Adhesion Property in Probiotic Strain Lactobacillus acidophilus
A probiotic is a product containing viable, defined microorganisms in sufficient numbers, which alter the microflora (by implantation or colonization) in a compartment of the host and by that exert beneficial effects on the host (10). Evidence for the use of probiotics date back thousands of years to the Old Testament where it was cited that Abraham owed his fertility and longevity to the continual consumption of yogurt (2). However, the concept of probiotics did not come to light until the 1900s when zoologist Elie Metchnikoff noted that Bulgarian peasants lived longer, healthier lives due to their regular consumption of fermented milk products (7).
The majority of probiotics are categorized as lactic acid-producing bacteria. As a result of carbohydrate fermentation, these bacteria produce lactic acid, thus their name. Lactic acid probiotics help aid in improving intestinal tract health, enhancing the immune system and reducing risk of certain cancers (7). While the beneficial effects of probiotics has been noted and heavily studied, the mechanisms of probiotics still remain unknown. However, studies have shown strong probiotics are characterized by the ability to adhere strongly to intestinal epithelium (2).
Lactobacilli are favorable in the production of strong probiotics due to their innate properties, which include the ability to adhere to cells, reduce pathogenic adherence, persist and multiply, produce acids, safe and nonpathogenic, and form a balanced flora (8). While all these properties make a strong probiotic, the main focus will be on the adhesion ability of Lactobacillus acidophilus. The ability to adhere to cells is the most important factor in making a good probiotic, because without this the probiotic strain would not be viable. Adhesion to intestinal epithelial cells is necessary for colonization of probiotic strains to prevent their immediate elimination by peristalsis (6).
Mechanisms of Action
The mechanisms of adherence to intestinal epithelial cells of not just Lactobacillus acidophilus but all other probiotic strains still remain largely unknown. This is not due to scientists not having done enough research but rather the conditions to perform proper experiments remains difficult. The major setbacks to understanding the genetic systems responsible for intestinal adhesion of probiotic bacteria are the arduousness in performing human trials, as well as, the complex and kinetic nature of the intestinal environment (3). What scientists have been able to do are comparison trials of untreated bacterial strains and mutant strains and record specific properties that are involved in adhesion.
S-layer
To define the exact function of an s-layer is a conundrum. Scientists know they play a vital role in bacteria since they are pervasive throughout them. But defining this role is difficult because it can differ among bacterial species. It is known that S-layers are involved in cell adhesion and surface recognition, cell protection, they can also function as scaffolding for enzymes and virulence factors (12).
The basic structure of an S-layer is a lattice alignment (oblique, square, or hexagonal symmetry) of proteins or glycoproteins subunits. The morphological units are composed of one, two, three, four, or six identical subunits (13). The center-to-center spacing of these morphological units ranges from 2.5-35 nm (12). The S-layer is porous with pore size ranging 2-8 nm (12). Sequence homologies of S-layer genes do not occur often. In closely related species, for example Lactobacillus acidophilus and Lactobacillus helveticus there was an 80% homology in their sequence, however there was minimal homology with Lactobacillus brevis (12).
While many studies have been performed to show the adhesive abilities of bacteria, the exact science behind their genetic systems remains unknown. The S-layer proteins of lactobacilli have been shown to function as adhesins, which are cell-surface components of bacteria that are able to adhere to other cells (4). By comparing untreated bacterial cells and ones treated with LiCl, in order to remove the S-layer, proved the adhesive nature of S-layer proteins in lactobacilli (4).
Additionally, once LiCl removed the S-layer, tests showed that coaggregation was significantly reduced. Coaggregation plays a role in competitive exclusion and reduces pathogenic load during mucosal flushing (1). This shows that the S-layer also plays an important role in strengthening the immune system.
Proteins within Lactobacillus acidophilus that bind to intestinal tissues
The following are surface proteins in Lactobacillus acidophilus, which differ from the S-layer proteins. It is believed that maximum probiotic effects can be achieved if the bacteria are able to adhere to mucus and intestinal epithelial cells (14). These adhesive proteins are not universal among the genus Lactobacillus. A study conducted by Bernet tested four different strains of Lactobacillus, only one strain showed adhesive capabilities (15). The following mucin and fibronectin-binding proteins are specialized for binding to the digestive tract, the main target for probiotics.
Mucin-binding protein
The mucosal layer that lines the digestive tract is involved in many important functions. A key function is its ability to protect or inhibit the host from pathogenic bacterial colonization (9). The digestive tract, for probiotics, is a key target for binding and colonization. It is important for probiotics to be able to bind to the mucosal layer because this layer comes into direct contact with digested food and is responsible for most digestive, absorptive, and secretory processes (17).
It is important to understand, whether a strong probiotic strain, such as Lactobacillus, can bind to mucin. Research showed that lactobacilli have the ability to bind to mucin through comparison trials of untreated lactobacilli and adhesion-inhibited lactobacilli (9). This shows Lactobacillus has the ability to deter pathogenic bacterial colonization and aid the immune system. However this study was done on porcine cells rather than human cells. The next step in this research would be to see how effective and beneficial this would be to humans.
