Lactobacillus (plural: lactobacilli) is a genus of Gram-positive, mostly rod-shaped bacteria from the family Lactobacillaceae. Lactobacillus belongs together with other bacterial genera to the lactic acid bacteria; they all produce lactic acid through fermentation. Lactobacillus species are important for the food industry.
Origin of the name: The generic name refers to they occurrence and appearance, lactis from Latin stands for “milk” and bacillus (Lat.) means “small rod”, Lactobacillus is therefore a “rod-shaped bacterium in milk”.
Many Lactobacillus species are used as probiotics.
Lactobacillus is regarded as a typical genus for a lactic acid bacterium. The representatives of the genus are Gram-positive bacteria, they do not form endospores and in most cases are not capable of active movement. The species of Lactobacillus are predominantly rod-shaped, the cells occur individually or in chains. However, curved and helical (e.g. Lactobacillus curvatus) variants may also occur. After the classification of different species, which were first classified in other genera, there are now some coccus among the lactobacilli as well as bacterial forms between coccus and rods.
On solid, carbohydrate-containing culture media, the cells grow into colonies, which are typically quite small in lactobacilli. In L. paralimentarius, for example, the diameter of the colonies is between 0.8 and 1.5 mm.
Growth and metabolism
As representatives of lactic acid bacteria, lactobacilli grow anaerobically but aerotolerantly, i.e. 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 cultivated on culture mediums containing haemins or blood. In this case, they show a positive reaction in the oxidase test. Lactobacillus, however, also has some species that do not tolerate oxygen at all. These species, which are considered obligatory anaerobic, include L. aviarius and L. ruminis. Another typical characteristic of lactobacilli is the need for complex growth factors and amino acids during cultivation.
The temperatures suitable for cultivation are in the range of 30-40 °C for most species, so Lactobacillus belongs to the mesophilic organisms. Some Lactobacillus species also grow well at 45 °C, this tendency to thermophilia is used to subdivide the genus and is important for use in the food industry. The optimum pH value for growth is a slightly acid pH value (pH 5 to 6), whereby acid pH values up to pH 4 are also tolerated. The lactic acid produced reduces the pH value of the nutrient medium, provided that no buffering additives are present. Some Lactobacillus species produce up to 2.3 % lactic acid in carbohydrate-rich nutrient media.
Lactic acid fermentation
Lactobacilli can utilize various carbohydrates for energy production in a fermentation process. The characteristic feature of fermentation is that the substrates are degraded without oxygen. The typical product of fermentation for lactic acid bacteria is lactic acid, hence this metabolic pathway is called lactic acid fermentation. A distinction is made between homofermentative and heterofermentative species. Homofermentative species produce almost exclusively lactic acid from glucose by fermentation, while heterofermentative species produce other end products in addition to lactic acid, mostly ethanol and carbon dioxide, sometimes also acetic acid. Heterofermentatives usually lack the enzyme aldolase.
The representatives of the genus Lactobacillus do not form a uniform group in this respect. The homofermentative species include Lactobacillus delbrueckii subsp. bulgaricus, where the typical metabolic pathway is explained. The following table gives an overview of the lactic acid fermentation of some Lactobacillus species.
Many lactobacilli form bacteriocins, toxic proteins or peptides that are secreted by bacteria and kill or inhibit the growth of other (competing) bacterial species. Bacteriocins produced by different Lactobacillus species include Lactacin-F and Bavaricin-A. Lactobacillus plantarum forms various plantaricins (A, S, T, and Plantaricin-SIK).
Lactobacillus is counted among the gram-positive bacteria with a low GC content (the proportion of the nucleic bases guanine and cytosine) in bacterial DNA. Lactobacillus species also show a high variability regarding this characteristic, the GC content lies between 32 and 53 mole percent. Lactobacillus mali has a GC content of 32-34 mole percent, while Lactobacillus panis has a GC content of 53-56 mole percent. Usually the content of guanine and cytosine in the DNA within a bacterial genus does not vary so much, which confirms that the lactobacilli form a rather heterogeneous group.
In addition to milk and dairy products, Lactobacillus species are found in or on plants and some form part of the natural intestinal flora of humans and other animals. Lactobacilli have been isolated from all parts of the digestive tract of humans, including the stomach.
