The Various Healthcare Applications of the Genus Lactobacillus

Abstract

The genus Lactobacillus is a heterogeneous group of lactic acid bacteria (LAB) with important implications in biotechnology. It is a predominant microorganism in the world of gastrointestinal health, but various other uses are being explored. They have long been considered in the generally recognized as safe (GRAS) category by the Food and Drug Authority (FDA). They have been extensively used in fermentation and there is growing interest regarding their use in gut health, vaccine production, and biomedical innovation. This chapter highlights the application of lactobacilli in healthcare.

1. Introduction

Lactic acid-producing bacteria (LAB) are gram-positive, catalase-negative organisms that constitute a diverse group extensively utilized in both medicinal and industrial domains, with the genus Lactobacillus emerging as a prominent member of this microbial consortium. Despite the plethora of organisms within LAB, lactobacilli stand out due to their unique attributes, including acid tolerance and regulatory capabilities in food production. The versatility of lactobacilli has fueled their integral role in the development and success of various applications such as probiotics, food fermentation, agriculture, vaccine production, and beyond.

This genus of organisms has garnered substantial recognition, owing to their role as prolific producers of lactic acid. This characteristic not only contributes to their predominance in the microbiota but also aligns with their extensive historical use in fermentation processes. They have existed in civilization throughout centuries in various forms, but their use has become more refined in the modern world. As mentioned earlier, they first started as integral partners in the world of fermentation but have grown to be vital for horticulture and healthcare. The Food and Drug Authority (FDA) has duly acknowledged the Lactobacillus organism’s long-standing presence in the Generally Recognized as Safe (GRAS) category, underscoring their safety and reliability in diverse applications. This chapter embarks on an exploration of Lactobacillus, shedding light on their current applications and delving into emerging areas of development in the healthcare industry [1, 2].

Due to the diverse number of organisms within the genus Lactobacillus, comprising 261 species at phenotypic, ecological, and genotypic levels, a study published in 2020 by Zheng et al. suggested a revised taxonomy of the genus Lactobacillus reflecting the phylogenetic position of the microorganisms, and grouping lactobacilli with shared ecological and metabolic properties [3]. As we navigate through the myriad applications of lactobacilli, it becomes evident that their significance extends beyond traditional areas of use. The capacity of this genus of organisms to influence probiotics, food fermentation, agriculture, vaccine development, and biomedical research underscores their multifaceted contributions. From their pivotal role in gastrointestinal health to their expanding influence in novel applications, lactobacilli stand as a testament to the dynamic interplay between science and biotechnology.

2. Role as probiotic and applications in health-related conditions

The human gut is a complex ecosystem teeming with trillions of microorganisms, collectively known as the gut microbiota. Many probiotic bacteria are used for human consumption, the most used species being Lactobacillus. Lactobacilli, a group of lactic acid bacteria, play a significant role as probiotics contributing to numerous health benefits and have demonstrated noteworthy applications in various health-related conditions (Table 1). Probiotics are live microorganisms, which, when administered in adequate amounts, confer health benefits on the host through modulation of the innate and adaptive immunity resulting in the alteration of the human gut microbiome [1]. Lactobacilli are instrumental in maintaining a balanced gut microbiota due to their ability to outcompete harmful bacteria for resources and adhesion sites preventing the overgrowth of pathogenic microorganisms [23]. Lactobacilli function as probiotics by exerting beneficial effects on the host through various mechanisms, including competitive exclusion of pathogens, production of antimicrobial substances such as bacteriocins, modulation of both innate and adaptive immunity, and maintenance of gut barrier function, leading to alterations in the composition and function of the gut microbiome [24]. The production of lactic acid by lactobacilli creates an inhospitable habitat for many pathogens by contributing to the acidification of the gut environment [17, 25]. This allows lactobacilli to modulate immune responses, promoting a balanced and appropriate reaction to various challenges [1]. This modulation enhances the host’s ability to protect against infections and manage inflammatory processes [1, 26]. The competitive advantage of Lactobacillus in outcompeting harmful bacteria for resources and adhesion sites is crucial for preventing the overgrowth of pathogenic microorganisms [17]. This competitive exclusion mechanism has been extensively studied and validated in both in vitro and in vivo models [17]. Moreover, certain strains of lactobacilli produce antimicrobial substances such as bacteriocins, which exhibit selective antimicrobial activity against pathogenic bacteria, further contributing to their role in maintaining gut health [9, 24, 27]. For example, Lactobacillus salivarius and Lactobacillus brevis have been shown to produce bacteriocin that inhibits the growth of various pathogenic bacteria, thus potentially reducing the risk of infections [9, 27]. LAB also have the ability to create additional compounds like hydrogen peroxide and biosurfactants to decrease the virulence of pathogens. Hydrogen peroxide works by preventing invasion and terminating invading bacteria by early cell death. Lactobacillus paracasei can create hydrogen peroxide and this property allows it to kill S. aureus in laboratory models. Similar LAB in the vaginal microbiome has been shown to decrease the incidence of bacterial vaginosis. Biosurfactants are secreted by LAB and can change the permeability of invading pathogens. This is particularly useful in the field of food safety as biofilms prevent food spoilage and foodborne pathogens. All these various mechanisms work together to promote immunity [20].

