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Staphylococcus (S.) pseudintermedius is part of the normal skin microbiota of dogs. As the most important canine coagulase-positive staphylococcal species, the opportunistic pathogen can cause a variety of different infections when the natural barrier of the host is lowered. The number of infections caused by multidrug resistant S. pseudintermedius increased in the last years – including infections with methicillinresistant S. pseudintermedius (MRSP) – limiting treatment options for veterinarians.
Different screening studies investigated the frequency and genetic variation of this opportunistic pathogen obtained from colonized and infected animals, revealing a high genotypic variability. However, there is no information available on the diversity of S. pseudintermedius in single patients that are frequently affected by infections with this pathogen although recurrent infections have been described as a serious problem for single dog patients in the past. Therefore, the aims of this study were to investigate the genetic and phenotypic variability of S. pseudintermedius isolated from multiple infections of a single patient.
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 S. pseudintermedius (MSSP) and MRSP isolated from multiple wound infections of one dog between 2008 and 2014. Both, MRSP and MSSP were obtained from the patient at 18 respectively 10 time points, suggesting that the dog of interest was highly susceptible for infections caused by S. pseudintermedius. Furthermore, mixed infections with MRSP and MSSP were determined on three occasions. Worryingly, these isolates expressed the same colony morphology. This finding is of major concern for routine bacteriological diagnostic since it was shown during this work that even phenotypically similar appearing colonies can exhibit different resistance profiles.
Genotypic characterization identified three distinct genotypes, whereas the two lineages ST71-MRSP and ST529-MSSP were predominant. Analysis of single nucleotide polymorphisms and the variability of mobile genetic elements showed only minor variation, revealing the isolation of very stable lineages during the investigated time period. Regarding the variability of the accessory genome content, ST71-MRSP showed the flexibility to adapt to environmental changes by the uptake of a doxycycline resistance gene encoding plasmid as well as by the loss of a phage after initial infection. The patient history revealed that the uptake of the plasmid was induced by treatment of the patient with doxycycline.
Phenotypic characterization showed opposing results for ST71-MRSP and ST529-MSSP with respect to biofilm formation as well as adherence to fibrinogen. These in vitro findings lead to the assumption that MSSP and MRSP are able to use different pathways to successfully infect their hosts. No respectively minor phenotypical differences were observed for isolates sharing the same genetic background. This outcome is in accordance with the stable genomes of strains isolated over six (ST71-MRSP) respectively five years (ST529-MSSP). A reasonable explanation for the lack of variability within the identified lineages might be recurrent auto-infection, persistent infection or re-infection due to an external source.