Figure 1.

Simplified diagram summarizing the mechanisms of the three defense lines of resistance to antimicrobial agents for bacteria. There are three lines of defense for bacterial cells to overcome death by antimicrobial agents. The first defense line is bacterial biofilms, which limits the penetration of antimicrobial agents. The cell wall, cell membrane and the encased efflux pumps construct the second defense line to limit the absorbance or elevate the excretion of antimicrobial agents. When antimicrobial agents enter the bacterial cells, they meet the third line of defense, involving the alteration of target sites, regulation of gene expression and production of certain enzymes.