Surviving as a Community: Antibiotic Tolerance and Persistence in Bacterial Biofilms

Abstract

Biofilms are surface-associated bacterial communities that play both beneficial and harmful roles in nature, medicine, and industry. Tolerant and persister cells are thought to underlie biofilm-related bacterial recurrence in medical and industrial contexts. Here, we review recent progress aimed at understanding the mechanical features that drive biofilm resilience and the biofilm formation process at single-cell resolution. We discuss findings regarding mechanisms underlying bacterial tolerance and persistence in biofilms and how these phenotypes are linked to antibiotic resistance. New strategies for combatting tolerance and persistence in biofilms and possible methods for biofilm eradication are highlighted to inspire future development.

Keywords: antibiotic resistance; bacteria; biofilm; persistence; tolerance.

Figure 1.

Biofilm mechanics. (A) Left: schematic of a rheometer setup for measuring biofilm mechanics. Biofilms (yellow) are sandwiched between a rotating (red arrow) shaft and a stationary plate (gray stripes). Right: Representative storage modulus G’ and loss modulus G” curves as a function of shear strain ε measured for a V. cholerae biofilm. G’ and G” correspond to the elastic and viscous responses of the biofilm, respectively. From the curve, the elastic modulus G’_p and the yield strain ε_Y are extracted. (B) Schematic of V. cholerae biofilm matrix components and how they contribute to biofilm mechanical properties. Cells (yellow cylinders) interact through surface lipopolysaccharides (black curvy lines) or by crosslinking via RbmA (green symbols). VPS (red wavy lines) is crosslinked by RbmC and Bap1, both depicted as blue dots. Removal of RbmC and Bap1 causes the VPS to swell. Images are adapted from Yan et al., 2018.

Figure 2.

Biofilm formation process. (A) Cross-sectional image of the bottom cell layer of a growing V. cholerae biofilm cluster at 18 h and (B) the corresponding segmented image with color-coding according to z position. Scale bar: 3 μm. (C) Schematic representation of the steps in the V. cholerae biofilm formation process. Cells are in yellow and the matrix is in pink. (D) Side views of 7 h old biofilms grown with 0.4 μg/mL A22 (magenta), without treatment (yellow) and with 4 μg/mL cefalexin (cyan). A22 and cefalexin cause the cells to become shorter and longer, respectively. Scale bar, 10 μm. Images in A-C are adapted from Yan et al., 2016. Images in D are adapted from Beroz et al., 2018.

Figure 3.

Tolerance and persistence in bacterial biofilms. (A) Experimental trajectory of E. coli cells from tolerant to resistant in response to periodic treatment with 50 μg/mL ampicillin. Wildtype E. coli cells transition to tolerant (blue arrow), and subsequently, to resistant (red arrow). MIC is defined as the minimum inhibitory antibiotic concentration that prevents bacterial growth. See Levin-Reisman et al., 2017. (B) Confocal images of P. aeruginosa biofilms following viability staining. Live cells are green and dead cells are red. Spontaneous cell death in the absence of antibiotics as shown in the figure, occurs in the interior, nutrient limited region of of ΔrelA ΔspoT biofilms but not in wildtype biofilms. The image in A is adapted from Levin-Reisman et al., 2017. Images in B are adapted from Nguyen et al., 2011.