Lactobacillus acidophilus (Neo-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 commonly found in humans, specifically the gastrointestinal tract and oral cavity as well as some speciality fermented foods such as fermented milk or yogurt, though it is not the most common species for this. 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 of 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 strain, Lactobacillus acidophilus NCFM. With the large amount of information discovered about L. acidophilus NCFM, the US Food and Drug Administration has adjudged the microbe to be an approved ingredient in beverages, dairy products, and other probiotic foods.

Biological and biochemical features

Morphology

thumb|Lactobacillus acidophilus image taken with a [[scanning electron microscope (SEM). ]]

thumb|A Lactobacillus acidophilus culture

Lactobacillus acidophilus is an immobile rod-shaped (bacillus), gram-positive organism that ranges in size from 2-10 μm in size. 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. 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 the problem. 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. 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. 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.

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. it is more common in the gut.

In experiments, L. acidophilus seemed to decrease Candida albicans<nowiki />' ability to adhere to vaginal epithelial cells; however, L. acidophilus<nowiki />' use 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

thumb|A capsule containing L. acidophilus used for vaginal health

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. Another study conducted at Maranatha 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 of Salmonella typhi, the microbe associated 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.

In 2023, Lactobacillus acidophilus was the 297th most commonly prescribed medication in the United States, with more than 400,000 prescriptions.

Dairy industry usage

thumb|An example of fermented milk, a dairy product L. acidophilus is commonly added to for probiotic effects

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." 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

  • Lactobacillus at MedlinePlus
  • Lactobacillus acidophilus at University of Maryland Medical Center
  • Lactobacillus acidophilus 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 acidophilus
  • Type strain of Lactobacillus acidophilus at BacDive – the Bacterial Diversity Metadatabase
  • Lactobacillus acidophilus - Benefits & Probiotic Uses at Pocket Reviewer