User:BottsE

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A Microbial Biorealm page on the genus BottsE

Classification

Higher order taxa

Bacteria; Actinobacteria; Actinobacteria; Bifidobacteriales; Bifidobacteriaceae; Bifidobacterium [Others may be used. Use NCBI link to find]

Species

NCBI: Taxonomy

Bifidobacterium longum

Description and significance

Tissier first discovered the genus Bifidobacterium in the feces of breast-fed infants in 1900, even though the name Bifidobacteria did not arise until the early 1960s (1). Bifidobacteria are Gram-positive, non-spore forming, rod-shaped bacteria. Specifically, Bifidobacterium longum are often found clumped together in irregular B or Y shaped branching patterns, although the morphology can vary depending on growth phase (figure 1) (5). Bifidobacteria average about 2.5 mm in diameter, they are non-motile, and anaerobic (except they may tolerate oxygen in the presence of CO2). In terms of metabolic tests, bifidobacteria are catalase negative, benzidine reaction negative, indole negative, and they do not reduce nitrates (3). They are also starch negative and lactose positive (1). They are typically found associated with hosts (primarily mammals), but they have also been found in insect and bird GITs, among other places (5). For humans, they play an extremely important role in forming a stable microbial colony in the GI tract, among other beneficial functions (8).

Currently, there are 25 known species of Bifidobacteria, including B. longum (1).

Genome structure

The circular genome of B. longum contains 2, 256, 656 base pairs, with 60% of them being G-C pairs. 86% of these genes are thought to be coding (1,730 coding regions), and 22% are specific to B. longum (unless they have yet to be identified in other species). The genome codes for 4 nearly identical rrn operons, 57 tRNAs that code for all 20 amino acids, and 16 intact insertion sequence elements (7). In terms of plasmids, they have been found in 70% of the sequenced B. longum strains, and 7 different types have been identified. Furthermore, some strains harbor defective lysogenic phages, which are dormant plasmids that still contain DNA (1).

The sequence of B. longum subspecies infantis shows a large inverted region (about 1.3 Mb) in the middle of the genome compared to both B. longum strains. This inversion could possibly be a recent evolutionary event (6).

Cell structure and metabolism

In terms of cell wall structure, there is a lot of variety in the genus in terms of peptidoglycan content. Bifidobacteria cell walls do contain a significant amount of polysaccharides (glucose and galactose accompanied by rhamnose) and lipoteichoic acids. The lipoteichoic acid is partly responsible for the hydrophobic nature of the bifidobacterial surface (1).

For carbon sources in the human GI tract, simple sugars are utilized in the upper GI tract and complex carbohydrates are consumed in the distal part. B. longum takes advantage of the complex carbohydrates, whether they are dietary compounds, like cellulose, or host derived compounds, like mucins. To metabolize sugar, Bifidobacteria use a metabolic pathway called the Fructose-6- Phosphate Phosphoketolase pathway, otherwise referred to as the “Bifido shunt” (5). Through this pathway, glucose is fermented primarily into acetic and L (+) lactic acid; no gas is produced. Due to this process, bifidobacteria are able to process a variety of indigestible components (3).

Bifidobacteria use ammonium salts as their sole source of nitrogen, and many strains can synthesize water-soluble vitamins such as thiamine, folic acid, nicotinic acid, pyridoxine, and vitamin B12 (1).

Ecology

Bifidobacteria prefer to live in an environment that is 36-38°C (3). They most commonly live in the alimentary tracts of human adults and infants, where they coexist with many other anaerobes. Bifidobacteria also inhabit other diverse ecological niches, including the rumen of cattle, sewage, the human vagina, dental caries, and honeybee intestines. Bifidobacterium bifidum and B. longum are the most ubiquitous species within the genus (1).