Organism/strain	Disorder/disease	Characteristics	References
Lactiplantibacillus – L. plantarum	Irritable bowel syndrome (IBS)	Modulation of gut motility, reduction in abdominal pain. Clinical trials by Guglielmetti et al. demonstrated significant alleviation of IBS symptoms.	[4, 5, 6, 7]
Lactobacillus rhamnosus GG	Inflammatory bowel disease (IBD)	Anti-inflammatory properties, maintenance of gut barrier. Prophylactic efficacy against pouchitis in a double-blind trial by Gionchetti et al.	[4, 5, 7, 8]
Lacticaseibacillus – L. casei group, Limosilactobacillus – L. reuteri group	Infectious diarrhea	Competitive exclusion of pathogens, antimicrobial activity. Effective in reducing antibiotic-associated diarrhea in children.	[4, 9, 10, 11]
Lactobacillus acidophilus, Limosilactobacillus – L. reuteri group	Necrotizing enterocolitis	Gut barrier protection, immunomodulation. Lower incidence in very low birth weight infants treated with Lactobacillus, as shown by Lin et al.	[4, 7, 12, 13]
Lacticaseibacillus paracasei, Limosilactobacillus fermentum	Immune modulation	Regulation of immune responses, cytokine modulation. Enhances influenza vaccination effects.	[4, 14, 15, 16]
Lactobacillus acidophilus, Lacticaseibacillus – L. casei group	Microbiome balance	Competition with pathogenic bacteria, microbiome modulation. Effective in managing functional abdominal pain in children.	[4, 17, 18, 19]
Ligilactobacillus – part of L. salivarius group; Liquorilactobacillus - part L. salivarius group, Levilactobacillus brevis	Antimicrobial properties	Production of bacteriocins, inhibition of pathogenic growth. Lactobacillus GG is associated with a reduced risk of bacteremia.	[9, 20, 21, 22]

Table 1.

Summarization of application of Lactobacillus in health-related conditions. Please note that the characteristics provided are summarized and should be explored further based on the referenced studies for a comprehensive understanding.

In addition to their antimicrobial properties, Lactobacillus species modulate immune responses, promoting a balanced and appropriate reaction to various challenges. This immune modulation occurs through interactions with immune cells and the production of immunomodulatory molecules such as cytokines [28]. By enhancing the host’s immune function, lactobacilli contribute to the protection against infections and the management of inflammatory processes [29]. The clinical application of lactobacilli extends to the management of various gastrointestinal disorders, including but not limited to irritable bowel syndrome (IBS), inflammatory bowel disease (IBD), infectious diarrhea, and necrotizing enterocolitis [1, 4]. Studies have demonstrated the ability of specific Lactobacillus strains, such as Lactiplantibacillus plantarum and Lacticaseibacillus rhamnosus GG, to modulate immune responses and alleviate symptoms associated with gastrointestinal disorders like irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD) [8]. Lactiplantibacillus plantarum emerges as a beneficial strain, exhibiting the ability to modulate gut motility and alleviate abdominal pain. Clinical trials, such as the study conducted by Guglielmetti et al., have indicated a significant reduction in IBS symptoms, such as pain, bloating, and urgency, highlighting the potential of Lactobacillus plantarum as a therapeutic intervention [5, 6]. Lacticaseibacillus rhamnosus GG showcases anti-inflammatory properties and contributes to the maintenance of gut barrier function. The findings from a double-blind trial conducted by Gionchetti et al. suggest that prophylactic use of Lacticaseibacillus rhamnosus GG can be effective in preventing the onset of pouchitis, underscoring its potential in managing IBD-related complications [5, 8]. Lacticaseibacillus rhamnosus GG has also been shown to promote favorable immune responses in extraintestinal conditions. The emerging Western world has opened a plethora of new allergens that increase the risk of certain atopic pathologies. The use of this LAB in genetically predisposed pregnant and lactating women helped reduce the prevalence of atopic diseases in their offspring in a double-blinded trial by Kalliomäki et al. [30]. Additionally, children who had atopic dermatitis were given LAB probiotics and had reduced clinical symptoms from their condition [26].

