Contents lists available at ScienceDirect Journal of Microbiological Methods journal homepage: www.elsevier.com/locate/jmicmeth Review Tailoring cellular metabolism in lactic acid bacteria through metabolic engineering Anshula Sharma a , Gaganjot Gupta a , Tawseef Ahmad a , Baljinder Kaur a, ⁎ , Khalid Rehman Hakeem b,c, ⁎⁎ a Department of Biotechnology, Punjabi University, Patiala 147002, India b Department of Biological Sciences, Faculty of Science, King Abdulaziz University, 21589 Jeddah, Saudi Arabia c Princess Dr. Najla Bint Saud Al-Saud Center for Excellence Research in Biotechnology, King Abdulaziz University, Jeddah, Saudi Arabia ARTICLEINFO Keywords: Central carbon metabolism Heterologous gene expression Synthetic metabolic engineering Systems biology Biosynthetic pathways Metabolite production ABSTRACT Metabolic engineering combines approaches of synthetic and systems biology for tailoring the existing and creating novel biosynthetic metabolic pathways in the desired industrial microorganisms for production of biofuels, bio-materials and environmental applications. Lactic acid bacteria (LAB) are gaining attention worldwide due to their extensive utilization in food, fermentation and pharmaceutical industries owing to their GRAS status. Well-elucidated genetics and regulatory control of central metabolism make them potential can- didates for the production of industrially valuable metabolites. With the recent advancements in metabolic engineering strategies, genetic manipulation and tailoring of cellular metabolism is being successfully carried out in various LAB strains as they are providing highly efcient and industrially competitive robust expression systems. Thus, this review presents a concise overview of metabolic engineering strategies available for the comprehensive tailoring of lactic acid bacterial strains for large-scale production of industrially important me- tabolites. 1. Introduction 1.1. Biological activities and industrial relevance of lactic acid bacteria Lactic acid bacteria (LAB) constitutes an important Clostridium re- lated clade of non-spore-forming, catalase-negative and micro- aerophilic Gram-positive species belonging to genera Aerococcus, Carnobacterium, Enterococcus, Lactobacillus, Lactococcus, Leuconostoc, Oenococcus, Pediococcus, Streptococcus, Tetragonococcus, Vagococcus and Weissella (Johnson-Green, 2002; Hutkins, 2006). Based on the mode of glucose fermentation, these are divided into two group i.e. homo-fer- mentative and hetero-fermentative organisms. Homo-fermentative lactic acid bacteria convert carbohydrates into lactic acid as a sole end product, while hetero-fermentative species also produce ethanol, acetic acid and carbon dioxide in addition to lactic acid (Halasz, 2009). Apart from the production of lactic acid, LAB also has the ability to produce industrially valuable metabolites having potential applications as nu- traceuticals, pharmaceutics, commodity chemicals, aroma, favour compounds and also in fermentation and waste processing industries as summarized in Fig. 1. Antimicrobial agents and metabolites such as acetaldehyde, acetone, bacteriocins, benzaldehyde, diacetyl, for- maldehyde, propanoic acid which contribute positively to aroma, fa- vour, stability, shelf life and texture of fermented foods (Lavermicocca et al., 2000; Leroy and De Vuyst, 2004; Routray and Mishra, 2011; Papagianni, 2012a). Since ancient times, lactic acid bacteria are used as starter cultures in fermented foods and introduced to the raw material as functional food ingredients for improving nutritive quality and safety of the end product besides conferring several health benefts being “Probiotic” in nature(Konings et al., 1999; Hansen, 2002; Shah, 2007). Besides food production, LAB are also reported to produce several industrially im- portant enzymes viz. glycosylase, peptidases, amylases (Novik et al., 2007; Patel et al., 2012; Guldfeldt et al., 2001), vitamins like folate, B12, K2, ribofavin and thiamine (LeBlanc et al., 2011). Several low- calorie sweeteners are vital food ingredients mainly marketed as “dia- betic foods” produced using special or modifed strains of LAB. For example, production of L-alanine, mannitol, sorbitol, xylitol, tagatose, and trehalose has been observed in LAB which can be incorporated directly to foods or can be generated in situ during LAB fermentation (Wisselink et al., 2002; Patra et al., 2009). https://doi.org/10.1016/j.mimet.2020.105862 Received 4 September 2019; Received in revised form 3 February 2020; Accepted 3 February 2020 ⁎ Corresponding author at: Baljinder Kaur, Department of Biotechnology, Punjabi University, Patiala 147002, India. ⁎⁎ Correspondence to: Khalid Rehman Hakeem, Department of Biological Sciences, Faculty of Science, King Abdulaziz University, 21589 Jeddah, Saudi Arabia. E-mail addresses: baljinderkaur@pbi.ac.in (B. Kaur), kur.hakeem@gmail.com (K.R. Hakeem). Journal of Microbiological Methods 170 (2020) 105862 Available online 05 February 2020 0167-7012/ © 2020 Elsevier B.V. All rights reserved. T