Lactic acid bacteria (LAB) are a class of anaerobic, Gram-positive, catalase-negative microorganisms that can survive and grow in the presence of oxygen. They have the appearance of cocci or rods and do not form spores; the main byproduct of sugar fermentation is lactic acid [19,20]. The benefits of LAB consumption became apparent in the early 20th century when a scientist proposed that consuming the live microorganisms found in yogurt would lengthen the consumer’s life by improving host health and lowering the number of bacteria in the digestive tract that cause spoilage and toxins [21]. Following their discovery, LAB have been the subject of extensive research, and an impressive amount of data has been gathered about their contribution to the final product’s organoleptic qualities and nutritional value, as well as how they extend the shelf life of fermented foods. This is because LAB produce a wide range of compounds, such as ethanol, hydrogen peroxide, bacteriocins, and organic acids, among others, which work in concert to prevent or eradicate microbial contamination [22,23]. The ability of LAB strains to produce bacteriocins can be seen as a benefit and a functional role in the food industry to enhance food safety and quality [24]. Additionally, LAB produce metabolites, which are technologically attractive and explain how LAB are used in food to produce a wide range of fermented products [25].

LAB can grow in the majority of raw foods and are typically found in nutrient-rich environments [26]. Meat, dairy products, wine, beverages, sourdoughs for baked goods, fruits, vegetables, fish, and sea products are examples of food products that contain LAB. Based on the type of food product, handling techniques, and environment, different microflora predominate in these food products [26,27,28]. Since the majority of LAB’s properties are strain-dependent, the variety of LAB is intriguing overall both at the species and strain levels [25].

Since LAB have been used for centuries in food production and fermentation without posing any health risks, the American Food and Drug Agency (FDA) has classified the majority of LAB as Generally Recognized as Safe (GRAS) at the strain level [19,29,30]. The Qualified Presumption of Safety (QPS) status was also granted by the European Food Safety Authority (EFSA) to the majority of LAB genera at the species level, including Lactococcus, Lactobacillus, Leuconostoc, Pediococcus, and Streptococcus thermophilus [19,20,30]. Oenococcus, which is used in wine; Lactobacillus, which is used in meat, vegetables, dairy, and cereals; and Lactococcus, which is used in dairy products, are some of the most frequently chosen genera for industrial application [31].

Gram-positive cocci known as Lactococcus are found solely in pairs or in chains. They are catalase-negative, and facultatively anaerobic L-lactic acid is the main byproduct of the fermentation of glucose during the glycolytic pathway of L. lactis [32]. The lactic acid produced by the anaerobic conditions throughout the glycolysis pathway adds to the sour taste of fermented foods like yogurt [33]. While there are two types of fermentation, homo-lactic acid fermentation and hetero-lactic acid fermentation, Lactococcus is an example of homo-lactic acid fermentation because it produces only lactic acid (pyruvate is created by the glycolysis process using glucose as a carbon source, and lactate dehydrogenase converts it to lactic acid) [33].

L. lactis as one of the best known and characterized species of LAB serves as a model organism for studying LAB [29]. Taxonomically, L. lactis is a mesophilic species that belongs to the Streptococcaceae family. It is further divided into four subspecies: L. lactis subsp. hordniae, L. lactis subsp. lactis (which includes the biovar diacetylactis), L. lactis subsp. tructae, and L. lactis subsp. cremoris [34,35,36]. Known as “dairy” lactococci, L. lactis subsp. lactis and L. lactis subsp. cremoris together constitute the primary LAB components of a dairy starter, a costarter, or flavoring adjunct culture systems used in the production of cheese [31,37]. Recently, Li et al. [36] proposed that L. lactis subsp. cremoris should be elevated to the species level as L. cremoris sp. and L. lactis subsp. tructae should be transferred to L. cremoris as L. cremoris subsp. tructae. This was included in the official list of recognized prokaryotic species [38].

Strains that are isolated from raw milk or even non-dairy environments are referred to as “wild” lactococci. According to studies, wild lactococci isolated from dairy and non-dairy sources can produce particular flavors that are different from those of industrial strains [35,39]. For a very long time, L. lactis has been utilized in milk fermentation, both in well-managed industrial applications and small-scale traditional operations [40]. Technological contribution of lactococci has always been associated with the manufacturing stage; however, because of their proteolytic and amino acid conversion pathways, they can also affect the final texture and flavor of dairy products [41,42].

A significant number of LAB components linked to dairy food fermentation are L. lactis strains. These strains are used in commercial mesophilic starter cultures or small-scale traditional processes like making artisanal cheese, or they are employed in carefully regulated industrial dairy fermentation to produce cheese and ripen it, as well as fermented milk products like buttermilk, butter, and sour cream [18,31,35,40,43,44]. In the dairy industry, mesophilic starter cultures are classified as D-, L-, DL-, or O-cultures according to the application of L. lactis biovar diacetylactis (D-), Leuconostoc (L-), or a combination of both species (DL-) for flavor formation in addition to L. lactis strain(s) that are the major contributors to acid formation. Likewise, O-cultures are those starter cultures that exclusively rely on L. lactis for acidification [18]. It is assessed that through the consumption of fermented dairy products, humans ingest up to 10¹⁸ lactococcal cells throughout the year [31]. In dairy starter cultures, the L. lactis primary function is to generate lactic acid at a sufficient rate and to aid in the fermentation process by breaking down milk proteins. These actions have a significant impact on the final dairy product’s microbiological quality and organoleptic characteristics (Table 1) [31].