Furthermore, Lactobacillus species play a crucial role in maintaining gut barrier function, which is essential for preventing the translocation of harmful pathogens and molecules from the gut lumen into systemic circulation. This barrier function is maintained through various mechanisms, including the reinforcement of tight junctions between intestinal epithelial cells and the production of mucins, which form a protective layer over the intestinal mucosa [31]. Clinical applications of lactobacilli extend beyond gastrointestinal disorders to include conditions such as infectious diarrhea and necrotizing enterocolitis, particularly in vulnerable populations like infants. Lacticaseibacillus casei and Limosilactobacillus reuteri demonstrate their efficacy through competitive exclusion of pathogens and antimicrobial activity. Studies, including those by Vanderhoof et al., have revealed a reduction in antibiotic-associated diarrhea in children supplemented with Lactobacillus, indicating a promising role in preventing and managing infectious diarrhea [10, 11]. Necrotizing enterocolitis, a critical concern in premature infants, sees potential intervention with Lactobacillus acidophilus and Limosilactobacillus reuteri. These strains contribute to gut barrier protection and immunomodulation, as demonstrated by Lin et al., resulting in a lower incidence of Necrotizing Enterocolitis in very low birth weight infants [12, 13]. Lactobacillus strains, including Lacticaseibacillus paracasei and Limosilactobacillus fermentum, are recognized for their immune modulation properties. Moreover, research suggests that Lactobacillus strains like Lacticaseibacillus casei and Limosilactobacillus reuteri may enhance the efficacy of influenza vaccination through their immune modulation properties [32]; through the regulation of immune responses and cytokine modulation, these strains have been associated with enhanced effects of influenza vaccination, as observed in studies such as that conducted by Olivares et al. [14, 15].

Cirrhosis affects over 160 million people and results in more than 1.3 million deaths a year [33]. It has been shown that the occurrence and progression of cirrhosis are associated with systemic inflammatory changes, which the gut microbiome contributes to [34]. A systemic review of 17 studies among cirrhotic patients with different degrees of hepatic encephalopathy (HE) compared probiotics versus placebo. These studies showed significant improvement in neurophysiological status and ammonia levels with probiotic use, most significantly with a combination containing Streptococcus, Bifidobacterium, and Lactobacillus (VSL#3) [35, 36]. Compared to other standard therapies, such as lactulose and rifaximin, there was a lower incidence of serious adverse events and less hospitalization, overt HE, and infections as well as a reduction in the incidence of ascites at 6-month follow-up. However, no differences were seen in terms of abdominal pain, bloating, or constipation. While improvements were noted in Model for End-Stage Liver Disease (MELD) scores, the impact on liver function tests and Child-Turcotte-Pugh classification remained inconclusive, thus making it unclear if they can play a role in improving liver function [37]. This latest systematic review and meta-analysis provide additional support for the benefits of probiotics in improving aspects of cirrhosis management; additionally, the observed safety profile indicates a promising avenue for intervention in cirrhosis treatment. However, further research is warranted to elucidate their precise role in enhancing liver function.

