In vitro Antibacterial Activity and Mechanism of Glycerol Monolaurate Against Streptococcus iniae Isolated from Pearl Gentian Grouper
[Objective]To investigate the in vitro antibacterial effect and mechanism of action of Glycerol monolaurate (GML) against Streptococcus iniae for providirg new evidence for the control of S.iniae infecion in fish farming.[Method]This study focused on S.iniae from pearl gentian grouper (♀Epinephelus fuscoguttatus ×♂Epinephelus lanceolatus),and the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of GML against S.iniae were determined using the microdilution method.The potential mechanism of action of GML against S.iniae was evaluated through detection of growth curves,conductivity,changes in bacterial morphology,bacterial protein expression,biofilm formation,and transcription of virulence-related genes.[Result]The MIC and MBC of GML against S.iniae were 16 mg/L and 32 mg/L,respectively.The inhibitory effect showed a concentration-dependent pattern,with higher concentrations resulting in stronger inhibition.Particularly,the inhibition of bacterial growth and biofilm formation were most pronounced at the MIC.After treatment with different concentrations of GML,bacterial cells exhibited loose arrangement,uneven sizes,increased intercellular spaces,and significant damage such as concavities and folds.Additionally,treatment with 1/2 MIC and MIC concentrations of GML led to a significant increase in the electrical conductivity of the bacterial suspension,indicating damage to the bacteria at these concentrations.The 1/4 MIC concentration promoted the increase in protein bands,while the MIC concentration exhibited an inhibitory effect causing the protein bands to become faint or disappear.[Conclusion]GML showed excellent in vitro antibacterial activity against S.iniae isolated from the pearl gentian grouper.It played an antibacterial role by destroying the integrity of cell structure,leading to the loss of contents,inhibiting the growth of bacteria and biofilm formation,and affecting the expression of bacterial virulence genes and proteins.
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