Fibronectin-binding protein
Previous research has shown that fibronectin plays an important role in bacterial adherence to epithelial cells (5). Fibronectin is a fibrous protein present in the gastrointestinal tract. If injuries were to occur in the GI tract, pathogenic bacteria would have great potential to infect the area. However, a good probiotic strain that has fibronectin-binding proteins can competitively exclude pathogenic bacteria (16). To test if fibronectin-binding protein plays a role in lactobacilli adhesion, comparison trials can be set up. An untreated Lactobacillus strain and a mutant strain where the S-layer was removed proved fibronectin plays a role in lactobacilli. In absence of the S-layer, lactobacilli were unable to adhere to the fibronectin present on epithelial cells (5). However, when the S-layer was present, the lactobacilli were able to bind to the fibronectin (5). This further reinforces the adhesive nature of the S-layer.
Further Reading
1. Altermann, E., W. M. Russell, M. A. Azcarate-Peril, R. Barrangou, B. L. Buck, O. McAuliffe, N. Souther, A. Dobson, T. Duong, M. Callanan, S. Lick, A. Hamrick, R. Cano, and T. R. Klaenhammer. 2005. Complete genome sequence of the probiotic lactic acid bacterium Lactobacillus acidophilus NCFM. Proc. Natl. Acad. Sci. USA 102:3906-3912.
2. Demuth, D. R., Y. Duan, W. Brooks, A. R. Holmes, R. McNab, and H. F. Jenkinson. 1996. Tandem genes encode cell-surface polypeptides SspA and SspB which mediate adhesion of the oral bacterium Streptococcus gordonii to human and bacterial receptors. Mol. Microbiol. 20:403-413.
3. Law, J., G. Buist, A. Haandrikman, J. Kok, G. Venema, and K. Leenhouts.1995. A system to generate chromosomal mutations in Lactococcus lactiswhich allows fast analysis of targeted genes. J. Bacteriol. 177:7011-7018.
References
1. Beganovic, J., Frece, J., Blazenka, K., Pavunc, A., Habjanic, K., Suskovic, J. (2011), Functinality of the S-layer protein from the probiotic strain Lactobacilus helveticus M92. Journal of Microbiology, 100.1: 43-53. [1]
2. Boyle, R., Robins-Browne, R., Tang, M. (2006), Probiotic use in clinical practice: what are the risks?. The American Journal of Clinical Nutrition. 83.6: 1256-1264. [2]
3. Buck, BL., Altermann, E., Svingerud, T., Klaenhammer, TR. (2005), Functional analysis of putative adhesion factors in Lactobacillus acidophilus NCFM. Applied and Environmental Microbiology, 71: 8344-8351. [3]
4. Frece, J., Kos, B., Svetec, IK., Zgaga, Z., Mrsa, V., Suskovic, J. (2005), Importance of S-layer proteins in probiotic activity of Lactobacillus acidophilus M92. Journal of Applied Microbiology, 98: 285-292. [4]
5. Hynonen, U., Westerlund-Wikstrom, B., Palva, A., Korhonen, T. (2002), Identification by Flagellum Display of an Epithelial Cell- and Fibronectin-Binding Function in the SlpA Surface Protein of Lactobacillus brevis. Journal of Bacteriology, 184.12: 3360-3367. [5]
6. Kos, B., Šušković, J., Vuković, S., Šimpraga, M., Frece, J. and Matošić, S. (2003), Adhesion and aggregation ability of probiotic strain Lactobacillus acidophilus M92. Journal of Applied Microbiology, 94: 981–987. [6]
7. Parvez, S., Malik, K.A., Ah Kang, S. and Kim, H.-Y. (2006), Probiotics and their fermented food products are beneficial for health. Journal of Applied Microbiology, 100: 1171– 1185. [7]
8. Reid, G. (1999), The Scientific Basis for Probiotic Strains of Lactobacillus. Applied and
Environmental Microbiology, 65.9: 3763-3766. [8]
9. Rojas, M., Ascencio, F., Conway, P. (2002), Purification and Characterization of a Surface Protein from lactobacillus fermentum 104R That Binds to Porcine Small Intestinal Mucus and Gastric Mucin. Applied and Environmental Microbiology, 68.5: 2330-2336. [9]
10. Schrezenmeir, J., de Vrese, M. (2001), Probiotics, prebiotics, and synbiotics— approaching a definition. The American Journal of Clinical Nutrition, 73.2: 361-364. [10]
11. Stålhammar-Carlemalm, M., Areschoug, T., Larsson, C. and Lindahl, G. (1999), The R28 protein ofStreptococcus pyogenes is related to several group B streptococcal surface proteins, confers protective immunity and promotes binding to human epithelial cells. Molecular Microbiology, 33: 208–219. [11]
Edited by Ashley Barnhill, a student of Nora Sullivan in BIOL168L (Microbiology) in The Keck Science Department of the Claremont Colleges Spring 2015.
