Abstract
Stenotrophomonas maltophilia is an emerging nosocomial pathogen. In many bacteria iron availability regulates, through the Fur system, not only iron homeostasis but also virulence. The aim of this work was to assess the role of iron on S. maltophilia biofilm formation, EPS production, oxidative stress response, OMPs regulation, quorum sensing (QS), and virulence. Studies were done on K279a and its isogenic fur mutant F60 cultured in the presence or absence of dipyridyl. This is the first report of spontaneous fur mutants obtained in S. maltophilia. F60 produced higher amounts of biofilms than K279a and CLSM analysis demonstrated improved adherence and biofilm organization. Under iron restricted conditions, K279a produced biofilms with more biomass and enhanced thickness. In addition, F60 produced higher amounts of EPS than K279a but with a similar composition, as revealed by ATR-FTIR spectroscopy. With respect to the oxidative stress response, MnSOD was the only SOD isoenzyme detected in K279a. F60 presented higher SOD activity than the wt strain in planktonic and biofilm cultures, and iron deprivation increased K279a SOD activity. Under iron starvation, SDS-PAGE profile from K279a presented two iron-repressed proteins. Mass spectrometry analysis revealed homology with FepA and another putative TonB-dependent siderophore receptor of K279a. In silico analysis allowed the detection of potential Fur boxes in the respective coding genes. K279a encodes the QS diffusible signal factor (DSF). Under iron restriction K279a produced higher amounts of DSF than under iron rich condition. Finally, F60 was more virulent than K279a in the Galleria mellonella killing assay. These results put in evidence that iron levels regulate, likely through the Fur system, S. maltophilia biofilm formation, oxidative stress response, OMPs expression, DSF production and virulence.
Highlights
Stenotrophomonas maltophilia is a widespread environmental, multidrug resistant bacterium
In order to assess the role of iron on the production of biofilms and factors potentially involved in biofilm formation and virulence of S. maltophilia the following studies were performed under iron-limiting and iron-replete conditions
Iron-dependent gene regulation in bacteria is generally mediated by the Fur system (Escolar et al, 1999)
Summary
Stenotrophomonas maltophilia is a widespread environmental, multidrug resistant bacterium. Factors that could be involved in the virulence of S. maltophilia include Smf1-fimbrial operon (de Oliveira-Garcia et al, 2003), protease StmPr1 (Windhorst et al, 2002; Nicoletti et al, 2011), exopolysaccharides and lipopolysaccharides (Huang et al, 2006), and siderophores (Garcia et al, 2012). Another important virulence factor of S. maltophilia is its capacity to form biofilms, communities of microbial cells that grow on biotic or abiotic surfaces embedded within extracellular polymeric substances (EPS) (Huang et al, 2006; Passerini de Rossi et al, 2007; Pompilio et al, 2008). S. maltophilia biofilms exhibit phenotypic characteristics that are distinct from those of planktonic organisms, including increased resistance to antimicrobial compounds (Di Bonaventura et al, 2004; Passerini de Rossi et al, 2009, 2012; Pompilio et al, 2010)
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