## 1 Introduction  
The production of fuels from renewable raw materials is an essential strategy to meet the global energy demand, providing a potential alternative to fossil fuels [1, 2]. Biodiesel is an example of a biofuel. A byproduct, glycerol, is formed during its production. The disposal of this byproduct causes damage to the environment, so there is considerable interest in developing studies looking at how to add value to glycerol [3, 4, 5]. Thus, there is an interest in the bio-conversion of the glycerol produced in higher-value products, improving the process economy and minimizing waste [6, 7]. Glycerol is already a raw material source of many high-value products such as polymers, fuel additives, antibiotics, and painkillers. Microorganisms can bioconvert glycerol into metabolites due to the size and structure of their molecules, with similar yields to those obtained with the sugar metabolism [3]. The glycerol molecules can cross the cytoplasmic membrane by passive diffusion. Fermentative glycerol metabolism has been studied in detail in bacteria, including _Citrobacter, Klebsiella, Clostridium, Lactobacillus_, and even _Shimwellia_, to obtain high-value products, such as 1,3-propanediol, 2,3-butanediol, ethyl alcohol and hydrogen. In _Klebsiella_, glycerol can be metabolized both oxidatively and reductively. _K. pneumoniae_, a facultative anaerobe bacterium, is capable of bioconverting glycerol molecules in both aerobic and anaerobic environments. The reductive anaerobic metabolism of glycerol produces 1,3-propanediol, while the oxidative anaerobic metabolism of glycerol produces pyruvate and contributes to cell (biomass cellular) growth. The end products of pyruvate-deriving are lactic acid, ethanol, and 2,3-butanediol [3; 4; 6; 8; 9; 10; 11]. _K. pneumoniae_, with its broad substrate characteristics, high yield, and easy cultivation, has been an efficient type of bacteria for 2,3-butanediol production [12]. However, there are also studies showing that such a microorganism could metabolize substances such as 1,3-propanediol, 2-ketogluconic acid, succinate, 3-hydroxypropionic acid and ethanol [12; 13; 14; 15; 16; 17; 18]. Figure 1 shows some possible end products of glycerol degradation. Lactic acid (LA), an essential monomer in the polymer industry, can be obtained by bioconversion through microbial fermentation (bacteria, fungi, and yeast) and chemical synthesis. In the chemical route, the isomers D-(\(-\))-lactic acid and L-(+)-lactic acid are produced. The main advantages of this method are the lower cost of the purification step and the absence of a residual substrate. The advantages of fermentation are the possibility of using low-cost raw materials to produce one specific stereospecific enantiomer of the isomers' lactic acid by controlling the process parameters. The L-lactic acid enantiomeric form is the most common for applications in the polymer industry. As a matter of fact, high optical and chemical purities are considered important requirements for commercial lactic acid applications [4; 10; 20; 21; 22; 23]. Figure 1: Schematic pathways of glycerol metabolism from some microorganisms. Adapted with permission from [PERSON] et al. [19]. Copyright 2008 © Elsevier Inc. This compound has diverse applications, including as a pH adjusting agent, active ingredient in cosmetics, and monomer in biodegradable polymer poly(lactic acid) production. Lactic acid polymers (PLA) can also be used in medicine due to their resorption capacity. Therefore, they are used as pins and plates for bone implants, sutures and tissue regeneration [4; 17; 19]. PLA is a thermoplastic biopolymer that can be semicrystalline or amorphous in nature. In this scenario, the bioconversion of renewable resources to produce lactic acid exhibiting high optical purity is fundamental to improving the physical characteristics of PLA, such as crystallization, thermostability, and mechanical performance. The mechanisms PLA biodegradation involve a two-step degradation process.