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
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. 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 the 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
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