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

SULIGOY CM; GONZALEZ CD; LÖFFLER B; SORDELLI DO; DOTTO CM; ROBINSON D; TUCHSCHERR L; BUZZOLA FR; LOMBARTE SERRAT A; SACHSE S; GOMEZ MI

Seul

Simposio; International Symposium on Staphylococci & Staphylococcal Infections - ISSSI 2016; 2016

Background: Staphylococcus aureus is a highly prevalent opportunistic,multifactorial pathogen. The evolution of S. aureus within affected tissueappears to lead to persistence of the microorganism and chronicity of theinfection. This study identified changes that occur in S. aureus exposed to thehost defense mechanisms during infection and evaluated whether thesechanges affect S. aureus virulence.Methods: S. aureus isolates from a single patient with chronic osteomielitisand from the same infection site were investigated. The strains weresequenced with Illumina Technology and assembled de novo with SPAdes.The genomes were annotated with Prokka and manually curated with Artemisand compared using ACT. Variant calling was made following currentlyrecommended protocols. Expression of the effector molecule of the globalregulator agr (RNAIII) was performed by qRT-PCR. S. aureus strains weretested 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 withantibodies to CP5 in a mouse model of mastitis infected with HU-1 asdescribed previously. Isolate HU-85a and strain HU-1 expressed alpha andbeta-haemolysins and capsular polysaccharide (CP8 and CP5, respectively)whereas strain HU-85c and HU-1NT did not due to drastic reduction of RNAIIIexpression. Genomic analysis revealed an insertion of C in position 471 ofagrC resulting in a frameshift truncated product. Both HU-85c and HU-1NTexhibited reduced virulence in a rat model of osteomyelitis as ascertained byreduction of the bacterial bone load and OI when compared with the parentalcounterparts.Conclusions: selective pressure exerted by prolonged permanence in thechronically infected host or experimentally with antibodies to an agr-dependentsecreted product in a mouse model of infection resulted in drastic reduction ofRNAIII expression by mutation. Whereas the presence of S. aureus agrdeficientmutants in clinical samples is well recognized the evolution fromconserved to permanent reduction of RNAIII expression appears to beadvantageous for persistence and may favor chronicity of infection.