acidophilus

''Lactobacillus acidophilus'' ( New Latin 'acid-loving milk-bacillus') is a rod-shaped, Gram-positive, homofermentative, anaerobic microbe first isolated from infant feces in the year 1900. The species is most commonly found in humans, specifically the gastrointestinal tract, oral cavity, and vagina, as well as various fermented foods such as fermented milk or yogurt. The species most readily grows at low pH levels (below 5.0), and has an optimum growth temperature of 37 °C. Certain strains of ''L. acidophilus'' show strong probiotic effects, and are commercially used in dairy production. The genome of ''L. acidophilus'' has been sequenced.

''L. acidophilus'' has antagonistic effects on the growth for ''Staphylococcus aureus, Escherichia coli, Salmonella typhimurium,'' and ''Clostridium perfringens''. Out of the four organisms, ''Staphylococcus aureus'' is the most affected. Along with ''S. aureus'', the other Gram-positive bacteria, ''C. perfringens,'' was affected more by ''L. acidophilus,'' than the two other bacteria that are Gram-negative. ''L. acidophilus'' is found to also reduce oral plaque formation by ''Streptococcus mutans''.

History

''Lactobacillus acidophilus'' was first isolated from the human gastrointestinal tract in 1900 by Ernst Moro with the original name ''Bacillus acidophilus.'' Over time, there have been many changes to the methods for characterizing taxonomy of organisms, leading to the genus distinction of ''Lactobacillus'' in 1929. Complication around finding the original strain arose when multiple strains of a single isolate were given a variety of names. Most studies on ''L. acidophilus'' was focused on one particular strand, ''Lactobacillus acidophilus'' NCFM. With the large amount of information discovered about ''L. acidophilus''NCFM, the US Food and Drug Administration has been adjudged the microbe to be an approved ingredient in beverages, dairy products, and other probiotic foods.

Biological and Biochemical Features

Morphology

''Lactobacillus acidophilus'' is an immobile rod-shaped (bacillus), gram-positive organism that ranges in size from 2-10 μm in size. Being, gram positive, ''L. acidophilus'' has one phospholipid bilayer membrane with a large cell wall consisting of peptidoglycan exterior to the membrane. The cell wall of ''L. acidophilus'' is interwoven with teichoic acids and surface proteins, with anionic and neutral polysaccharides as well as an S-layer lining the exterior of the cell. The S-layer proteins of ''L. acidophilus'' have been shown to adhere to epithelial cells as well as mucus and other extracellular proteins. The S-layer is made of two structural domains. The C-terminal domain is responsible for cell wall anchoring, while the N-terminal domain is responsible for interacting with the cell environment, as well as S-layer self assembly. In the ''L. acidophilus'' species, the N-terminal region shows high amino acid variability along with low sequence homology (31-72%). However, the C-terminus shows low amino acid variability and high amino acid sequence homology (77-99%).''L. acidophilus'' does not have any extracellular means of motion like a flagellum or pilli, and therefore is an immobile microbe.

Metabolism

''L. acidophilus'' is a homofermentative anaerobic microorganism, meaning it only produces lactic acid as an end product of fermentation; and that it can only ferment hexoses (not pentoses) by way of the EMP pathway (glycolysis). ''L. acidophilus'' has a slower growth time in milk than when in a host due to limited available nutrients. Because of its use as a probiotic in milk, a study done by the American journal of dairy science examined the nutrient requirements of ''L. acidophilus'' in an effort to increase its low growth rate. The study found that glucose and the amino acids Cysteine, Glutamic acid, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tyrosine, Valine, and Arginine are essential nutrients to the growth of ''L. acidophilus'', with Glycine, Calcium-pantothenate, and Mn2+ acting as stimulatory nutrients. The study helps to explain the low growth rate of ''L. acidophilus'' in milk, as some of the amino acids necessary to ''L. acidophilus'' growth are lacking in milk. Adding amino acids with high rates of consumption to fermented milk is a possible solution to this problem.