The capacity of Lactobacillus acidophilus and Lacticaseibacillus casei to maintain microbiome balance is crucial, particularly in addressing functional abdominal pain in children. Clinical trials revealed the effectiveness of these strains in promoting a balanced gut microbiota and managing gastrointestinal discomfort [18, 19]. Furthermore, the antimicrobial properties of L. salivarius and L. brevis are notable, attributed to their production of bacteriocins that inhibit the growth of pathogenic bacteria. Research has suggested a potential role in reducing the risk of bacteremia associated with these strains, emphasizing their significance in safeguarding against infections [20, 21]. LAB not only work against other competing bacteria but also assist in viral infections. Lactobacillus plantarum LRCC5310 found in certain Asian fermented foods has been shown to prevent the growth and replication of rotavirus. Similarly, Lactobacillus curvatus showed a decreased incidence of norovirus [2]. Collectively, these findings underscore the diverse and promising applications of Lactobacillus in promoting gut health and addressing various health-related conditions.

In humans, the gastrointestinal tract supports trillions of microbial cells creating an ecosystem that is unique to each person. As research in the field of probiotics advances, the exploration of personalized probiotic interventions based on individual microbiome composition is becoming an exciting avenue for future applications [38]. Understanding the unique interactions between Lactobacillus species and the host microbiota can inform the development of tailored probiotic therapies for improved health outcomes [24]. Currently, there are studies looking to evaluate the use of probiotics to improve certain commonly found ailments like obesity, hyperlipidemia, and cardiovascular disease [2]. Additionally, advancements in probiotic delivery systems and formulations are enhancing the efficacy and stability of probiotic supplements, further expanding their potential applications in clinical settings [38].

The genus Lactobacillus has emerged as an essential guardian of gut health, employing a diverse array of mechanisms to promote well-being, and addressing various health-related conditions. Their role as probiotics encompasses immune modulation, digestive benefits, and protection against pathogens, all while contributing to the maintenance of a balanced gut microbiota. These multifaceted contributions highlight the intricate interplay between Lactobacillus and the human gut, underscoring the importance of understanding and harnessing the potential of these probiotic microorganisms for optimizing health outcomes.

3. Lactobacillus-based vaccination strategies

Vaccination is one of the most effective public health strategies and stands as a cornerstone in the arsenal to combat infectious diseases. Researchers are exploring innovative methods to bolster its efficacy. Among these approaches is the utilization of live bacteria as vectors for the delivery of vaccine antigens directly to the immune system. LAB have emerged as promising candidates as potential bacterial carriers of compounds with prophylactic and therapeutic effects, due to their long-standing use as starter strains in food and fermentation processes, coupled with their GRAS status conferred by the FDA [39]. This designation ensures that LAB are nonpathogenic, and their application is safe for human and animal consumption; thus, making them safe and ideal vehicles for mucosal vaccination. Most infections occur primarily by targeting mucosal surfaces and hence, stimulating a local immune response at these sites can effectively block pathogen entry. Immunization via the mucosal route is simpler than standard injections and live bacterial vaccines can induce mucosal as well as systemic immune responses when delivered via mucosal routes [40]. This is attributed to their capacity to elicit antigen-specific secretory immunoglobulin A (IgA) response [41], which promotes the entrapment of antigens and microorganisms within the mucus layer. Certain strains can adhere to the intestinal epithelium, which makes them an attractive candidate as bacterial carriers, while other strains have properties that allow them to enhance the immune response caused by the carrier antigens. Their ability to be stored at room temperature via lyophilization also allows for delivery worldwide.