Overall bacterial colonization of the intestine begins at delivery, continues into breastfeeding and weaning, and is complete at the age of 18 months. Aerobic and facultatively anaerobic bacteria, such as enterobacteria, enterococci, and staphylococci, originally colonize the infant GI tract. Once they consume the oxygen present in the intestine, anaerobic bacteria can come, including clostridia, bacteroides, and bifidobacteria. During breastfeeding, various factors in milk help enhance microbial development, including immunoglobulins, lactoferrin, lysozyme, bioactive lipids, leukocytes, and various milk glycans. The specific oligosaccharide glycans are especially important for bifidobacteria growth and are further discussed in the current research section (4).

GI bifidobacteria play the biggest role in infant gut microbiota development through expanding diversity and keeping harmful bacteria in check. As people grow older, the number of bifidobacteria decreases significantly, while the number of other bacteria (including harmful Escherichia coli and clostridia) increases. Bifidobacteria may regulate detrimental bacteria by producing antimicrobial compounds such as organic acids, iron-scavenging compounds, and bacteriocins. Secondly, bifidobacteria can lower the pH in the intestine through production of acetic and lactic acids, which blocks the growth of some harmful bacteria. They also may provide protection against some immune-based disorders by stimulating their host’s innate immune response (6). Specifically, it is hypothesized that bifidobacteria reduce the incidence of GIT diseases such as necrotizing enterocolitis, rotavirus infection, and antibiotic-linked GIT distress (8). Finally, as mentioned above, bifidobacteria might play a role in the production of water-soluble vitamins and the absorption of B complex vitamins (2).

Pathology

Some species of bifidobacteria show antibiotic resistance to polymyxin B, nalidixic acid, kanamycin, gentamicin, and metronidazole, but in general bifidobacteria are not pathogenic (1).

Bifidobacteria are actually used for therapeutic purposes. They have been used to treat digestive disturbances in infants that are not breastfed, enterocolitis, constipation, liver cirrhosis, disturbed balance of the intestinal flora following antibiotic therapy, and promotion of intestinal peristalsis. More on medical intervention can be found in the current research section (1).

Current Research and or Application to Biotechnology

Breast milk is a topic of much research in regards to its connection with bifidobacterial growth. As a whole, breast milk plays a strong role in negatively selecting against bacterial groups not beneficial to the human gut and encouraging beneficial microbes that participate in host metabolism, immune development, or other critical physiological processes. In terms of bifidobacterial growth specifically, human milk oligosaccharides (HMOs) are key. Infants that are breastfed have significantly larger populations of bifidobacteria in their feces (representative of their guts) than non-breastfed infants and adults. Consumption of HMOs is highly conserved throughout the entire B. longum subspecies infantis lineage, but other subspecies contain HMO-related gene clusters throughout their chromosome and can consume to a lesser extent (8).

HMOs are a heterogeneous mix of soluble glycans (mainly glycolipids and glycoproteins) that vary by individual. The amount in the breast milk declines over the lactation cycle, but in the colostrum (early milk) it can be present as high as 4 g/L; HMOs are the third most abundant component of human milk. To the bifidobacteria, HMOs are calorie dense molecules in an environment with a high microbial concentration and limited carbon sources (8). Bifidobacteria import the intact HMO and metabolize it through the fructose-6-phosphate phosphoketolase pathway discussed above. Some even speculate that the HMOs not only feed the bifidobacteria, but that the brest milk originally inoculates the infant in the first place (8).

In addition to HMOs and their relation to bifidobacteria, it is important to note that HMOs play many additional roles in the body. They may possess anti-adhesive effects that reduce the binding of pathogenic bacteria to colon cells, they regulate inflammatory processes in gut-associated lymphoid tissue, they may decrease intestinal permeability in preterm infants in the first postnatal month, they may assist with neonate brain development and myelination, and finally they may protect against diarrhea (4).

In studying HMOs and bifidobacteria, it can be a difficult task to separate what exactly is providing the benefit - the HMOs or the bifidobacteria they help spread throughout the colon. Regardless, a second line of research has focused on developing infant formula that comes as close as possible to mimicking the benefits of breast milk. It is important that infant formulas not only provide nutritional needs, but also facilitate the development of intestinal microbes (4).

