Tetracycline Resistance Among Canine Methicillin-Resistant Staphylococcus pseudintermedius (MRSP) Clinical Isolates: Is Minocycline a Viable Treatment Option?

<b>Background/Objectives</b>: Infections caused by multidrug-resistant methicillin-resistant <i>Staphylococcus pseudintermedius</i> (MRSP) strains are becoming increasingly common in veterinary medicine. Depending on which <i>tet</i> genes are present, MRSP isolates may exhibit resistance to all tetracyclines or resistance to tetracycline and doxycycline with susceptibility to minocycline. As minocycline may be a potential treatment option, our study aimed to verify this hypothesis. We have compared phenotypic resistance to tetracyclines with the presence of the <i>tet</i>(K), <i>tet</i>(L), <i>tet</i>(M), and <i>tet</i>(O) genes and conducted the molecular characterization of 50 clinical MRSP isolates of canine origin. <b>Methods</b>: The presence of the <i>tet</i> genes was determined by PCR. Molecular characterization included multilocus sequence typing (MLST) and staphylococcal cassette chromosome <i>mec</i> (SCC<i>mec</i>) typing. <b>Results</b>: Among the 50 examined clinical MRSP isolates, sequence type ST71 with the SCC<i>mec</i> II-III cassette was predominant (n = 27; 54%). Among these isolates, the <i>tet</i> genes were either absent or present only as the tet(K) gene, resulting in susceptibility to all tetracyclines, tetracycline and doxycycline resistance, and minocycline susceptibility. In contrast, isolates carrying the SCC<i>mec</i> type V cassette (n = 20; 40%) were resistant to all tetracyclines tested and belonged to ST551, ST2851 (new), ST2869 (new) and ST672. These genotypes were most often characterized by the presence of a single <i>tet</i>(M) gene; two genes, <i>tet</i>(M) and <i>tet</i>(K); or three genes, <i>tet</i>(M), <i>tet</i>(K) and <i>tet</i>(O). Notably, 28 out of 50 isolates (56%) showed minocycline susceptibility, and 19 (38%) were minocycline-susceptible and resistant to tetracycline and doxycycline. <b>Conclusions</b>: The obtained results indicate that genotype ST551 and its related ST2851 carry the SCC<i>mec</i> type V and typically contain two or even three <i>tet</i> genes with the <i>tet</i>(M) gene, which confers resistance to all tetracyclines, including minocycline. These genotypes are becoming more common in Poland, and thus, minocycline may be unsuitable for the treatment of MRSP infections in our geographical region. However, in other countries, distinct MRSP genotypes exhibiting minocycline susceptibility may predominate, such as those belonging to clonal complex 71 and carrying the SCC<i>mec</i> type II-III cassette. In the case of such strains, minocycline may be a therapeutic option. Therefore, it is advisable to monitor the spread of MRSP clones carrying different <i>tet</i> genes and exhibiting varying minocycline resistance profiles.