case, they then exhibit a positive reaction in the oxidase test. Further, a typical lactic acid bacteria, the need for complex flag growth factors and amino acids in the culture.

Under aerobic, anaerobic and microaerophilic conditions, a heterofermentative lactic acid fermentation is performed. D-glucose or D-fructose is converted via the pentose phosphate pathway to an equimolar amount of D- lactic acid, ethanol, and carbon dioxide. Only a few species of the genus are able to break down lactose ( milk sugar) on this route. For this purpose, the disaccharide lactose is digested with the help of the enzyme β -galactosidase own bacteria into its two components, glucose and galactose. In contrast, include L -arabinose, D-mannose and maltose to the carbohydrates that can be utilized by most of Leuconostoc species.

All types can also degrade sucrose, typically this is the education of dextrans from the substrate sucrose. Due to the formation of dextrans sucrose-containing solutions can be transformed into a jelly - like mass. The polysaccharide formed dextran accumulates around the cells as so-called mucus. If Leuconostoc species cultured on a saccharose-containing solid nutrient medium, they form large, slimy colonies as they grow on a glucose-containing culture medium as small colonies without mucus.

Other pathways include the conversion of acetyl -CoA to acetoin and diacetyl, wherein the latter is of importance as a flavor in the manufacture of lactic acid foods.

For cultivation of these bacteria, the MRS medium is used in liquid form ( MRS broth ) or with the addition of agar - agar as a solid growth medium. This is not a Selective culture, but also offers all lactic acid bacteria a suitable nutrient supply, because in addition to glucose and meat extract, yeast extract, peptone, and various minerals are included. The optimal temperature for the incubation is about 30 ° C.

The representatives of the genus Leuconostoc may contain high levels of glucose tolerate without their growth is affected. Similar to the yeast osmophilic they can reproduce in habitats with low water activity. Bacteria are referred to in this context as xerotolerant. Leuconostoc species grow in a medium that contains 30-40 % glucose and are therefore xelotolerant.

Occurrence

Milk is the natural habitat for numerous Leuconostoc species. Leuconostoc mesenteroides can be isolated from many plant surfaces, this being the case in intact, as well as in decomposing plant parts. In the production of sauerkraut naturally present on the cabbage Leuconostoc species are often involved, an isolated from sauerkraut strain was identified in 1992 as a new species.

Importance

They play an important role in various fermentations. For example, the subspecies of Leuconostoc mesenteroides subsp. cremoris as starter culture for the production of butter and cheese in dairies. Other types are involved in the production of silage and sauerkraut, as well in the lactic acid fermentation is important. In the fermentation of coffee beans many different microorganisms interact with, studies from 2007 showed that there Leuconostoc is involved and led to the discovery of a new species Leuconostoc species but can also lead to food spoilage, if the formation of lactic acid is not is desired. In addition to the spoilage of fresh milk by " souring " are also vacuum-packed meat and sausages affected because the bacteria can grow there under anaerobic conditions. The same goes for under a protective gas atmosphere ( such as carbon dioxide ) packaged products.

System

Outer systematics

Leuconostoc is similar to the cellular appearance of the members of the genus Lactococcus. Both species also exhibit similarities in their DNA composition. The GC content (the proportion of guanine and cytosine nucleobases ) in the bacterial DNA is in each case 38 to 41 mol percent. However, they differ in their metabolism, as Leuconostoc performs a heterofermentative lactic acid fermentation, while for the homofermentative Lactococcus lactic acid fermentation is typical.

Inside systematics

The genus includes numerous species, which partly still in subspecies ( subspecies ) are divided. The classification of bacteria changed regularly, as new research results make another classification necessary or new species are discovered and described. The current status can with in the List of Prokaryotic names standing be viewed in Nomenclature ( LPSN ). Also, the Leibniz Institute DSMZ - German Collection of Microorganisms and Cell Cultures GmbH maintains a corresponding list.

Then (as of 2013) include the following 13 species to the genus Leuconostoc:

  • Leuconostoc carnosum Shaw & Harding 1989
  • Leuconostoc citreum Farrow et al. 1989
  • Leuconostoc fallax Martinez - Murcia & Collins 1992
  • Leuconostoc gasicomitatum Björk Roth et al. 2001
  • Leuconostoc gelidum Shaw & Harding 1989
  • Leuconostoc holzapfelii De Bruyne et al. 2007
  • Leuconostoc inhae Kim et al. 2003
  • Leuconostoc kimchii Kim et al. 2000
  • Leuconostoc lactis Garvie 1960
  • Leuconostoc mesenteroides ( Tsenkovskii 1878) van Tieghem 1878 ( type species ), now divided into the subspecies Leuconostoc mesenteroides subsp. cremoris ( Knudsen and Sorensen 1929) Garvie 1983
  • Leuconostoc mesenteroides subsp. dextranicum ( Beijerinck 1912) Garvie 1983
  • Leuconostoc mesenteroides subsp. mesenteroides ( Tsenkovskii 1878) Garvie 1983
  • Leuconostoc mesenteroides subsp. suionicum Gu et al. 2012

Four types of bacteria that were previously assigned to the genus Leuconostoc are, since 2008, to the new genus Fructobacillus:

  • Fructobacillus durionis ( Leisner et al., 2005 ) Endo and Okada, 2008 (previously Leuconostoc durionis )
  • Fructobacillus ficulneus ( Antunes et al. 2002) Endo and Okada 2008 ( vorherLeuconostoc ficulneum )
  • Fructobacillus fructosus ( Kodama 1956) Endo and Okada, 2008 (previously Leuconostoc fructosum )
  • Fructobacillus pseudoficulneus ( Chambel et al. 2006) Endo and Okada 2008 ( previously Leuconostoc pseudoficulneum )

Besides genetic studies also showed metabolic physiological characteristics that the assignment to the genus Leuconostoc was inconclusive. The Fructobacillus species do not produce ethanol, but which is a typical product of metabolism of heterofermentative lactic acid fermentation. In them, glucose is reduced to lactic acid and acetic acid instead. Also, the image in the scanning electron microscope shows rod-shaped cells.

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