There are two main options for promoting microbial growth. The first option is to add bifidobacteria directly (along with lactobacilli) as probiotic additives to formula. The second option is to add bifidobacteria indirectly by adding prebiotic oligosaccharides to the formula that arrive undigested in the colon. To date, this second option cannot be reproduced on a scale large enough for inclusion in infant formulas using HMOs, but animal milk seems to be a great alternative for the oligosaccharides (4).

Bovine milk appears to be the most promising at the moment. The bioactivity of oligosaccharides from bovine milk (BMOs) and human milk are similar, and are good candidates because of the large size of the existing dairy industry. The known downsides of this approach are that BMOs have low sialic acid content and no fucosylation compared to HMOs. In is known that both fucosylation and sialyation play a role in the prevention of pathogens and the promotion of beneficial bacteria. Regardless, formulas containing BMOs are on the market. The next line of research plans to focus on goat milk, which actually contains higher levels of oligosaccharides than bovine milk (4).

Bifidobacteria continues to play a role outside of normal post-natal development; in fact much research has focused on its use in a medical setting. In terms of premature infants, oral supplementation of bifidobacteria probiotics may decrease the time needed for its population to develop, but it has yet to reduce the time needed for a feeding tube. In the elderly, supplementation may contribute to colon regularity by treating constipation through a lubricant effect that reduces fecal passage time. Related to that, bifidobacteria has been suggested to prevent diarrhea, but so far the evidence is not conclusive. For people who are lactose intolerant, bifidobacteria can ferment lactose and may be able to alleviate the symptoms experienced. In terms of cholesterol reduction, it may reduce serum levels because bifidobacteria assimilates cholesterol into its cell membrane. It may be involved in cancer prevention, especially cancers of the gut (6). Finally, bifidobacteria supplementation can reduce the symptoms of allergic reaction to Japanese cedar pollen (2).

Commercially, Bifidobacterium longum can be found in Cascade Fresh yogurts, among other fermented dairy foods. There are also multi-probiotic capsules, such as Jarro-Dophilus EPS and VSL #3, that contain the bacteria (2).

References

1) Biavati, Bruno, Barbara Sgorbati, and Vittorio Scardovi. "The Genus Bifidobacterium."The Prokaryotes - Second Edition 1 (1992): 811-29. Print.

2) "Bifidobacterium Longum." Probiotics. Web. 06 May 2012. <http://www.probiotics-lovethatbug.com/bifidobacterium-longum.html>.

3) Buchanan, R.E., and N.E. Gibbons. "Bergey's Manual of Determinative Bacteriology. Eighth Edition." (1974) Baltimore, MD: Williams & Wilkins: 669-671

4) Chichlowski, Maciej, J. German, Carlito B. Lebrilla, and David A. Mills. "The Influence of Milk Oligosaccharides on Microbiota of Infants: Opportunities for Formulas."Annual Review of Food Science and Technology 2 (2011): 331-51. Print.

5) Klijn, Adrianne, Annick Mercenier, and Fabrizio Arigoni. "Lessons from the Genomes of Bifidobacteria." FEMS Microbiology Reviews 29.3 (2005): 491-509. Print.

6) Lee, Ju-Hoon, and Daniel J. O'Sullivan. "Genomic Insights into Bifidobacteria."Microbiology and Molecular Biology Reviews 74.3 (2010): 378-416. Print.

7) Schell, Mark A., Maria Karmirantzou, Berend Snel, David Vilanova, Bernard Berger, Gabriella Pessi, Marie-Camille Zwahlen, Frank Desiere, Peer Bork, Michele Delley, R. Pridmore, and Fabrizio Arigoni. "The Genome Sequence of Bifidobacterium Longum Reflects Its Adaptation to the Human Gastrointestinal Tract." Proceedings of the National Academy of Sciences 102.26 (2005): 9430. Print.

8) Sela, David A. "Bifidobacterial Utilization of Human Milk Oligosaccharides."International Journal of Food Microbiology 149 (2011): 58-64. Print.

9) "Taxonomy Browser." NCBI. U.S. National Library of Medicine. Web. 23 Apr. 2012. <http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi>.


Edited by student of Dr. Lynn M Bedard, DePauw University http://www.depauw.edu