# Nutrient Sensing and Biofilm Modulation: The Example of L-arginine in _Pseudomonas_
## 1 Introduction  
Although bacteria can be found as free-living individuals, biofilms are considered their prevailing mode of life in the environment [1, 2]. These are multicellular communities, usually bound to a solid surface, in which microorganisms live embedded in a self-produced extracellular matrix with structural and protective roles [3]. Biofilm development is a widespread trait in prokaryotes that was probably acquired early during their evolution [4]. It is a key strategy for microorganisms to colonise, adapt and survive in very different and changing habitats for several reasons. Firstly, bacteria establish social interactions within the biofilm, such as competition or metabolic cooperativity, where they can exchange, remove and redistribute nutrients [2]. Secondly, biofilms constitute a favourable environment to attain new traits through horizontal gene transfer, which can confer an adaptive advantageby increasing genetic diversity and accelerating population evolution [5]. Finally, the extracellular matrix provides protection against a variety of environmental challenges, from dehydration to antimicrobial compounds [6]. Much of our current knowledge on the molecular mechanisms involved in the process of biofilm development comes from studies in _Pseudomonas_ species, particularly of opportunistic pathogen _P. aeruginosa_. Planktonic and surface-attached cells show significant differences in terms of global gene expression and protein profile. As an example, a proteomic study in _P. aeruginosa_ revealed clear modifications between those two lifestyles, including proteins involved in signal transduction, second messenger systems, appendages and outer membrane components, among others [7]. Gene expression changes have been observed at different stages of biofilm development, or even at the same stage depending on the surrounding conditions and the spatial distribution of cells within the biofilm [8; 9]. Although the regulatory network involved in biofilm formation is very complex and far from being fully understood, showing variations from one species to another, it is possible to establish three broad levels of modulation of the process by different types of signalling mechanisms. The first level is due to environmental cues. These include surface characteristics like charge or roughness and external factors like pH, temperature or nutrient availability. The importance of nutrients in biofilm development is discussed in more detail in the following section. Other compounds and microelements like calcium, magnesium, inorganic phosphate or iron have been reported to influence attachment of _Pseudomonas_ to solid surfaces [10; 11; 12]. In many cases, detection of and response to a signal are carried out by two-component systems formed by a sensor histidine kinase which senses the environmental stimuli and by a response regulator that transduces the signal inside the cell. The second level of regulation comes from intracellular small molecules known as the second messengers. Usually, these are cyclic nucleotides, mono- or dinucleotides, which are widely distributed in both prokaryotes and eukaryotes. In terms of biofilm formation and transition between lifestyles, cyclic di-guanosine monophosphate (c-di-GMP) has emerged as a key element in many bacteria. Variations in local and global cellular contents of this molecule modulate different functions, including motility, secondary metabolism, or surface attachment [13; 14]. As a rule, high cellular levels of c-di-GMP correlate with the sessile lifestyle, and low levels promote motility. Production of the structural elements of the biofilm matrix (adhesins, exopolysaccharide and possibly also extracellular DNA) is controlled by c-di-GMP in many microorganisms, including _P. aeruginosa_[15], _P. fluorescens_[11] and _P. putida_[16]. Finally, the third level of modulation is derived from intercellular communication via quorum sensing (QS).