TABLE 2.

Substances synthesized in food by lactic acid bacteria.

Substance	Metabolic Engineering works	Expanding applications in the food industry	Lactic acid bacteria strains (References)
Lactic acid	Heterologous expression of gene encoding short-chain dehydrogenase for higher yield of D-lactic acid	Use dairy industry waste as a substrate to reduce costs	Pediococcus acidilactici (Qiu et al., 2020), Lacticaseibacillus rhamnosus B103 (Bernardo et al., 2016) Lacticaseibacillus casei, Lactiplantibacillus pentosus and Lactobacillus sp. (Shirai et al., 2001) Enterococcus faecalis (Deibel and Niven, 1964)
	Improve the yield of lactic acid by adding different nutrients such as the substrate glucose or vitamin B compounds or adopting pH control strategies	Fermentation strategies and metabolic engineering are often used to improve the yield and purity of lactic acid	Lacticaseibacillus rhamnosus HN001 (Wang et al., 2019), Pediococcus acidilactici ZY271 (Han et al., 2019) Lactiplantibacillus pentosus CECT4023T (Cubas-Cano et al., 2019)
Other organic acids	The organic acid (formic acid, acetic acid, propionic acid, butyric acid, and succinic acid) production of lactic acid bacteria in fish infusion broth	Detection of organic acids produced by lactic acid bacteria and improvement of food quality and safety	Lactobacillus lactis subsp. Lactis (Sezen et al., 2016)
	3-Hydroxypropionic acid produced through glycerol metabolism	3-Hydroxypropionic acid is an important platform chemical	Limosilactobacillus reuteri (Kumar et al., 2013)
	The production of lactic acid, propionic acid was and succinic acid in fermented silages	The production of organic acids in fermented fish silages replaces the need of the addition of chemical additives for acidification	Levilactobacillus brevis, Lactiplantibacillus plantarum, Pediococcus acidilactici, and Streptococcus spp.
	Heterologous expression of mvaES gene of Enterococcus faecalis	Synthesize mevalonate	Enterococcus faecalis (Wada et al., 2017)
Bacteriocin		Inhibit the growth of Listeria monocytogenes in raw minced beef and gilthead sea bream	Lactiplantibacillus plantarum TN8 (Trabelsi et al., 2019), Latilactobacillus sakei CTC494 (Costa et al., 2019)
	Gasserins has antibacterial activity against Listeria monocytogenes or Bacillus cereus	Gassericin A can be an important tool for food preservation	Lactobacillus gasseri (Pandey et al., 2013)
	Sakacin P has antibacterial activity against Listeria monocytogenes or Bacillus cereus	Sakacin P exerts its antibacterial effect in fermented sausage	Latilactobacillus sakei (Chen et al., 2012)
Vitamins	Add passion fruit by-product and oligofructose to soy milk can produce folic acid	Synthesize folic acid in dairy products	Streptococcus, Lactobacillus and Lactococcus (Khalili et al., 2020), Lactococcus lactis NZ9000 (Wegkamp et al., 2007),
	Insert a 1059-bp DNA fragment into the upstream regulatory region of the rib operon of Lactiplantibacillus plantarum	Induce the overexpression of riboflavin biosynthesis	Lactiplantibacillus plantarum (Ge et al., 2020)
	Purine biosynthesis can trigger riboflavin secretion more effectively in Lactococcus lactis		Lactococcus lactis JC017 (Chen et al., 2017)
Extracellular polysaccharides	Synthesize glucan using sucrose	Synthesize isomalto-/malto-polysaccharides by using different substrate	Leuconostoc mesenteroides (Yan et al., 2018)Lactobacillus crispatus (Hidalgo-Cantabrana et al., 2019)Limosilactobacillus reuteri 35-5 (Bai et al., 2016)
		Increase the extracellular polysaccharide content of yogurt	Streptococcus thermophilus zlw TM11 and Lactobacillus delbrueckii subsp. bulgaricus 34.5 (Han et al., 2016)
