Lactobacillus salivarius
1. Classification
a. Higher order taxa
Lactobacillus salivarius belongs to the terrabacteria group in the class Firmicutes. It is in the class Bacilli, of the order Lactobacillales, and belongs to the family Lactobacillus (“European Bioinformatics” 2018).
2. Description and significance
Lactobacillus salivarius is a well characterized, rod-shaped, Gram-positive species of probiotic bacteria (Chapot-Chartier et al. 2014). It is commonly isolated from human, porcine, and avian gastrointestinal tracts. L. salivarius is a lactic acid bacterium that has antimicrobial activity due to the production of bacteriocins, which directly inhibit pathogens (Messaoudi et al. 2013). Modern discoveries have demonstrated the effectiveness of the bacteriocins of L. salivarius against specific pathogens such as those that are responsible for foodborne illness, acne and other skin conditions, and irritable bowel syndrome in humans (Messaoudi et al. 2013, Corr et al. 2007, Deidda et al. 2018, Peran et al. 2005). L. salivarius has also been proven to fight the pathogen Staphylococcus aureus; L. salivarius can target both planktonic cells as well as biofilms of S. aureus through the secretion of anti-Staphylococcus proteins. (Kang et al. 2017) The bacteriocin properties of L. salivarius also are effective in food preservation (Messaoudi et al. 2013). New knowledge regarding the beneficial antibacterial properties of L. salivarius have led to an increase in the use of the strain as a replacement to previous topical and oral antibiotics (Corr et al. 2007). Further exploration of the mechanisms of L. salivarius’ probiotic and antimicrobial properties has the potential to yield new discoveries beneficial to human health.
3. Genome structure
The genome of L. salivarius is a circular chromosome of approximately 2,028,405 base pairs with a 32.7% G/C content (Ayala et al. 2017). Of all of the chromosomal genes, the function of 71% of these genes are known (Claesson et al. 2006). There are 1,982 protein coding sequences along with 64 tRNA and 31 rRNA sequences (Ayala et al. 2017). Additionally, this bacterial genome has a tetracycline resistance gene (Ayala et al. 2017). Of the entire genome of L. salivarius, 20% consists of four plasmids, two of which are megaplasmids (Raftis et al. 2014, Seol et al. 2011). Evidence shows these megaplasmids may be a possible mechanism used by the bacterium to expand and/or contract its genome to adjust to varying environmental conditions (Claesson et al. 2006). One of the megaplasmids identified encodes proteins involved in metabolic processes and expresses characteristics that can help L. salivarius survive within the gastrointestinal tract of humans (Claesson et al. 2006). However, the function of only 50% of the genes of the megaplasmid are known (Claesson et al. 2006).
4. Cell structure
L. salivarius bacteria are rod-shaped cells (bacilli). Along with all lactic acid bacteria, L. salivarius are Gram-positive due to the structure of their cell wall (Chapot-Chartier et al. 2014). The specific amino acid sequence of the peptide chains of the peptidoglycan layer impacts the bacteria’s interaction with host cells (Chapot-Chartier et al. 2014). One particular amino acid chain of the peptidoglycan in L. salivarius Ls33, for instance, is associated with the strain’s anti-inflammatory properties in intestinal cells (Macho et al. 2011). The cell wall also contains lipoteichoic acids (LTAs) and wall teichoic acids (WTAs) which are anionic molecules that play various functional roles in the cell. LTAs and WTAs can help establish a pH gradient across the cell wall, maintain cell shape, and recognize bacteriophages (Chapot-Chartier et al. 2014). The final component of the cell wall, the S layer, has been found to play a crucial role in the adhesion process of L. salivarius (Wang et al. 2017). Choline binding protein A (CbpA), an S-protein in the cell wall of L. salivarius REN, can recognize CbpA receptors on human intestinal host cell to facilitate the binding process. This adhesion allows the bacteria to exhibit its probiotic functions by protecting the epithelial lining of the gut and excluding pathogens (Wang et al. 2017).
5. Metabolic processes
L. salivarius is characterized as a lactic acid bacterium (LAB), which denotes its ability to produce lactic acid by fermentation (Kuratsu et al. 2010). Specifically, L. salivarius is a homofermentative bacterium, meaning it can only utilize carbohydrates to ferment sugars via the Embden-Meyerhof-Parnas (EMP) pathway. The sugars it can ferment include the monosaccharides glucose, ribose adonitol, galactose, fructose, mannose, mannitol sorbitol and N-acetylglucosamine, in addition to the disaccharides maltose, lactose, sucrose and trehalose (Claesson et al. 2006). Each of these sugars are commonly found in or produced from human dietary components, which is why L. salivarius thrives in the human gastrointestinal system (Claesson et al. 2006). L. salivarius is a facultative-anaerobe for these processes (Stern et al. 2006).
L. salivarius is also able to metabolize bacteriocins, or small peptides with antimicrobial activity (Messaoudi et al. 2013). This property provides L. salivarius with beneficial probiotic properties in humans. Abp118, a bacteriocin of L. salivarius, has been shown to exhibit direct antagonism against human pathogens, such as those that cause foodborne illnesses (Corr et al. 2007). Other bacteriocins produced by this microbe have proven successful in combating the common skin condition, acne (Deidda et al. 2018). Lactic-acid bacterium bacteriocins have increasingly been used as oral and topical antibiotics and disinfectants as well (Messaoudi et al. 2013).
6. Ecology
The ecology of specific strains of the genus Lactobacillus have not been studied. Lactobacilli have been found to inhabit the gastrointestinal tract of mammals and thus are part of the gut microbiota. These gut microbiota, when compared to “germ-free” animals, have shown to have substantial influence over the physiology, immunology, and resistance to pathogens of their host (Walter, 2008). They can persist in the stomach, small intestine, or large intestine. The Lactobacilli that inhabit the gut must be well-adapted to their habitats in order to persist and have an impact on their host. L. salivarius has not proven to be able to permanently reside in the gut and thus have lasting effects on the physiology or immunology of its host. Seven-day consumption of the probiotic, L. salivarius, is enough for the bacteria to establish and flourish in the human intestinal tract only transiently (Bonetti et al. 2002).
7. Pathology
L. salivarius has proved to be an effective probiotic with at least temporary influence on gut microbiota. The bacteria can reduce host colonization by pathogenic bacteria (Ayala et al. 2017). L. salivarius L28 possesses the ability to fight against several foodborne pathogens, including Escherichia coli O157:H7, Salmonella spp., and Listeria monocytogenes (Ayala et al. 2017). This strain L28 has been isolated from ground beef and its genome was analyzed to pinpoint its specific antagonistic mechanisms or unique genetic markers (Ayala et al. 2017). Tests revealed potential virulence factors and potential genes for tetracycline resistance. In addition, inducing factors for bacteriocin synthesis were found, which explain how L28 can kill or inhibit closely related bacteria (Ayala et al. 2017).
8. Current Research
Include information about how this microbe (or related microbes) are currently being studied and for what purpose
9. References
It is required that you add at least five primary research articles (in same format as the sample reference below) that corresponds to the info that you added to this page. [Sample reference] Faller, A., and Schleifer, K. "Modified Oxidase and Benzidine Tests for Separation of Staphylococci from Micrococci". Journal of Clinical Microbiology. 1981. Volume 13. p. 1031-1035.