Genomics

The specialization of prokaryotic genomes is distinguishable when recognizing how the prokaryote replicates its DNA during replication. In ''L. acidophilus,'' replication begins at an origin called oriC and moves bi-directionally in the form of replication forks. The DNA is synthesized continuously on the leading strand and in discontinuous Okazaki fragments on the lagging strand with help from the DNA polymerase III enzyme. An RNA primer is needed to initiate the DNA synthesis on the leading and lagging strands. DNA polymerase III follows the RNA primer with the synthesis of DNA in the 5' to 3' direction. ''L. acidophilus'' consists of a small genome with a low guanine-cytosine content, approximately 30%. A study comparing 46 genomes of varying strains of ''L. acidophilus'' found the genome size ranged from 1.95 Mb to 2.09 Mb, with an average size of 1.98 Mb. The average number of coding sequences in the genome was 1780, with the strains isolated from fermented foods and commercial probiotics having more coding sequences on average than those isolated from humans. ''L. acidophilus'' has an open state pan-genome (all of the genes within a species), meaning that the pan-genome size increased as the number of genomes sequenced increased. The core-genome (the genes shared by all individuals of a species) consist of around 1117 genes in the case of ''L. acidophilus.'' Genetic analysis also revealed that all ''L. acidophilus'' strains contained at least 15 families of glycosyl hydrolases, which are the key enzymes in carbohydrate metabolism. Each of the 15 GH families were involved in metabolizing common carbohydrates, such as glucose, galactose, fructose, sucrose, starch, and maltose. Genes encoding antibiotic resistance by means of antibiotic efflux, antibiotic target alteration, and antibiotic target protection were present in all ''L. acidophilus'' strains, providing protection against 18 different classes of antibiotic across all strains. Fluoroquinolone, glycopeptide, lincosamide, macrolide and tetracycline were the five classes of antibiotic to which ''L. acidophilus'' displayed the highest level of tolerance, with more than 300 genes relevant to these classes.

Environment

''L. acidophilus'' grows naturally in the oral, intestinal, and vaginal cavities of mammals. Nearly all Lactobacillus species have special mechanisms for heat resistance which involves enhancing the activity of chaperones. Chaperones are highly conserved stress proteins that allow for enhanced resistance to elevated temperatures, ribosome stability, temperature sensing, and control of ribosomal function at high temperatures. This ability to function at high temperatures is extremely important to cell yield during the fermentation process, and genetic testing on ''L. acidophilus'' in order to increase its temperature tolerance is currently being done. When being considered as a probiotic, it is important for ''L. acidophilus'' to have traits suitable for life in the gastrointestinal tract. Tolerance of low pH and high toxicity levels are often required. These traits vary and are strain specific. Mechanisms by which these tolerances are expressed include differences in cell wall structure, along with other changes is protein expression. Changes in salt concentration have been shown to affect ''L. acidophilus'' viability, but only after exposure to higher salt concentrations. In another experiment highlighted by the American Dairy Science Association, viable cell counts only showed a significant reduction after exposure to NaCl concentrations of 7.5% or higher. Cells were also observed to distinctly elongate when grown in conditions of 10% NaCl concentration or higher. ''L. acidophilus'' is also very well suited for living in a dairy medium, as fermented milk is the ideal method of delivery for introducing ''L. acidophilus'' into a gut microbiome. The viability of ''L. acidophilus'' cells encapsulated by spray drying technology stored at refrigerated condition (4 °C) is higher than the viability of cells stored at room temperature (25 °C).

Quorum Sensing

Quorum sensing among cells is the process among which cell signaling can lead to coordinated activities which can ultimately help bacteria control gene expression in a consecutive sequence. This is accomplished via detection of small autoinducers which are secreted in response to increasing cell-population density. In ''Lactobacillus acidophilus,'' which can be found in the gastrointestinal tract, quorum sensing is important for bacterial interaction when considering biofilm formation and toxin secretion.  In ''L. acidophilus'', along with many other bacteria, the ''luxS''-mediated quorum sensing is involved in the regulation of behavior. In monoculture, the production of '' luxS'' increased during the exponential growth phase and started to plateau as it progressed to the stationary phase. Up-regulation of ''luxS'' can occur when ''L. acidophilus'' is placed in co-cultivation with another ''Lactobacillus'' species.

