Diversity in regulatory regions of icaADBCR and fnbAB genes among Staphylococcus aureus strains isolated from periprosthetic joint infections

Introduction: The ability of bacteria to colonize implant surfaces and tissues as a biofilm plays a relevant role in medical-device-associated infections. Staphylococcus aureus strains can produce a biofilm matrix made of the poly-N-acetylglucosamine (PIA/PNAG) exopolysaccharide and/or proteins. PIA/PNAG is synthesised by enzymes encoded by the icaADBC operon whose expression is repressed by the transcriptional regulator IcaR, while the protein-dependent biofilm is commonly associated to fibronectin-biding proteins, FnBPA and FnBPB, encoded by fnbA and fnbB genes. The aim of this work was to identify common genetic features in the regulatory regions of biofilm-related genes among clinical S. aureus strains derived from periprosthetic joint infections (PJI).  

Material and Methods: Genomes of 45 S. aureus strains from PJI were sequenced. Firstly, the sequence comprising the entire icaADBC regulatory region (5’UTR of icaADBC and icaR, the icaR coding sequence and its 3’UTR region) and secondly, the sequence of the promoter region of fnbAB were compared to those of S. aureus MW2 strain. Regulatory regions containing distinctive features were identified, fused to a reporter gene and introduced in a reference strain to analyze differences in gene expression.

Results: In the case of the icaADBC operon, single nucleotide polymorphisms (SNPs) in the icaADBC regulatory region allowed clustering of the strains in five groups from which a representative strain was chosen for further studies: S. aureus MIC 6924 (20% of isolates), MIC 6934 (13%), MIC 6936 (7%), MIC 6948 (2%) and MIC 7018 (4%). Of note, MICs 6948 and 7018 contained mutations in the icaR coding sequence. In this respect, a single nucleotide mutation in icaR (Val176Glu) caused a significant increase in icaADBC transcription and thus, in PIA/PNAG production and biofilm formation. In contrast, none of the rest of the SNPs found in the icaADBC regulatory region modified the transcription levels of the reporter gene. With respect to fnBPA and fnBPB genes and in agreement with previous studies, 100% of the strains contained the fnbA gene whereas only 69% contained the fnbB gene. The promoter region of fnbA was found to be highly conserved. SNPs in the promoter region of fnbB allowed clustering the strains in five groups. From these, the most frequently identified pattern was represented by S. aureus MIC 6948 (53%) and correlated with a lower level of reporter expression, whereas the group containing SNPs in the LexA binding sites was represented by MIC 7014 (4%) and correlated with higher expression levels.

Conclusion:  Our results suggest that S. aureus isolates from periprosthetic joint infections do not share specific features in cis regulatory regions of icaADBC and fnbB genes that may help to predict a higher expression levels of biofilm matrix compounds.

Acknowledgement: This project has received funding from the European Union's H2020 research and innovation programme under Marie Sklodowska-Curie grant agreement No 801586.