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Lactobacillus rhamnosus is a bacterium that originally was considered to be a subspecies of L. casei, but later genetic research found it to be a species of its own. It is a short Gram-positive heterofermentative facultative anaerobic non-spore-forming rod that often appears in chains. L. rhamnosus can generate ATP by aerobic respiration and switch to fermentation or anaerobic respiration if oxygen is not present. L. rhamnosus' is non-motile, but has pilli as cellular appendages. On the surfaces of their pilli, mucin-binding proteins have been identified. All L. rhamnosus strains have a singular circular chromosome. Some strains of L. rhamnosus bacteria are being used as probiotics, and are particularly useful in treating female-related infections, most particularly very difficult to treat cases of bacterial vaginosis (or "BV"). The Lactobacillus rhamnosus and L. reuteri species are most commonly found in the healthy female genito-urinary tract and are most helpful to supplement in order to regain control over dysbiotic bacterial overgrowth during an active infection. L. rhamnosus sometimes is used in yogurt and dairy products such as fermented and un-pasteurized milk and semi-hard cheese. While frequently considered a beneficial organism, L. rhamnosus may not be as beneficial to certain subsets of the population; in rare circumstances, especially those primarily involving weakened immune system or infants, there may be no advantage.

Different strains of Lactobacillus rhamnosus are very similar. For instance, studies indicate that L. rhamnosus strains GG and LC705 both have a GC content of 47% and AT content of 53%. In addition, the American Society of Microbiology sequenced and annotated the genome of L. rhamnosus strain 2166, which has a GC content of 46.6% and AT content of 53.4%. The genome sequence was annotated using NCBI GenBank and RAST software. In addition, the annotation revealed that L. rhamnosus 2166 has a large amount of sugar-metabolism-related genes.

Lactobacillus rhamnosus GG (ATCC 53103) is a strain of L. rhamnosus that was isolated in 1983 from the intestinal tract of a healthy human being; filed for patent on 17 April 1985, by Sherwood Gorbach and Barry Goldin, and the 'GG' derives from the first letters of their surnames. The patent refers to a strain of "L. acidophilus GG" with American Type Culture Collection (ATCC) accession number 53103; later reclassified as a strain of L. rhamnosus. The patent claims the L. rhamnosus GG (ATCC 53103) strain is acid- and bile-stable, has a great avidity for human intestinal mucosal cells, and produces lactic acid. Since the discovery of the L. rhamnosus GG (ATCC 53103) strain, it has been studied extensively on its various health benefits and currently L. rhamnosus GG (ATCC 53103) strain is the world's most studied probiotic bacterium with more than 800 scientific studies. In one study, Rasinkangas et al's genomic characterization of non-mucus-adherent derivatives of L. rhamnosus GG strain indicated that it had pilus production machinery. This machinery is affected by its spaCBA-srtC1 gene cluster. Furthermore, genome sequencing revealed that the strand Lactobacillus rhamnosus GG (ATCC 53103) has a higher number of proteins invovled in carbohydrate utilization in comparison with instestinal lactobacilli.

In regards to quorum sensing, Lactobacillus rhamnosus GG produces a molecule similar to autoinducer-2 (AI-2), which is significant because AI-2 has been suggested to be the signaling molecule for bacteria on a universal level. Another study by Moslehi-Jenabian, et al, showed that the AI-2 activity increased when they lowered the pH level. Where the pH level is 4, the luxS gene reached a maximum level and then quickly decreased. Conclusively, this study displayed that the luxS gene greatly influences cell-to-cell communication among the microbes within the intestine. Furthermore, this response by probiotic lactobacilli has been interpreted to be integral to survival in the gastrointestinal tract.

The genome sequence of Lactobacillus rhamnosus GG (ATCC 53103) has been decoded.