Bacteria naturally release extracellular vesicles (EVs) during growth. These EVs are known as membrane vesicles (MVs) in Gram-positive bacteria or outer membrane vesicles (OMVs) in Gram-negative bacteria. EVs are known to store and to protect proteins, small molecules, and genetic materials and to deliver them. They play diverse roles, such as enabling bacterial survival during stress conditions and communications within bacterial communities.
Here, this study shows that EVs from Gram-positive bacteria are possible to transmit antibiotic resistance substances unto antimicrobial-sensitive Gram-negative bacteria. We collected EVs from a methicillin-resistant Staphylococcus aureus (MRSA) called ST541. Using these EVs, we performed vesicles-mediated transformation of antimicrobial-sensitive Escherichia coli called RC85 based on established protocols. The transformant RC85 colonies that acquired antibiotic resistance were re-select on antibiotic selection plates. One of them was named RC85-T. The resulting transformant RC85 was exhibited logarithmic phase growth but slightly slower than RC85. RC85-T showed a higher value of resistance against β-lactam antibiotics when compared with minimum inhibitory concentrations (MICs) of RC85. OMVs from the RC85 and the RC85-T were purified, and their quantitative production rates and β-lactamase activity were compared. The production of both OMVs was nearly equal. The β-lactamase activity of OMVs from the RC85-T was higher than that of the RC85 OMVs per equal protein concentration. We performed a comparative proteomic analysis to investigate the difference in β-lactam resistive compounds carried by OMVs and investigated whether both types of OMVs could protect susceptible E.coli from β-lactam-induced death. Several proteins that can be involved in degrading β-lactam antibiotics were more abundant in OMVs from RC85-T. Also, OMVs from RC85-T dose-dependently rescued β-lactam-susceptible E. coli from β-lactam antibiotic-induced growth inhibition.
In conclusion, we surmise that certain substances in Gram-positive ST541 EVs may be transferred to Gram-negative bacteria E. coli RC85, resulted in β-lactam antibiotic resistance. The findings of the present study might provide the importance of MVs in promoting the emergence of resistant strains out of a mixed community of bacteria.