Maximizing Polylactic Acid Bioplastic Yield from Lactobacillus rhamnosus L6 via Response Surface Methodology: Evaluating Antimicrobial Effectiveness, Physical, and Chemical Properties of PLA

Polylactic acid (PLA) is a biodegradable and biocompatible polymer usually used in various scales (cosmetics, pharmaceuticals, food packaging, and environmental applications) due to its sustainability and mechanical properties. The objective of this research was to investigate the mechanical, physical, and antibacterial characteristics of improved PLA from Lactobacillus rhamnosus L6, utilizing response surface methodology (RSM). The data revealed that the highest values of both bacterial PLA production and lactic acid content (0.99 and 1.94 g/L) were achieved using RSM with a C/N ratio of 52.50 and an inoculum size of 10.62%. The produced PLA exhibited a higher tensile strength (6.20 MPa) than the control (3.44 MPa) and also a higher elongation (60.3%), indicating greater flexibility and increased moisture content (15.67%). Fourier transform infrared spectra analysis of this spectrum suggested that the polymer contained functional groups such as OH, CH, C═O, CH3, and CO.OH groups in the polymer backbone as well as the presence of ester or ether groups. The X-ray diffraction patterns of the biopolymer showed diffraction peaks (2θ) at 20–25°. According to scanning electron microscopy, the PLA surface was smooth and rougher at higher magnification, indicating inner microfibrillar densification and cracking. The produced PLA established a broad-spectrum antimicrobial agent (antibacterial and antifungal), with a higher efficacy against Gram-positive bacteria, especially Bacillus cereus ATCC 33018 (inhibition diameter = 33 mm), compared to Gram-negative bacteria and molds. The tested PLA exhibited no significant cytotoxicity against Vero cells with a viability rate of 98.82–99.60% up to 25 μg/mL and displayed an IC₅₀ value of 43.7 ± 0.34 μg/mL.