Lactobacillus acidophilus

Lactobacillus acidophilus (New Latin 'acid-loving milk-bacillus') is a species of Gram-positive bacteria in the genus Lactobacillus. L. acidophilus is a homofermentative, microaerophilic species that ferments sugars into lactic acid. Lactic acid helps prevent the drop of pH which aids in L. acidophilus' growth.[1] This species grows readily at low pH (below pH 5.0) and has an optimum growth temperature of around 37 °C (99 °F).[2] L. acidophilus is found in the human and animal gastrointestinal tract and mouth.[3] Some strains of L. acidophilus may be considered to have probiotic characteristics.[4] These strains are commercially used in many dairy products, sometimes together with Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus in the production of acidophilus-type yogurt, or acidophiline. Its genome has been sequenced.[5]

Lactobacillus acidophilus
Lactobacillus acidophilus, Numbered ticks are 11 μm
Scientific classification
Domain: Bacteria
Phylum: Bacillota
Class: Bacilli
Order: Lactobacillales
Family: Lactobacillaceae
Genus: Lactobacillus
Species:
L. acidophilus
Binomial name
Lactobacillus acidophilus
(Moro 1900) Hansen & Mocquot 1970
Lactobacillus acidophilus under microscope with dark light background.
Lactobacillus acidophilus image was taken with a scanning electron microscope (SEM).

L. acidophilus was found to lower serum cholesterol and raise cholesterol in fecal matter when fed to pigs.[6] These pigs were fed the same amount of food, with the same nutritional content, but one group received saline solution with L. acidophilus while the other group received just the saline. The group given the saline with the bacteria had lowered serum cholesterol compared to the control group. Pigs were chosen because their digestive system is similar to that of humans.

L. acidophilus is also correlated with antagonistic actions upon growth for Staphylococcus aureus, Escherichia coli, Salmonella typhimurium, and Clostridium perfringens.[7] The S. aureus, out of the four organisms, was the most affected by L. acidophilus. However, 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.[8] Although some research has been done, more testing is needed to determine how strong the effect is on S. mutans, what the effect is, and how exactly the effect is executed on L. acidophilus.

The viability of L. acidophilus cells encapsulated by spray drying technology stored at refrigerated condition (4 °C) was found to be higher than the viability of cells stored at room temperature (25 °C).[9]

Taxonomy

L. acidophilus is a rod-shaped (bacillus), Gram-positive organism that ranges in size from 2-10 μm in size. The cell wall of L. acidophilus consists of peptidoglycan, interwoven with teichoic acids and surface proteins, with anionic and neutral polysaccharides as well as an S-layer lining the exterior of the cell.[10]

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).[10]

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.[11] 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.[12] 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.[12] L. acidophilus consists of a small genome with a low amount of guanine-cytosine content, approximately 30%.[11] Another common characteristic of L. acidophilus' genome is the specific growth requirements needed.[11]

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.[5][13][14][15] 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.[16]

Therapeutic applications

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. L. acidophilus can be found dairy products. Because of this, dairy products containing L. acidophilus can act as a valuable dietary supplement.[17]

Many claims are made for Lactobacillus acidophilus, as for many probiotic supplements, of health benefits when consumed, generally by improving or restoring the gut flora.[18][19][20] Probiotics are considered generally safe to consume, but may cause bacteria-host interactions and unwanted side effects in rare cases.[21][22]

Some strains of L. acidophilus have been studied extensively for health effects. The Mayo Clinic publishes a list of disorders for which L. acidophilus has been tested, grading the evidence for each use from strong evidence of effectiveness, through unclear, down to strong evidence of ineffectiveness. According to the list there is good (rather than strong) evidence supporting the use of L. acidophilus or yogurt enriched with it for the treatment of some vaginal infections; effectiveness for other conditions ranges from unclear to fair negative evidence.[23]

A blend of bacterial strains including L. acidophilus NCFM decreased the incidence of pediatric diarrhea. L. acidophilus led to a significant decrease in levels of toxic amines in the blood of dialysis patients with small bowel bacterial overgrowth. At adequate daily feeding levels, L. acidophilus may facilitate lactose digestion in lactose-intolerant subjects.[24]

The Mayo Clinic lists use of L. acidophilus for heart disease among those "based on tradition or scientific theories" that "often have not been thoroughly tested in humans, and safety and effectiveness have not always been proven."[23]

Treating Lactose Intolerance

There are many fermented dairy products that use L. acidophilus including yogurt and some types of cheese. Sweet acidophilus milk is consumed by individuals who suffer from lactose intolerance or maldigestion, which occurs when enzymes (specifically lactase) cannot break down lactose (milk sugar) in the intestine. Failure to digest lactose results in discomfort, cramps and diarrhea.[25] Some bacteria have been shown to improve lactose digestion by providing β-galactosidase, while some L. acidophilus strains have been linked to improvement in symptoms and indicators of lactose indigestion.[26]

Treating Gastrointestinal Symptoms in Hemodialysis Patients

There is 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.[27]

See also
  • Lactic acid fermentation

References
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