So far, Lactococcus Lactis remains the model LAB microorganism in vector research given that it lacks lipopolysaccharides in the cell wall, which eliminates the risk of endotoxic shock, while its acid resistance enables it to survive passage through the stomach. In addition, L. lactis exhibits low immunogenicity, further enhancing its suitability as a vaccine vector [42]. The first use of it as a vaccine vector was in 1990 when killed L. lactis was combined with PAc, a structural gene for a surface protein antigen on S. mutans that had been shown to prevent colonization of S. mutans on animal and human tooth surfaces in previous studies. The L. lactis vector was then introduced intranasally in mice to immunize them against S. mutans, which resulted in the production of IgG and IgA and reduced infection rates [42, 43]. In 1993, a live L. lactis vaccine producing tetanus fragment C (TTFC) to protect mice against the challenge of tetanus toxin was created [44]. The pneumococcal antigen pneumococcal surface protein A (PspA) is a target in protective antibody production and implicated in preventing pneumococcal infection in humans and mice. Intranasal vaccination with a strain of L. lactis that expressed pneumococcal surface protein A (PspA) led to better protection against a respiratory challenge with S. pneumoniae than PspA given intranasally or via injection [45]. These studies showcased the potential of live L. lactis vaccines against diseases. Moreover, other bacteria have been targeted using similar methods demonstrating L. lactis’ versatility as a vaccine vector. Helicobacter pylori, specialized in the colonization of the human stomach, infects around 50% of the world’s population and leads to a variety of issues from gastritis to cancer. H. pylori produces a special urease to buffer the pH of the stomach and harms the gastric mucosa through recruitment of neutrophils leading to a pro-inflammatory response. L. lactis expressing subunit B (UreB) antigen reduced the H. pylori count in the stomachs of mice vaccinated via oral route [46]. A similar reduction in H. felis load was found in the stomachs of mice inoculated with UreB-producing L. plantarum [47].

Expanding beyond bacterial infections, LAB-based vaccines have shown promise against viral and parasitic diseases. For instance, recombinant L. lactis expressing antigens from human papillomavirus (HPV) demonstrated efficacy in preventing HPV-induced tumors. More than 50% of HPV-related cervical cancers have been associated with HPV type 16 [48]. In one detailed experiment, a viral protein HPV-16 E7 antigen, which is produced during the malignant progression of HPV type 16 cervical lesions and thus considered to be a major candidate antigen for vaccines against HPV-related cervical cancer, was utilized as a target and recombinant L. lactis expressing cell wall-anchored E7 antigen and a secreted form of interleukin (IL)-12 were introduced mucosally to mice models. Increased levels of E7 and a secreted form of IL-12 that is used to treat HPV-16 induced-tumors were seen in mice models and prevented TC-1 tumor formation [49]. In HIV-1, the envelope protein mediates binding to CD4 and a coreceptor, which leads to conformational changes in the envelope protein resulting in fusion between the viral and cellular membranes [50]. L. lactis engineered to express the V2–V4 loop of the envelope protein of HIV-1 on the cell surface was constructed and introduced to mice via oral immunization. Higher levels of HIV-specific serum IgA and fecal IgA antibodies were seen. Mice were then challenged with vaccinia virus expressing HIV envelope proteins and the viral load in the immunized mice was 350-fold lower than the control mice [51]. This demonstrated that L. lactis elicited robust immune responses and conferred protection against viral challenge.

Malaria is a global parasitic disease that remains the leading cause of morbidity and mortality in the developing world. Zhang et al. showed that L. lactis expressing a merozoite surface protein 1 (MSP1) fragment, which is present in all species of Plasmodium and has been shown to inhibit the invasion of P. falciparum into erythrocytes in vitro, had reduced parasitemia and increased survival against P. yoelli in mice models [52]. Giardia lamblia is a flagellated unicellular eukaryotic microorganism that commonly causes diarrheal disease throughout the world. Water contaminated with their cysts leads to giardiasis. Cyst wall protein 2 (CWP2) is a major component of their cyst wall structure. Recombinant L. lactis expressing CWP2 led to the induction of CWP2 mucosal IgA antibodies in mice intestines through oral immunization. When challenged with G. muris, recombinant L. lactis expressing CWP2-treated mice showed 63% less cyst output than non-immunized mice [53]. These recombinant LAB-based vaccines targeting malaria and giardiasis exhibited significant reductions in parasite burden and enhanced survival rates.

The ability of LAB to be used for heterologous protein delivery is not limited to infectious disease but can also be expanded to certain non-infectious conditions such as inflammatory bowel disease (IBD). Studies employing L. lactis to deliver anti-inflammatory cytokines or protease inhibitors have shown promising results in ameliorating colitis symptoms in animal models. Tumor necrosis factor alpha (TNFα) is one of the most studied pro-inflammatory cytokines in IBD and therapies exist targeting this protein. L. lactis expressing murine anti-TNFα orally administered to dextran sulfate sodium (DSS)-induced colitis mice led to decreased stool TNFα as well as general mucosal improvement [54]. Another study involving recombinant L. lactis producing IL-10, an anti-inflammatory cytokine capable of suppressing the pro-inflammatory response of immune cells, showed a 50% reduction in colitis in DSS-induced colitis mice and prevented the onset of colitis in IL-10 knockout mice [55]. Other studies involving L. lactis and elafin, an endogenous protease inhibitor, as well as anti-inflammatory molecules such as secretory leukocyte protease inhibitor (SLPI) and the enzyme 15-lipooxygenase-1 (15-LOX-1), showed a reduction in inflammation of colitis-induced mice models [56, 57]. These early studies suggest that the usage of bacterium vectors for localized delivery reduces the required therapeutic dosage of these anti-inflammatory molecules, which in turn leads to fewer systemic side effects as well as being more cost-effective for the long-term management of IBD.

