Biofilm formation - what we can learn from recent developments

Abstract

Although biofilms have been observed early in the history of microbial research, their impact has only recently been fully recognized. Biofilm infections, which contribute to up to 80% of human microbial infections, are associated with common human disorders, such as diabetes mellitus and poor dental hygiene, but also with medical implants. The associated chronic infections such as wound infections, dental caries and periodontitis significantly enhance morbidity, affect quality of life and can aid development of follow-up diseases such as cancer. Biofilm infections remain challenging to treat and antibiotic monotherapy is often insufficient, although some rediscovered traditional compounds have shown surprising efficiency. Innovative anti-biofilm strategies include application of anti-biofilm small molecules, intrinsic or external stimulation of production of reactive molecules, utilization of materials with antimicrobial properties and dispersion of biofilms by digestion of the extracellular matrix, also in combination with physical biofilm breakdown. Although basic principles of biofilm formation have been deciphered, the molecular understanding of the formation and structural organization of various types of biofilms has just begun to emerge. Basic studies of biofilm physiology have also resulted in an unexpected discovery of cyclic dinucleotide second messengers that are involved in interkingdom crosstalk via specific mammalian receptors. These findings even open up new venues for exploring novel anti-biofilm strategies.

Keywords: antimicrobial strategies; biofilm formation; cyclic di-nucleotide second messengers; extracellular matrix; underlying diseases.

Fig. 1

(a) P. aeruginosa (red) and S. aureus (green) grown together in a ‘wound-like’ media that supports their co-culture as patchy distinct microcolonies. A three-dimension segment of the medium is shown from different angles. Image by Cody Fell (Rumbaugh laboratory), unpublished. (b) Visualization of P. aeruginosa biofilms in a CF lung of a CF male, 41 years of age, chronic P. aeruginosa mucoid and nonmucoid infection for 28 years, 46 precipitating antibodies, 114 2-week anti-P. aeruginosa treatment courses. Intraluminal P. aeruginosa biofilms surrounded by polymorphonuclear leukocytes (PMNs) visualized using peptide nucleic acid – fluorescence in situ hybridization (PNA-FISH) and DNA staining with 4′, 6-Diamidin-2-phenylindol, (DAPI). Adapted from [9]. (c) P. aeruginosa grown in artificial sputum medium. Adapted from [95] with permission.

Fig. 2

Novel molecular mechanisms involved in regulation of biofilm formation. (a) The giant protein PsrP of Streptococcus pneumoniae promotes colonization in the airways through multiple binding events. PsrP, via its BR domain located outside of the capsule, binds to biofilm-associated eDNA and adheres to surface accessible keratin (KRT) 10/KRT 1 as well as self-associates via BR domains. (b) The cellulase BcsZ located in the periplasm regulates cellulose production independently of cyclic di-GMP in S. typhimurium. Cyclic di-GMP (c-di-GMP) regulates expression of csgD encoding the major biofilm regulator. Subsequently, csgD activates transcription of the diguanylate cyclase AdrA to synthesize the cyclic di-GMP involved in production of the exopolysaccharide cellulose via binding to the cyclic di-GMP receptors BcsE and a PilZ domain at the C-terminal end of the cellulose synthase BcsA.

Fig. 3

Light directed control of bacterial behavior. (a) Absorption range of characterized photoreceptor domains that are coupled to downstream bacterial cyclic dinucleotide signalling domains [88]. Chromophores such as biliverdine/bilin derivatives, flavin derivatives such as FAD or cumarin that sense light of different wave length are covalently or noncovalently coupled to the protein scaffold of photoreceptors in phytochromes (including bacteriophytochromes and cyanobacteriochromes), LOV/BLUF and xanthopsin proteins, respectively [–98]. Phytochrome/phytochrome-like proteins contain PAS/GAF/PHY domains in various combinations [99]. (b) Engineering of an optogenetic system to regulate cyclic di-GMP levels bidirectionally. A red light-activated diguanylate cyclase (based on a bacteriophytochrome photoreceptor with a biliverdine chromophore) and a blue light-activated phosphodiesterase (based on a BLUF domain photoreceptor with an FAD chromophore) to regulate motility (left) and Congo red stained biofilm formation (right) [92].