Proteolytic activity, lactose fermentation capabilities, exopolysaccharide (EPS) production, flavor production, and other isolate-dependent traits are among the many traits of L. lactis used as starter cultures for commercial purposes [2]. L. lactis subsp. cremoris and L. lactis subsp. Lactis, also L. lactis subsp. lactis biovar. diacetylactis, are the most common L. lactis subspecies found in dairy environments. L. lactis subsp. lactis is usually differentiated from L. lactis subsp. cremoris by its tolerance to higher temperatures and salt concentrations.

From the technological perspective, two characteristics of L. lactis strains are of significant importance—they can grow at 10 °C and at 40 °C, but not at 45 °C, and can also tolerate high concentrations of NaCl [32]. The strains’ ability to tolerate NaCl allows them to survive the negative effects of high osmotic pressure in the gastrointestinal tract’s high-salt environment and preserve a relative osmotic pressure balance in such circumstances. Because bacterial cells grown in high salt concentrations will experience a loss of turgor pressure that would affect the physiology, enzyme activity, water activity, and cell metabolism, the NaCl tolerance test indicates the degree of osmotolerance exhibited by a given LAB strain. It is known that homofermentative strains like L. lactis are more resistant to NaCl than heterofermentative strains [18]. LAB are described as homofermentative when an end product of fermentation is numerous amounts of lactic acid alone and heterofermentative when lactic acid is produced together with acetic acid, ethanol, and carbon dioxide [19].

The primary criterion for selection of L. lactis subsp. lactis starter cultures is acidification activity, as these cultures are frequently employed in the production of cheese, butter, and other fermented milk products. Slow or medium acid-producing strains are only employed if they have additional desirable qualities. Fast acid-producing strains are most commonly chosen as starter cultures [2]. Additionally, the dairy industry must consider the capacity for acid production during storage because low post-fermentation acidification is a crucial factor for commercial starters used to produce fermented milk because it impacts the quality of the finished product [2,49].

Some Lactococcus strains, such as L. lactis subsp. lactis biovar diacetylactis, exhibit additional fermentation capabilities, such as the fermentation of citrate, which results in the production of diacetyl, a flavor and aroma enhancer. Citrate can be converted by these strains into carbon dioxide and aroma compounds (C4), which enhance the organoleptic properties of fermented foods [2,47]. Diacetyl plays a crucial role in the flavor of many dairy products because, even in small amounts, it gives certain cheeses their distinctive creamy and buttery aroma [47].

Probiotics are defined as live microorganisms, which, when administered in adequate amounts, confer a health benefit on the host [30]. Probiotic-containing food products are also referred to as functional foods (the fastest-growing category of functional food development is probiotic foods) [50,51] and have a number of positive health effects, such as lowering blood pressure and serum cholesterol levels [52,53], improvement in lactose intolerance [54], production of numerous vital bioactive molecules, such as vitamins, amino acids, and gamma-aminobutyric acid, demonstrating health-promoting activity, stimulation of the growth of beneficial microorganisms and reduction in the amount of pathogens, and improving the intestinal microbial balance of the host and lowering the risk of gastro-intestinal diseases [29,55]. Thus, there is a great deal of interest in the isolation of novel probiotic strains that have the potential to enhance the quality of already-existing food products and promote health [56,57]. The optimal daily dosage of probiotic bacteria to achieve the desired effect is between 10⁶ and 10⁹ viable organisms [58,59], although studies have been performed that demonstrate dead cells can also have positive immunological effects [57].

Interesting probiotic potential has also been shown by certain L. lactis strains. Several previous studies have shown that some L. lactis strains that were isolated from raw and fermented milk were resistant to bile salts and acids (pH 2.5–3) and exhibited other properties like antimicrobial activity or growth in cholesterol, classifying them as probiotics [60,61]. The primary component of bile, bile salts, are toxic to living cells because they have the ability to alter the structure of cell membranes [62]. Consequently, in order for probiotic strains to reach the small intestine or colon alive, they must endure exposure to bile [63]. Human bile’s physiological concentration falls between 0.3 and 0.5% [64]. The concentration at which the growth lag time of LAB strains is measured is 0.3%; L. lactis has been reported to be resistant to 0.3–1.0% (w/v) of bile salt [60,65,66].

L. lactis has exhibited resilience in the severe conditions of the gastrointestinal tract, as well as the capacity to remain viable in the intestine and generate lactic acid, which contributes to the preservation of an acidic environment in the gut and prevents the growth of pathogenic bacteria [67]. It has been observed that L. lactis strengthens the tight junctions between intestinal epithelial cells, thereby enhancing the integrity of the gut barrier. This characteristic lowers intestinal permeability and stops harmful compounds from entering the bloodstream [68,69]. Bacteriocins, which inhibit the growth of pathogenic bacteria, are among the antimicrobial substances that L. lactis produces. In order to keep the gut microbiota balanced and healthy, these antimicrobial properties are essential [65]. Detailed information about L. lactis, isolated from dairy products, probiotic characteristics is given in Table 2.