The species Bifidobacterium bifidum, formerly classified as lactobacilli, occurs in the intestines of adults and infants (fed on breast milk). The obligate anaerobic bacterium is an important component of the intestinal flora. The term bifidus flora generally stands for the totality of the different species of bifidobacteria in the human intestine. The intestinal flora of infants breastfed with breast milk is specifically referred to here.
Lactobacillus salivarius and Lactobacillus ruminis belong to the autochthonous intestinal flora of humans. Autochthonous bacteria are regularly detectable within the respective habitat. L. salivarius and L. ruminis are thus “permanent inhabitants” of the human intestine. The species L. paracasei, L. brevis, L. fermentum, L. plantarum and L. rhamnosus occur only temporarily in the intestine.
In the stomachs of various animals such as mice, pigs and rats, lactobacilli form cell layers which are connected to the epithelial cells of the stomach. Lactobacillus amylovorus, L. johnsonii and L. reuteri occur mainly in the digestive tract of pigs. Furthermore, some lactobacilli form dense layers on the epithelium in the crop of birds, especially the species L. salivarius.
Lactobacillus reuteri forms antibiotic substances and is used commercially as a probiotic in poultry farming to prevent salmonella infections, for example. The intestinal bacterium from the animal world, for which the Swedish company BioGaia has applied for a patent, is also used in human medicine and is used there as a dental health remedy, as a diarrhoea remedy for small children and as a stomach remedy to combat Helicobacter pylori. The bacterium, which is used in mass livestock farming to save synthetic antibiotics, spreads quite quickly in the human digestive tract after appropriate ingestion and can not only be found in the intestines, but also in the acid environment of the stomach. It is transmissible through mother’s milk, infects infants and can be found in raw milk products. Since 2018, L. reuteri has been suspected of migrating from the digestive tract to the liver. There it is believed to trigger the autoimmune disease systemic lupus erythematosus. Normally, migration to other organs is an exclusion criterion for the use of probiotics. The immunobiologist Martin Kriegel, who uncovered the scandal, found L. reuteri not only in the liver of mice suffering from systemic lupus, but also in the liver of lupus patients. The researcher recommends that those affected include resistant starch in their diet in order to displace L. reuteri with safer bacterial strains.
Different types of Lactobacillus form the so-called Döderlein bacteria or Döderlein rods. The Döderlein bacteria are part of the natural vaginal flora of women. Through fermentation, the bacteria in the vagina create an acidic environment and thus protect the vagina from other pathogenic bacteria that do not tolerate a low pH value. Among the most frequently identified species in various studies are Lactobacillus crispatus, L. iners, L. gasseri and L. jensenii. Previously, Lactobacillus acidophilus was identified as the dominant species in the vaginal flora of healthy women.
The location of Lactobacillus selangorensis (formerly Paralactobacillus selangorensis) is a Malaysian food ingredient called Chili bo.
Furthermore, lactobacilli can be isolated from many plant surfaces, whereby this is the case with intact, but also with decomposing plant parts. There for example L. brevis, L. delbrueckii, L. fermentum and L. plantarum have been detected.
Lactobacillus is a type genus of the family Lactobacillaceae and of the order Lactobacillales. The family currently (2013) comprises three genera: Lactobacillus, Pediococcus and Sharpea. Genetic and cell morphological investigations of Haakensen, among others, on the species initially classified as Pediococcus dextrinicus showed that it is better assigned to the Lactobacilli. Besides the renaming of the species to Lactobacillus dextrinicus, this also led to an extended description of the genus Lactobacillus in 2009.
The genus Lactobacillus comprises numerous (about 80) species. Currently (2013) 218 Lactobacillus species and subspecies are listed by the Leibniz Institute DSMZ – Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH in the Prokaryotic Nomenclature up-to-date. This compilation includes all names validly published according to the Bacteriological Code and takes into account the validation list of the International Journal of Systematic and Evolutionary Microbiology. For other species the classification to other genera is suggested.