Moreover, LAB have been investigated for their ability to deliver DNA vaccines, offering a versatile platform for immunization. DNA immunization leads to both a cellular and humoral immune response, and the use of live bacteria for DNA delivery has been studied for over 20 years. Initial studies of lactobacilli in cow milk allergy, which affects 2–3% of infants and young children, were with L. lactis harboring the plasmid DNA β-lactoglobulin (BLG), a dominant allergen in cow milk. This delivery vehicle was then incubated into mammalian epithelial cells and resulted in the transfer of the plasmid to the cells as well as the expression of the plasmid [58]. When epithelial cells were co-incubated with purified plasmid alone or mixed with L. lactis, no production of BLG was observed, suggesting that the plasmid requires internalization by the bacterium to achieve transfer. Chatel et al. conducted in vivo studies with mice via oral administration of L. lactis producing BLG which led to an increase in BLG and BLG cDNA in the epithelial membrane of the small intestine [59]. Targeting allergens such as BLG has demonstrated the feasibility of using ALB to induce oral tolerance. Studies on L. lactis have been done to see if modification of the bacterium can increase DNA vaccine delivery. L. lactis that extracellularly expresses FnBPA protein (fibronectin-binding protein A of S. aureus) or InlA (internalin of L. monocytogenes), both of which aid in internalization into non-phagocytic cells, was explored in vivo and in vitro; both studies showed an increased amount of cDNA in eukaryotic cells but did not increase the number of antigens produced [60, 61] suggesting a different mechanism for in vitro and in vivo entry.

While much of the current research focuses on L. lactis, the diverse physiology of other LAB strains presents the opportunity for discovering novel vaccine delivery systems. The potential application of LAB shows promise in the field of immunoprophylaxis for a variety of pathogens. Further exploration into the interaction between different LAB strains and the host immune system is warranted to fully harness their potential in immunoprophylaxis against a broad spectrum of pathogens.

4. Biomedical uses and therapeutic potential

Recent research has highlighted the role of LAB as catalysts in the formation of functional peptides, unveiling a potential avenue for their utilization in various medical applications. It has been observed that many proteins present in food matrices exist as inactive metabolites, inhibited from exerting their biological activities due to structural constraints imposed by surrounding compounds. The action of LAB in cleaving these bonds can liberate bioactive peptides, facilitating a range of physiological effects. Various studies have identified specific peptide sequences within proteins such as albumin, β-lactoglobulin, and α-lactalbumin in whey, which exhibit anti-hypertensive properties. The operational ingredient in these substances functions primarily through the inhibition of angiotensin I-converting enzyme (ACE) activity, thereby modulating the renin-angiotensin-aldosterone system contributing to the reduction of blood pressure levels [62]. Studies have shown that when LAB like L. acidophilus, L. brevis, L. animalis, and L. lactis are added to these protein mixtures, the active metabolites are cleaved away and the release of the active metabolites can have anti-hypertensive actions [62]. This suggests their potential use as a starter culture to develop antihypertensive functional foods. The bioavailability of these compounds is still under research but early animal studies, particularly in rat models, have demonstrated promising results regarding the potential of these peptides in preventing vascular damage and mitigating end-organ dysfunction through inhibition of ACE activity as well as stimulation of nitric oxide production in cells leading to a vasodilatory effect [63].

