**2.1 Taxonomic classification of lactobacillus**

Lactobacillus was first classified by scientists based on its observable type of characteristics such as optimal growth and sugar utilization [10]. According to Ayivi et al. [1], new classifications of Lactobacillus have recently been approved by scientist due to the extent to which the new lactobacilli genera made it difficult to differentiate and categorize between the various lactobacilli species. These novel classifications are made of 25 genera, and this genus is of 262 species (as of March 2020), all of which are phenotypically, ecologically, and genotypically varied [10]. The reclassification comprises host-adapted species *L. delbrueckii* group and *Paralactobacillus* with the other 23 new species being *Amylolactobacillus, Apilactobacillus, Companilactobacillus, Lapidilactobacillus, Agrilactobacillus, Schleiferilactobacillus, Loigolactobacilus, Lacticaseibacillus, Latilactobacillus, Holzapfelia, Dellaglioa, Liquorilactobacillus, Ligilactobacillus, Lactiplantibacillus, Furfurilactobacillus, Bombilactobacillus, Paucilactobacillus, Limosilactobacillus, Fructilactobacillus, Acetilactobacillus, Levilactobacillus, Secundilactobacillus, and Lentilactobacillus*. This new classification represents the microorganism's evolutionary relationships, and environmental, biological, and metabolic properties [10].

#### **2.2 Ecological niches of** *Lactobacillus*

Lactobacillus species are among the most "sensitive" and fastidious microorganisms, which are usually located in nutrient-rich settings and other environments. These nutrient-rich habitats are classified as fermented/spoiled foods and animal

feed, and the environment includes plant surfaces, soil, and the bodies of vertebrates, invertebrates, and humans [8]. Lactobacilli are the most common microorganism found in the microbiota of fermented foods. Lactobacillus is prominent in fermented foods such as cheese, yogurt, and milk (dairy products) because lactobacilli species are the primary bacteria employed in dairy fermentation. Lactobacillus has also been identified in fermented foods such as meats, veggies, and sourdough [8]. Due to that, the Lactobacillus species *L. plantarum* is the most common bacterial species that occurs naturally in vegetables like cabbage and lettuce [11]. Lactobacilli can also be found in the natural microbiota of the host animals, occupying numerous niches. They are healthy bacteria and therefore found in the human's gastrointestinal, urinary, and genital systems without causing any harm to the human body or producing disease [2]. They are also present in the mouth cavity and vaginal canal in humans forming part of the human microbiome and imparting great benefits to the human system such as the prevention of the growth of other harmful bacteria [12].

Lactobacillus can also be found in sewage and soils, where it causes fecal pollution. Soil samples have yielded isolates of *L. plantarum, Lactobacillus paracasei* subsp. *paracasei,* and *Lactobacillus brevis* species [13]. Lactobacillus has also been found and isolated in plants where sugar traces can encourage their growth.

#### **2.3 Characterization of** *Lactobacillus*

Lactobacillus is a LAB that is known for producing lactic acid as a byproduct of glucose metabolism. Considering the characteristics of microorganisms, Lactobacillus is perhaps the most important LAB being utilized widely in the food industries as starter cultures to perform key responsibilities such as the production of fermented foods and their possible probiotic effects on humans [14]. These LAB (Lactobacillus) cells have rod-shaped with a size in the range of 0.5–1.2 × 10−10 μm; however, under certain growth conditions, the cells assume a coccoid-like shape [15]. Lactobacilli are auxotrophic and therefore grow best in nutrient-rich media, with the best growth temperature being 30–40°C; however, they can survive in a range of 5–53°C. The best (optimum) growth pH range for these LAB is 5.5–5.8 but can also grow at a pH < 5 [15]. The survival and viability of probiotic bacteria in adequate amounts in a product are the most significant requirements when considering the characteristics of probiotic strains in food. These Lactobacillus probiotic strains are not deficient in these characteristics and therefore make them a very good strain for the food industries. The characteristics of Lactobacillus that make them suitable probiotic microbe for use in the food industry include their resistance to bile and acid, adherence to human epithelial cells, colonization of the human intestine, production of antimicrobial substances, favorable growth characteristics, and favorable effects on human health. All these "rich" characteristics make it ideal for the effective utilization of the Lactobacillus strain in industries [16].