Due to the diversity within the genus, it is to be expected that new species will be added or no longer belong to the genus through reclassification and that new Lactobacillus species will be newly described. In order to structure the rather heterogeneous group of lactobacilli, a division into three subgroups has proved its worth:
A selection of the species of Lactobacillus (Lactobacillus Beijerinck 1901 emend. Haakensen et al. 2009):
- Lactobacillus delbrueckii (Leichmann 1896) Beijerinck 1901 (type species)
- Lactobacillus delbrueckii subsp. bulgaricus (Orla-Jensen 1919) Weiss et al. 1984
- Lactobacillus delbrueckii subsp. delbrueckii (Leichmann 1896) Weiss et al. 1984
- Lactobacillus delbrueckii subsp. lactis (Orla-Jensen 1919) Weiss et al. 1984
- Lactobacillus acidophilus (Moro 1900) Hansen et Mocquot 1970
- Lactobacillus alimentarius (ex Reuter 1970) Reuter 1983
- Lactobacillus amylolyticus Bohak et al. 1999
- Lactobacillus amylovorus Nakamura 1981
- Lactobacillus aviarius Fujisawa et al. 1985
- Lactobacillus bifermentans (ex Pette et van Beynum 1943) Kandler et al. 1983
- Lactobacillus brevis (Orla-Jensen 1919) Bergey et al. 1934
- Lactobacillus buchneri (Henneberg 1903) Bergey et al. 1923
- Lactobacillus casei (Orla-Jensen 1916) Hansen et Lessel 1971
- Lactobacillus coryniformis Abo-Elnaga et Kandler 1965
- Lactobacillus crispatus (Brygoo et Aladame 1953) Moore et Holdeman 1970 emend. Cato et al. 1983
- Lactobacillus crustorum Scheirlinck et al. 2007
- Lactobacillus curvatus (Troili-Petersson 1903) Abo-Elnaga et Kandler 1965 emend. Klein et al. 1996
- Lactobacillus dextrinicus (Coster et White 1964) Haakensen et al. 2009
- Lactobacillus fermentum Beijerinck 1901 emend. Dellaglio et al. 2004
- Lactobacillus gasseri Lauer et Kandler 1980
- Lactobacillus helveticus (Orla-Jensen 1919) Bergey et al. 1925
- Lactobacillus hilgardii Douglas et Cruess 1936
- Lactobacillus iners Falsen et al. 1999
- Lactobacillus jensenii Gasser et al. 1970
- Lactobacillus johnsonii Fujisawa et al. 1992
- Lactobacillus kefiri corrig. Kandler and Kunath 1983,
- Lactobacillus lindneri (ex Henneberg 1901) Back et al. 1997
- Lactobacillus mali Carr et Davies 1970
- Lactobacillus oryzae Tohno et al. 2013
- Lactobacillus panis Wiese et al. 1996,
- Lactobacillus parabuchneri Farrow et al. 1989,
- Lactobacillus paracasei Collins et al. 1989
- Lactobacillus paralimentarius Cai et al. 1999
- Lactobacillus plantarum (Orla-Jensen 1919) Bergey et al. 1923
- Lactobacillus reuteri Kandler et al. 1982
- Lactobacillus rhamnosus (Hansen 1968) Collins et al. 1989
- Lactobacillus rossiae corrig. Corsetti et al. 2005
- Lactobacillus ruminis Sharpe et al. 1973
- Lactobacillus sakei corrig. Katagiri et al. 1934 emend. Klein et al. 1996
- Lactobacillus salivarius Rogosa et al. 1953 emend. Li et al. 2006
- Lactobacillus sanfranciscensis corrig. (ex Kline et Sugihara 1971) Weiss et Schillinger 1984,
- Lactobacillus selangorensis (Leisner et al. 2000) Haakensen et al. 2011
In the food industry, lactic acid fermentation is mainly used in the production of dairy products such as cheese and yoghurt. Without lactic acid bacteria there would be practically no dairy products. But they are also involved in the production of other acidified food and feed. In addition to the Lactobacillus species, other bacterial species are usually also involved.
Types of Lactobacillus and Pediococcus are also known as pests in beverage production. The formation of lactic acid and other products leads to undesirable acidification and taste changes, e.g. in beer, wine and fruit juices. Even with heated drinking milk (UHT milk, pasteurised milk), the lactobacilli are undesirable if they get back into the product after heating due to contamination.
Bacteria: Firmicutes, Cyanobacteria. In: Martin Dworkin, Stanley Falkow, Eugene Rosenberg, Karl-Heinz Schleifer, Erko Stackebrandt (Ed.): The Prokaryotes, A Handbook of the Biology of Bacteria. 3rd edition. Volume 4 Springer Verlag, New York, USA 2006, ISBN 978-0-387-25494-4.