The intricate ecosystem of microorganisms inhabiting the human gut plays a crucial role in essential functions like nutrient synthesis, digestion and immune regulation (Jandhyala et al., 2015). Microorganisms like Lactic Acid Bacteria (LAB) and Bifidobacteria provide a range of benefits, including nutrient absorption and immune regulation. However, the balance maintained by these beneficial microorganisms can be disrupted by harmful strains, leading to conditions like inflammatory bowel disease, obesity, and more (Jandhyala et al., 2015). The two most prevalent lactic acid bacteria in the human intestine are Lactobacillus and Leuconostoc spp. supporting the host's immune system by preventing the invasion of pathogens and antigens into mucosal tissues as they can regulate the pH of the gut environment through the generation of lactate and short-chain fatty acids (SCFAs) (Zhang et al., 2015).
Researchers are delving into the therapeutic potential of bacteriocins, small antimicrobial peptides synthesized by bacteria as they possess targeted antimicrobial properties against specific bacterial strains, which could be potential substitutes for antibiotics (Guo et al., 2020). These bioactive molecules could have various applications, including their role as biological preservatives in the food industry and as agents for preventing and treating infectious diseases in both humans and animals. The safety of bacteriocins, attributed to their breakdown in the gastrointestinal system, has spurred increased interest in their use as food bio-preservatives (Kassaa et al., 2019).
However, the process of bacteriocin production and purification encounters challenges. Scaling up production and isolating bacteriocins at high quantities can be challenging due to the complexity of growth media and purification procedures. Efficient bacteriocin production requires precise control over factors like pH and temperature within optimized, intricate growth conditions is essential. Conventional methods, which adjust one variable at a time while keeping others constant, can be time-consuming, costly, and yield unsatisfactory outcomes (Taghiyeva, 2020).
The traditional method of culturing 3 in MRS media to produce bacteriocin is expensive. Therefore, researchers aim to simplify and reduce the cost of culture media by using natural sources such as molasses, corn syrup, soy milk, cheese whey, and millet. (Gutiérrez-Cortés et al., 2018; Sridhar et al., Unpublished results). Millets, like foxtail millet and proso millet, are renowned for their nutrient-rich profile and have demonstrated their ability to facilitate Lactobacilli growth and yield substantial lactic acid production, serving as a cost-effective and accessible substitute for commercial media in cultivating lactic acid bacteria (Sridhar et al., Unpublished results).
A strategy like co-culturing bacteriocin-producing microorganisms with pathogenic counterparts has been investigated as a potential strategy to control the growth of pathogenic ones in food systems (Chang et al., 2007; Ng & Bassler, 2009). Lactobacillus spp. exhibits quorum sensing, a communication mechanism involving autoinducer signalling molecules' generation, detection, and reaction (Ventura & Sagi, 2012). This coordination based on population density is vital for activities like colonization, biofilm formation, and virulence and also regulates various cellular processes, including bacteriocin production, and stress response (Sturme et al., 2007; Kleerebezem et al., 2010). Co-culturing of microorganisms creates an environment where the growth of the pathogenic microorganisms is inhibited, although it depends on factors like bacteriocin type, microorganisms used, culture conditions, and media composition (Todorov & Dicks, 2004).
Enhanced metabolite delivery using nanoparticles, including bacteriocins, has been reported multiple times. Usually, a nanovesicle or nanocapsule is made which is loaded with the desired bioactive compound. The size of the nanoparticles facilitates targeting greater surface area of the body thereby enhancing drug delivery (Reis et al., 2006). These biocompatible, biodegradable, and non-toxic polymers can protect and control the release of encapsulated bacteriocins (Rezaei et al., 2019). Since the functioning of bacteriocin as antimicrobial agents is limited by their instability and susceptibility to environmental factors like temperature and pH, nano-encapsulation counters these challenges, enhancing the stability and efficacy of bacteriocin (Bali et al., 2016). This is observed in studies with nisin against Listeria monocytogenes and pediocin against Staphylococcus aureus, Escherichia coli and Listeria monocytogenes (Benkerroum & Sandine, 1988; Rodrı́guez et al., 2004).
This study aims to re-establish bacteriocin production through Lacticaseibacillus rhamnosus mono-culture in a modified millet medium, induce/enhance bacteriocin production via co-culture with Staphylococcus aureus, evaluate the antimicrobial activity of bacteriocin extracts in mono-culture and co-culture using the agar well diffusion method, and examine the synergistic antimicrobial effects of bacteriocins and nano-conjugates.