# Nutrient Sensing and Biofilm Modulation: The Example of L-arginine in _Pseudomonas_
## 4 L-arginine Sensing and Biofilm Regulation in _Pseudomonas_
Accordingly, the RmcA mutant accumulates more c-di-GMP than the wildtype PAOI when L-arginine is the sole carbon source [64]. Recent data indicate that RmcA is involved in nutrient perception to sustain the maintenance of mature biofilm (rather than formation) once the attachment and the community is well-established; biofilms lacking RmcA or another phosphodiesterase named MorA cannot support nutrient limitation and finally die [66]. The name RmcA stands for the redox modulator of c-di-GMP [67]; this protein is involved in redox homeostasis (see below for more details); RmcA was found to inhibit colony wrinkling, a colony morphology which may improve the exchange of gases with the environment [67]. A similar strategy linking L-arginine sensing and c-di-GMP tuning has been observed in the DGC CdgH from _Vibrio cholerae_ whose periplasmic PBP recognises L-arginine from the extracellular milieu, likely to regulate the intracellular levels of c-di-GMP via the cytoplasmic DGC domain [68]. Among the _Pseudomonas_, _P. aeruginosa_ is an environmental organism able to infect a diverse range of life forms thanks to its metabolic versatility. In humans, _P. aeruginosa_ is an opportunistic pathogen targeting immunocompromised and nosocomial patients [69]; moreover, this bacterium represents the leading cause of death in cystic fibrosis (CF) patients due to the chronic lung infections dominated by its biofilm [70]. The CF lung sputum is a nutrient-rich milieu able to support the high-density growth of _P. aeruginosa_, in part through the perception of amino acids to be assimilated and catabolised [71; 72]. Multiple virulence-related phenotypes are modulated by CF lung components. In case of the environmental L-arginine, this amino acid may control the _P. aeruginosa_ biofilm, which is relevant for infections, virulence [73; 74; 75] and antibiotic resistance [76]. A screening of the PA14 mutant library for mutants exhibiting an altered biofilm phenotype indicated that L-arginine metabolism is essential for _P. aeruginosa_ biofilms [77]; the authors also found that the initial steps of pyrimidine biosynthesis were crucial, being necessary to sustain the pool of carbamoyl phosphate, a metabolite at the crossroads of both L-arginine anabolism and catabolism [77]. _P. aeruginosa_ and other pathogens (both intracellular and extracellular) may alter the host L-arginine metabolism to sustain their homeostasis and mitigate the L-arginine-derived NO defense response [78]. In contrast, in the oral environment, L-arginine (and its catabolism) buffers the pH decrease due to the microbiota metabolism, thus protecting against the dental plaque biofilm [79]. L-arginine supplementation was also found to decrease the _P. aeruginosa_ spread and sepsis in burn wound infections by influencing the virulence and may represent a promising phenotype-modulating tool for future therapeutic strategies [80]. Besides L-arginine, other polyamine precursors such as agmatine or exogenous putrescine upregulate biofilm formation in _Pseudomonas_, although the response to polyamines differs between the taxa [81]. Interestingly, the authors demonstrate that putrescine and L-arginine supplementation contributes to biofilm development (in terms of the extracellular structure) rather than to bacterial growth (cells) [81]. _P. aeruginosa_ PA14 strain harbors an extra agmatine-induced _agu_ operon responsible for agmatine utilization required for biofilm development, thus possibly providing further polyamines for maintenance or reparative processes or stabilization of DNA synthesis [82]. Given the relevance of L-arginine fermentation (i.e., the ADI pathway) in fueling ATP reservoirs under the anaerobic conditions that are frequently found at least in the innermost layers of the _P. aeruginosa_ biofilm, this aspect was discussed in a dedicated paragraph.