Hacking into bacterial biofilms: a new therapeutic challenge

Abstract

Microbiologists have extensively worked during the past decade on a particular phase of the bacterial cell cycle known as biofilm, in which single-celled individuals gather together to form a sedentary but dynamic community within a complex structure, displaying spatial and functional heterogeneity. In response to the perception of environmental signals by sensing systems, appropriate responses are triggered, leading to biofilm formation. This process involves various molecular systems that enable bacteria to identify appropriate surfaces on which to anchor themselves, to stick to those surfaces and to each other, to construct multicellular communities several hundreds of micrometers thick, and to detach from the community. The biofilm microbial community is a unique, highly competitive, and crowded environment facilitating microevolutionary processes and horizontal gene transfer between distantly related microorganisms. It is governed by social rules, based on the production and use of "public" goods, with actors and recipients. Biofilms constitute a unique shield against external aggressions, including drug treatment and immune reactions. Biofilm-associated infections in humans have therefore generated major problems for the diagnosis and treatment of diseases. Improvements in our understanding of biofilms have led to innovative research designed to interfere with this process.

Figure 1

Temporal evolution of biofilm. Schematization of the four-stage universal growth cycle of a biofilm with common characteristics, including initiation (I), maturation (II and III), maintenance (IV), and dissolution (V). Steps in P. aeruginosa are presented labelled with DAPI (A-C), chromosomal GFP (D) (personal data), or LIVE/DEAD BacLight kit (E) (Boles et al., 2005), observed with confocal microscopy and in S. aureus (F-H) in scanning electron microscopy (personal data). Potential hacking strategies are presented, including limiting 1) switch from planktonic to biofilm lifestyle (protein engineering of key players including c-di-GMP proteins, global regulators), 2) initial adhesion and interaction (glycomimetics), 3) communication (compounds interfering with QS autoinducers), 4) reactivating metabolic activity for increasing antibiotic efficiency (iron chelating procedure as an adjunct to conventional antibiotics), 5) developing anti-adhesive surfaces (silver or antiseptic-coated surfaces for endotracheal tubes), and 6) promoting dispersion (NO, capsules or dispersin-like molecules, phages).

Figure 2

Regulatory networks controlling transition between planktonic and biofilm lifestyle. The external frames illustrate the bacterial envelope with one or two membranes (OM: outer membrane, IM: inner membrane) according to Gram-positive (C) and Gram-negative bacteria (A, B, and D), respectively. A Control of biofilm formation in P. aeruginosa through the TCS GacS (HK)/GacA (RR) mediated by sRNA rsmY and rsmZ gene transcription and modulated by RetS and LadS, two additional HK in P. aeruginosa. B Control of EPS alginate in P. aeruginosa, which further impacts biofilm architecture by the system ECF sigma factor AlgU - anti-sigma MucA - AlgP (IM)-AlgW (periplasmic) complex: 1) activation of AlgW/AlgP, 2) cleavage of MucA, 3) release of AlgU, 4) activation of the alg UmucABCD operon. C Control of S. aureus biofilm formation through the Agr QS system: 1) AgrD autoinducer production, 2) AgrD autoinducer accumulation in the extracellular medium where it reaches a threshold, 3) activation of the TCS AgrCA by AgrD at the threshold concentration, 4) AgrA-dependent activation of the sRNA RNA III expression repressing expression of genes involved in biofilm formation together with amplification loop of agrABCD. D Control of P. aeruginosa biofilm formation through the intracellular second messenger c-di-GMP level controlled by the FimX protein having DGC and PDE domains, a RR domain, and a PAS domain. Note that in FimX protein only PDE activity is detectable (continuous arrow), whereas DGC activity is undetectable (dotted arrow).