**1. Introduction**

One of the most significant, and extensively used lactic acid bacteria (LAB) is *lactobacillus.* This genus comprises a large number of species that can be found in diverse environments, such as in plants, food products, and mucosal surfaces of the human body. Lactobacilli are characterized by the formation of lactic acid as the primary end product of carbohydrate metabolism. Species of *lactobacillus* are Grampositive, homofermentative, thermophilic, and non-spore-forming rods. *Lactobacillus* species ferment a broad array of carbohydrates and can ferment extracellular fructans, starch, or glycogen depending on the strain. All organisms formerly assigned to the *Lactobacillus delbrueckii* group are now included in the emended description of the genus [1].

Among LAB, *lactobacillus* is one of the genera considered as Generally Regarded as Safe (GRAS) by the U.S. Food and Drug Administration (FDA) as they are a safe means to generate products for a variety of industries. The European Food Safety

Authority (EFSA) also grants Qualified Presumption of Safety (QPS) status to many *lactobacillus* species. The EFSA notes that some LAB strains are susceptible to acquiring virulence and antibiotic resistance genes and have opportunistic properties and hence excluded in the QPS list [2]. Lactobacilli are broadly applied in the food industry as both technical starters in fermented goods and probiotics due to their unique health and nutritional benefits [3, 4]. These microorganisms play an indispensable role in many foods fermentation. For instance, many lactobacilli species have been identified as primary microorganisms in sauerkraut fermentation [5]. Lactobacilli also play a significant part in malolactic fermentation, critical in winemaking [6]. In Chinese Maotai-flavored liquor production, *lactobacillus* accelerates flavor component conversion from alcohol (ethanol) to acid (lactic acid and acetic acid) [7]. Lactic acid fermentation using *L. plantarum* was found optimal in improving pea protein isolates aroma and taste [8]. Fermentation with *lactobacillus* also improved the foaming capabilities of egg white [9]. Many *lactobacillus* strains are also important in the manufacture of many fermented meat and meat products. Moreover, probiotic *lactobacillus* was given special attention as well for its ability to stimulate or modulate the immune system [10]. It also has possible applications in health-related areas such as intestinal inflammation [11], prevention of urinary tract infection [12], and treatment against cancer cells [13].

Indigenous LAB is constantly exposed to extreme conditions such as varying temperature, pH, and nutrient levels [14]. As a result, native LAB has been linked to higher competitive metabolic capacities, which encourage their growth as a competitive microbiota for other microorganisms in their natural habitat. The synthesis of a large number of bioactive metabolites is one of these capacities. Among LAB genera, lactobacilli are known producers of diverse bioactive molecules that offer a wide range of benefits to food, agricultural, industrial and clinical fields. They have long been exploited in food and animal feed as natural preservatives. Their antimicrobial action is mostly due to the production of organic acids, hydrogen peroxide, inhibitory compounds, as well as competition for nutrients and the development of antimicrobial compounds like bacteriocin [15]. Several studies have shown that the organic acids produced by *lactobacillus* like lactic acid kill pathogens at sufficient concentrations, such as *Campylobacter jejuni* by disrupting its membrane [16]. It also secretes ethanol and fatty acid as antimicrobial molecules. It can produce acetic acid, formic acid, and other acids [17].

Lactobacilli exhibit their beneficial properties through a wide range of processes that include a large spectrum of bioactive compounds. In this chapter, the utility of these microorganisms and their bioactive compound by-products for the promotion of better health and nutrition are summed up. Lactobacilli and their by-products can be utilized in technology and product development geared towards sustainable approaches for the improvement of human conditions targeted by the United Nations 2030 Agenda and its Sustainable Development Goals. The well-established functions of lactobacilli and their bioactive molecules in food fermentation could play a key role in ensuring that people around the world have access to safe and nutritious food by improving the current food production, safety, and preservation. Its application in the clinical setting also has the potential to address major health concerns, including sexual, reproductive, newborn, and environmental diseases which will be discussed in the following sections. Thus, this chapter will center the attention on the different bioactive molecules from the genus *lactobacillus*, such as bacteriocins, bioactive peptides, SCFAs, vitamins, enzymes, EPs, immune-modulating compounds, bioconverted molecules, and its probiotic properties.
