Long exposure of Staphylococcus aureus to the host or selective pressure to an agr-dependent secreted product results in reduced virulence.

SULIGOY LOZANO CM; DOTTO C; LOMBARTE SERRAT A; ROBINSON D.A.; LOEFFLER B; TUCHSCHERR LPN; SORDELLI DO; BUZZOLA FR

Seoul

Simposio; International Symposium on Staphylococcus and Staphylococcal Infections (ISSSI); 2016

ISSSI and Korean Society for Chemotherapy

Background: Staphylococcus aureus is a highly prevalent opportunistic, multifactorial pathogen. The evolution of S. aureus within affected tissue appears to lead to persistence of the microorganism and chronicity of the infection. This study identified changes that occur in S. aureus exposed to the host defense mechanisms during infection and evaluated whether these changes affect S. aureus virulence.Methods: S. aureus isolates from a single patient with chronic osteomielitis and from the same infection site were investigated. The strains were sequenced with Illumina Technology and assembled de novo with SPAdes. The genomes were annotated with Prokka and manually curated with Artemis and compared using ACT. Variant calling was made following currently recommended protocols. Expression of the effector molecule of the global regulator agr (RNAIII) was performed by qRT-PCR. S. aureus strains were tested for virulence in a rat model of osteomyelitis and the bacterial load (CFU/tibia) and the morphometric osteomyelitic index (OI) were determined.Results: isolates HU-85a and HU-85c are isogenic and were isolated 13-months apart. Strain HU-1NT was obtained by exerting selective pressure with antibodies to CP5 in a mouse model of mastitis infected with HU-1 as described previously. Isolate HU-85a and strain HU-1 expressed alpha and beta-haemolysins and capsular polysaccharide (CP8 and CP5, respectively) whereas strain HU-85c and HU-1NT did not due to drastic reduction of RNAIII expression. Genomic analysis revealed an insertion of C in position 471 of agrC resulting in a frameshift truncated product. Both HU-85c and HU-1NT exhibited reduced virulence in a rat model of osteomyelitis as ascertained by reduction of the bacterial bone load and OI when compared with the parental counterparts.Conclusions: selective pressure exerted by prolonged permanence in the chronically infected host or experimentally with antibodies to an agr-dependent secreted product in a mouse model of infection resulted in drastic reduction of RNAIII expression by mutation. Whereas the presence of S. aureus agr-deficient mutants in clinical samples is well recognized the evolution from conserved to permanent reduction of RNAIII expression appears to be advantageous for persistence and may favor chronicity of infection.