Vaginal Microbiota

''Lactobacillus acidophilus'' is part of the vaginal microbiota along with other species in the genus including '' Lactobacillus crispatus'', ''Lactobacillus gasseri'', ''Lactobacillus jensenii'', and ''Lactobacillus iners''.
In experiments, ''L. acidophilus'' seemed to decrease '' Candida albicans''’ ability to adhere to vaginal epithelial cells; however, ''L. acidophilus''’ role in preventing yeast infections is unclear because this species of ''Lactobacilli'' has also been found not to have a very strong ability to adhere to (and thereby colonize) the vaginal cells.

Therapeutic Uses

Research has shown that the presence of ''L. acidophilus'' can produce a variety of probiotic effects in humans, such as; acting as a barrier against pathogens, assisting in lactose digestion, enhancing immune response, and reducing cholesterol level. ''L. acidophilus'' must exist in concentrations of 10^5 - 10^6 c.f.u (colony-forming units) per mL in order for these effects to be seen. A study conducted at the Wake Forest School of Medicine examined the affects of ''L. acidophilus'' on the structure and composition of the gut microbiome of mice with respect to the age of the mice. The research established the importance of the interactions between microbes within a gut microbial environment on the overall health of the organism, and the data showed that mice supplemented with ''L. acidophilus'' had reduced proteobacteria levels, and increased levels of other probiotic bacteria when compared to other mice of similar age. Another study conducted at Marantha Christian University studied the impact of ''L. acidophilus'' cell free supernatants (a liquid medium containing the metabolites produced by microbial growth) on the growth pattern ''Salmonella typhi'', the microbe assiciated with Typhoid fever. The study showed that the presence of ''L. acidophilus'' metabolites significantly inhibited the growth curves displayed by ''S. typhi'', supporting the idea that ''L. acidophilus'' presence has a positive impact on the species makeup of a gut microbial community, providing the organism with intestinal health benefits. The innate immune system of ''L. acidophilus'' also produces antimicrobial peptides. The group of short peptides found there have shown antimicrobial properties such as their strength against viruses and other cell types, including cancer cells. There is also some evidence supporting the use of a symbiotic gel (containing ''L. acidophilus'') in treating gastrointestinal symptoms in patients who had received a hemodialysis treatment. This gel also reduced the occurrence of vomit, heartburn, and stomachaches. Further study concerning this subject is needed to draw firm conclusions.

Dairy Industry Usage

As stated in a journal from the American Dairy Science Association, "''Lactobacillus acidophilus'' is a commercial strain and probiotic that is widely used in the dairy industry to obtain high-quality fermentation products." Increased levels of beneficial bacteria, and decreased levels of pathogenic bacteria within the intestine due to the consumption of fermented milk containing strains of ''L. acidophilus'' has a range of probiotic effects. Reduced serum cholesterol levels, stimulated immune response, and improved lactic acid digestion are all probiotic effects associated with intestinal ''L. acidophilus''presence''. L. acidophilus'' was also effective in reducing ''Streptococcus mutans'' levels in saliva, as well as decreasing risk factors associated with the development of nonalcoholic fatty liver disease. The strain of ''L. acidophilus'' that has been most widely researched, and is most widely used as an antibiotic is referred to as NCFM.

Side Effects

Although probiotics are generally safe, when they are used by oral administration there is a small risk of passage of viable bacteria from the gastrointestinal tract to the blood stream (bacteremia), which can cause adverse health consequences. Some people, such as those with a compromised immune system, short bowel syndrome, central venous catheters, cardiac valve disease and premature infants, may be at higher risk for adverse events.

See also

* Lactic acid fermentation

References

External links

''Lactobacillus''
at MedlinePlus

''Lactobacillus acidophilus''
at University of Maryland Medical Center


from the U. of Wisconsin

Global analysis of carbohydrate utilization by ''Lactobacillus acidophilus'' using cDNA microarrays

Safety and protective effect of ''Lactobacillus acidophilus'' and ''Lactobacillus casei'' used as probiotic agent in vivo

Pictures and research on Lactobacillus acidophilusType strain of ''Lactobacillus acidophilus'' at Bac''Dive'' – the Bacterial Diversity Metadatabase

''Lactobacillus acidophilus - Benefits & Probiotic Uses''
at Pocket Reviewer

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Probiotics
Lactobacillaceae
Bacteria described in 1970
Bacteria used in dairy products
Gram-positive bacteria