		Has strong inhibitory activity with a variety of pathogenic bacteria	Lactococcus lactis F-mou (Nehal et al., 2019), Lactiplantibacillus plantarum BR2 (Sasikumar et al., 2017)
	Two glycosyltransferases participate in the formation of glucan	Exploration of a new way of glucan biosynthesis	Lactobacillus johnsonii (Mayer et al., 2020)
	Glucan will extend to the crumb porosity of bread	Improvement of bread texture	Limosilactobacillus reuteri (Leemhuis et al., 2014)
γ-aminobutyric acid	Mutations in the GadA or gadR gene facilitate the conversion of L-monosodium glutamate (MSG) to GABA	Increase the GABA content in fermented cereals	Levilactobacillus brevis (Lyu et al., 2019) Levilactobacillus brevis D17 (Gong et al., 2019)
	GadC transports L-glutamate into the cell		Lactococcus lactis (Small and Waterman, 1998)
	Glutamate decarboxylase and pyridoxal-5′-phosphate participate in the decarboxylation reaction of L-glutamate		Lactococcus lactis (Cui et al., 2020)
	The cell immobilization technology increase GABA production		Levilactobacillus brevis RK03 (Hsueh et al., 2017) Levilactobacillus brevis (Shi et al., 2017)
Flavor substances	SHMT gene encodes a serine hydroxymethyltransferase with threonine aldolase activity	Produce flavor substances (2,3-butanedione and 2,3-pentanedione, etc.) in wine, vinegar, bread, sourdough and cheese	Streptococcus thermophilus (Chaves et al., 2002), (Bancalari et al., 2017)
	Heterologous expression of thl, hbd, and crt which encode thiolase, β-hydroxybutyryl-CoA dehydrogenase, and crotonase, and the Treponema denticola for higher yield of N-butanol		Levilactobacillus brevis (Li et al., 2020),Lacticaseibacillus casei, Lacticaseibacillus rhamnosus and Streptococcus thermophilus (Bancalari et al., 2017),Streptococcus thermophilus and Lacticaseibacillus casei (Chammas et al., 2006)
Antioxidant substances	Lactiplantibacillus plantarum fermentation significantly enhanced the ability to scavenge free radical’s DPPH when the fermenting conditions were optimized by the method of responsive surface design in fermenting sheep bone	Produce antioxidant substances (active phenol metabolites, chlorogenic acid glucoside, sulforaphane) have a variety of beneficial effects on the human body	Lactiplantibacillus plantarum (Ge et al., 2019; Mu et al., 2019; Ryu et al., 2019), Lacticaseibacillus rhamnosus, Lactobacillus acidophilus (Kêska and Stadnik, 2018), Leuconostoc mesenteroides (Nam et al., 2017)
	Metabolize phenolic acid by decarboxylase and reductase	Reduce the damage of phenolic substances to the plasma membrane and cell wall of lactic acid bacteria	Levilactobacillus brevis, Limosilactobacillus fermentum and Lactiplantibacillus plantarum (Filannino et al., 2018)
	Hydroxycinnamic acid (P-coumaric, ferulic acid and caffeic acid) can be degraded.		Lactiplantibacillus plantarum NC8 (Barthelmebs et al., 2000)
	Hydroxybenzoic acid (gallic acid and protocatechuic acid) can be degraded.		Lactiplantibacillus plantarum CECT 748T (Rodriguez et al., 2008) Lactiplantibacillus plantarum (Whiting and Coggins, 1971)
	Convert oxidized glutathione taken from the environment into reduced glutathione	Promotion of glutathione synthesis in industry	Limosilactobacillus fermentum CECT 5716 (Surya et al., 2018) Streptococcus thermophilus (Qiao et al., 2018)
	Mutant strain Fructilactobacillus sanfranciscensis DSM20451 ΔgshR lacking the glutathione reductase gene	Increase dough rheology; promote the hydrolysis of egg white protein; improve the acid resistance of lactic acid bacteria	Latilactobacillus sakei and Fructilactobacillus sanfranciscensis (Loponen et al., 2008) Fructilactobacillus sanfranciscensis (Xu et al., 2018) Ligilactobacillus salivarius (Lee et al., 2010)