Lactobacillus
Description
Lactobacillus is a genus of Gram-positive, facultative anaerobic or microaerophilic, rod-shaped, non-spore-forming bacteria. They are a major part of the lactic acid bacteria group (i.e. they convert sugars to lactic acid). In humans, they constitute a significant component of the microbiota at a number of body sites, such as the digestive system, urinary system, and genital system. In women of European ancestry, Lactobacillus species are normally a major part of the vaginal microbiota. Lactobacillus forms biofilms in the vaginal and gut microbiota, allowing them to persist during harsh environmental conditions and maintain ample populations. Lactobacillus exhibits a mutualistic relationship with the human body as it protects the host against potential invasions by pathogens, and in turn, the host provides a source of nutrients. Lactobacillus is the most common probiotic found in food such as yogurt, and it is diverse in its application to maintain human well-being as it can help treat diarrhea, vaginal infections and skin disorders such as eczema.
Function
Many lactobacilli operate using homofermentative metabolism (they produce only lactic acid from sugars), and some species use heterofermentative metabolism (they can produce either alcohol or lactic acid from sugars). They are aerotolerant despite the complete absence of a respiratory chain. This aerotolerance is manganese-dependent and has been explored (and explained) in Lactobacillus plantarum. Many species of this genus do not require iron for growth and have an extremely high hydrogen peroxide tolerance.
Pathology
Interactions with other pathogens
Lactobacillus species produce hydrogen peroxide which inhibits the growth and virulence of the fungal pathogen Candida albicans in vitro and in vivo. In vitro studies have also shown that Lactobacillus sp. reduce the pathogenicity of C. albicans through the production of organic acids and certain metabolites. Both the presence of metabolites, such as sodium butyrate, and the decrease in environmental pH caused by the organic acids reduce the growth of hypha in C. albicans, which reduces its pathogenicity. Lactobacillus sp. also reduce the pathogenicity of C. albicans by reducing C. albicans biofilm formation. Biofilm formation is reduced by both the competition from Lactobacillus sp., and the formation of defective biofilms which is linked to the reduced hypha growth mentioned earlier. On the other hand, following antibiotic therapy, certain Candida species can suppress the regrowth of Lactobacillus sp. at body sites where they cohabitate, such as in the gastrointestinal tract. \nIn addition to its effects on C. albicans, Lactobacillus sp. also interact with other pathogens. For example, Lactobacillus reuteri can inhibit the growth of many different bacterial species by using glycerol to produce the antimicrobial substance called reuterin. Another example is Lactobacillus salivarius, which interacts with many pathogens through the production of salivaricin B, a bacteriocin.
Probiotics
Lactobacillus species administered in combination with other probiotics benefits cases of irritable bowel syndrome (IBS), although the extent of efficacy is still uncertain. The probiotics help treat IBS by returning homeostasis when the gut microbiota experiences unusually high levels of opportunistic bacteria. In addition, Lactobacillus species can be administered as probiotics during cases of infection by the ulcer-causing bacterium Helicobacter pylori. Helicobacter pylori is linked to cancer, and antibiotic resistance impedes the success of current antibiotic-based eradication treatments. When Lactobacillus probiotics are administered along with the treatment as an adjuvant, its efficacy is substantially increased and side effects may be lessened. Also, Lactobacillus is used to help control urogenital and vaginal infections, such as bacterial vaginosis (BV). Lactobacillus produce bacteriocins to suppress pathogenic growth of certain bacteria, as well as lactic acid and H2O2 (hydrogen peroxide). Lactic acid lowers the vaginal pH to around 4.5 or less, hampering the survival of other bacteria, and H2O2 reestablishes the normal bacterial flora and normal vaginal pH. In children, Lactobacillus strains such as L. rhamnosus are associated with a reduction of atopic eczema, also known as dermatitis, due to anti-inflammatory cytokines secreted by this probiotic bacteria.