In Vitro Evaluation of the Antibacterial Effect and Influence on the Bacterial Biofilm Formation of Glutamic Acid and Some Structural Analogues

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Abstract
Background/Objectives: Glutamic acid (GLA) is an essential amino acid with a key role in human metabolism. A potential involvement in anticancer therapy and possible antibacterial and anti-biofilm effects were also observed. Glutamine (GLN) and monosodium glutamate (MSG) are GLA structural derivatives for which the last two effects were evaluated, with contradictory results. Therefore, this study aimed to assess the antibacterial activity and the influence on the biofilm formation of GLA, GLN, MSG, and glutamic acid diethyl ester (GLADE) on clinically relevant bacteria. Methods: Gram-positive and Gram-negative bacterial reference strains were used to test the antibacterial and anti-biofilm effects of GLA, GLN, MSG, and GLADE. The antibacterial properties were assessed by detecting the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC). The influence on biofilm formation was assessed by the crystal violet method, reading the optical densities (ODs) by spectrophotometry. Results: GLN did not demonstrate an inhibitory capacity at the maximum tested concentration (2.86 mg/mL); GLA showed inhibitory activity at 1.76 mg/mL and 0.88 mg/mL; MSG inhibited the growth of all bacterial strains at 112 mg/mL; GLADE had the most promising results on all bacterial strains (MICs of 12.75 mg/mL and 25.5 mg/mL). GLADE showed satisfactory MBC values on all bacterial strains (at 51 mg/mL and 25.5 mg/mL). Conclusions: GLA and some structural analogues are attractive options for possible antibacterial activity; optimizing GLADE to increase its antibacterial activity could be a new approach.

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