Furthermore, the addition of L. acidophilus to milk products aids with fermenting the milk with subsequent cholesterol-lowering effects. However, the exact mechanism of cholesterol reduction is unclear. In vitro experiments have shown that LAB can secrete bile salt hydrolase, which contributes to the deconjugation of bile salts. These compounds are less soluble and thus evacuated via feces, forcing the body to use cholesterol reservoirs to make new conjugated bile salts, resulting in a reduction in the total serum cholesterol [64]. Another proposed mechanism is that milk lactose is metabolized by lactobacilli which in turn produce lactic acid and short-chain fatty acids as metabolites, which leads to a reduction in pH levels and subsequently stimulates bile salt hydrolase activity, further aiding in cholesterol reduction [65]. Additional biochemical uses are being studied for pain control and immune system modification; these studies are working to provide medical alternatives and allow the food industry to enrich our diet with health-directed nourishment [66].

Beyond their effects on cardiovascular health, lactobacilli have also been found to produce various metabolic by-products, including biosurfactants, which hold promise in combating pathogenic infections. Biosurfactants may play a role in reducing the adherence capacity of several pathogens, a crucial step for biofilm formation and proliferation, through the mechanism of competitive exclusion [67]. These biosurfactant molecules are also advantageous over their synthetic counterparts due to decreased toxicity and increased biodegradability, rendering them attractive candidates for medical applications. Gan et al. showed that L. fermentum and its secreted biosurfactant can limit surgical implant infection caused by S. aureus in rat models [68]. Candida spp. cause 10% of intravenous catheters and prosthetic valve infections, 21% of urinary catheter infections, and frequently evade treatment through the creation of biofilms. Biosurfactants produced by L. brevis led to a decreased rate of biofilm growth on silicone [69], suggesting a potential role in the prevention of fungal infection. In another study, L. gasseri, L. fermentum, and L. crispatus have also been shown to adhere to HeLa cells and produce a biofilm as well as exhibit antimicrobial activity against a variety of multidrug-resistant urogenital pathogens, suggesting their potential utility in combating infections and maintaining urogenital health [70].

The multifaceted biochemical activities of lactobacilli and their metabolic by-products offer promising avenues for medical interventions and food industry innovations aimed at enhancing health outcomes. Further research into the mechanisms underlying these effects and their translation into practical applications is warranted to fully harness the therapeutic potential of LAB-derived compounds.

5. Challenges and outlook of Lactobacillus applications

Despite their numerous applications, several challenges persist in harnessing the full potential of lactobacilli. A crucial hurdle lies in comprehending the intricate interplay between lactobacilli and the human microbiome, especially concerning their efficacy as probiotics. The applications of lactic acid bacteria, shedding light on strategies against food spoilage microorganisms and foodborne pathogens, have been emphasized [2]. One of the foremost challenges in utilizing lactobacilli is ensuring their survival and viability during processing and storage. Factors such as pH, temperature, oxygen exposure, and interactions with other microorganisms could influence the stability of the lactobacilli. To address this issue, strategies like microencapsulation, freeze-drying, and selection of robust strains to enhance their survival rate are being explored [71]. Harnessing lactobacilli for health-related applications presents challenges in ensuring standardized production methodology and quality control measures to ensure their survival through the gastrointestinal tract. Variability in strain characteristics, fermentation conditions, and manufacturing processes can impact the efficacy and safety of Lactobacillus-based products. The inherent diversity in individuals’ microbiomes necessitates personalized approaches, propelling research into tailoring probiotic interventions based on individual microbial compositions [1]. In addition to this, establishing standardized protocols and quality assurance systems is essential for ensuring consistency and reproducibility in clinical outcomes.

In spite of the growing interest in probiotics, including lactobacilli, clinical evidence supporting their efficacy in specific medical conditions remains inconclusive. Challenges in designing rigorous clinical trials, including appropriate endpoints, patient selection criteria, and standardized intervention protocols, hinder the assessment of the efficacy of LAB. While lactobacilli exhibit various functional properties, including antimicrobial activity, immune modulation, and production of bioactive compounds, optimizing these properties for practical applications remains a challenge. Due to this, a deeper understanding of the immunomodulatory effects of lactobacilli is pivotal for enhancing therapeutic potential in the therapy of and optimizing clinical outcomes in inflammatory and infectious conditions [1, 23]. Understanding the mechanisms underlying these functions and their interaction with host systems is essential for enhancing their efficacy in functional foods and therapeutic interventions. Additionally, exploration into biocontrol agents against fungal diseases demands comprehensive insights into lactobacilli’s antifungal activity [26]. Their role as vectors for mucosal vaccination faces challenges in optimizing the delivery of vaccine antigens. Exploration of their ecological role in the gastrointestinal tract aligns with ongoing efforts to develop strains capable of efficient antigen expression and delivery [27, 58, 60, 61].

