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Introduction: The opportunistic pathogen S. pseudintermedius mainly causes purulent infections in dogs. Recurrent infections have been described in the past. In order to unravel the microevolution as well as the phenotypic diversity of this pathogen within one patient phenotypic and genotypic characterization was performed for methicillin-susceptible (MSSP) and methicillinresistant S. pseudintermedius (MRSP) isolated from multiple wound infections of a dog during a seven-year time period.
Material and Methods: Between 2008 and 2014, S. pseudintermedius was isolated from 25 wound swabs of one patient. In total, 38 isolates (up to eight colonies / swab) were sequenced. Clonal relationship was determined based on the allele diversity of 1064 target genes (Ridom SeqSphere + 2.3.1). Variability within isolates of distinct genotypes (gene content and SNPs [single nucleotide polymorphisms]) was analyzed (geneious 6.1.5). To display the phylogenetic relationship for isolates of each lineage neighbor-joining trees were built. The binding capacity to fibrinogen and fibronectin as well as biofilm formation was determined for eleven isolates.
Results: MLST+ typing revealed three distinct genotypes (I, II, III). All MRSP (n=21) belonged to genotype I. MSSP clustered into two genotypes II (n=15) and III (n=2). Within each of the two predominant lineages only minor variations were detected regarding the gene content and SNPs. Despite the low number of identified SNPs (MRSP-I n=26; MSSP-II n=27), an accumulation was observed over the time for MSSP-II. While MRSP-I showed only weak adherence to fibrinogen, MSSPIII had a moderate and MSSP-II a strong binding capacity. Strong biofilm formation was observed for all MRSP. Isolates sharing the same genetic background displayed a comparable phenotypic profile.
Discussion: Sampling of 25 wound infections from one patient revealed two different successful genetic lineages. Interestingly, isolates sharing the same genetic background showed only minor genetic variation even though the strains were isolated over seven years. While mixed infections with MRSP-I and MSSP-II were determined twice, exchange of mobile genetic elements was not detected. Phenotyping revealed opposing abilities for MRSP-I and MSSP-II regarding adherence to fibrinogen and biofilm formation, indicating that none of these tested mechanisms is essential for S. pseudintermedius to successfully infect dogs. The lack of phenotypic variability of isolates sharing the same genetic background is in accordance with the stable genome of these strains. A reasonable explanation for the lack of variability within the identified lineages might be recurrent auto-infections or a persistent infection rather than re-infections due to an external source.