The lack of clear guidelines and standardized criteria for evaluating probiotic safety and efficacy complicates the regulatory approval process. Streamlining regulatory procedures and establishing evidence-based criteria for probiotic evaluation are necessary to facilitate the translation of Lactobacillus-based interventions into clinical practice. In addition to this, education, training, and advocacy efforts are needed to promote the evidence-based use of lactobacilli in healthcare settings.

While there are several challenges, the outlook for Lactobacillus applications in the healthcare industry is promising. Advancements in genetic engineering techniques and microbiome research, coupled with a deeper understanding of microbiome dynamics, will likely lead to the development of more targeted and effective Lactobacillus-based interventions. By leveraging omics technologies and computational modeling, researchers can identify specific Lactobacillus strains and formulations optimized for targeted therapeutic interventions. Personalized medicine approaches, harnessing the potential of lactobacilli tailored to individual microbiomes, could revolutionize health-related applications addressing diverse health conditions, ranging from gastrointestinal disorders to metabolic diseases and immune-mediated conditions. Lactobacilli plays a pivotal role in modulating host-microbiome interactions as the emerging field of microbiome-based therapeutics is reshaping the landscape of medical interventions. Moreover, ongoing research into the use of lactobacilli in non-healthcare industries such as agriculture and livestock feeding holds the potential to enhance sustainability and productivity in these domains. As challenges are addressed and knowledge expands, Lactobacillus applications are poised to play an increasingly pivotal role in diverse fields, contributing to human health, food production, and environmental sustainability.

6. Conclusion

In conclusion, the exploration of lactobacilli applications in healthcare and biomedical industries reveals a multifaceted landscape rich with promise, yet riddled with challenges. LAB, particularly lactobacilli, have emerged as versatile organisms with diverse applications, ranging from probiotics to vaccine delivery systems and beyond. Their historical use in food fermentation, coupled with their safety profile and regulatory approval, has paved the way for their integration into various medical interventions.

The role of lactobacilli as probiotics, particularly in gastrointestinal health, is well-established. Their ability to regulate immune responses, inhibit pathogenic overgrowth, and maintain gut barrier integrity underscores their importance in addressing health-related conditions, encompassing gastrointestinal disorders and infectious diseases. Additionally, the prospect of personalized probiotic interventions, tailored to individual microbiome compositions, opens new avenues for customized therapeutic approaches. This is particularly noteworthy in the context of cirrhosis, where recent evidence suggests that the systemic inflammatory changes associated with cirrhosis, influenced by the gut microbiome, can be effectively mitigated with probiotic supplementation.

Lactobacilli-based vaccination strategies present an innovative approach to enhance vaccine efficacy, particularly through mucosal delivery systems. Studies demonstrate their effectiveness against a wide range of pathogens, including bacteria, viruses, and parasites, showcasing their versatility and potential in immunoprophylaxis.

In biomedical applications, lactobacilli show promise in catalyzing the formation of functional peptides with various physiological effects, from anti-hypertensive properties to cholesterol-lowering effects. Additionally, their production of metabolic by-products like biosurfactants holds potential in combating infections and promoting health outcomes.

However, challenges persist in harnessing the full potential of lactobacilli. Ensuring their survival and viability during processing and storage, standardizing production methodologies, and addressing variability in strain characteristics pose significant hurdles. Moreover, the lack of clear guidelines for evaluating probiotic safety and efficacy complicates regulatory approval processes.

Despite these challenges, the outlook for Lactobacillus applications in healthcare is promising. Advances in genetic engineering, microbiome research, and personalized medicine approaches are poised to revolutionize Lactobacillus-based interventions. As research continues to expand and knowledge deepens, lactobacilli are expected to play an increasingly pivotal role in diverse fields, contributing to human health, food production, and environmental sustainability.

Conflict of interest

The authors declare no conflict of interest.