In Vitro Activity of Mupirocin Against Methicillin-Resistant Staphylococcus aureus (MRSA) in Two Periods of Time (2016 and 2021–2022) in a Large University Hospital in Germany

Colonization with methicillin-resistant <i>Staphylococcus aureus</i> (MRSA) increases the risk of adverse health outcomes, and it is estimated that 10-30% of carriers subsequently develop infection. Mupirocin is currently widely used as a topical decolonization measure against nasal methicillin-resistant <i>S. aureus</i> (MRSA) carriers. The present study was carried out in order to determine the prevalence rate of high-level (HLMuR) and low-level (LLMuR) mupirocin-resistant MRSA strains among patients treated at our hospital over two periods of time (2016 and 2021-2022) and to observe possible changes in MRSA susceptibilities against mupirocin after a six-year period of use. This is a retrospective study carried out on MRSA isolated from various clinical specimens from inpatients. A total of 474 MRSA isolates found in nasal, nasopharyngeal, throat, wound, urine and prostheses swabs were examined. All <i>S. aureus</i> isolates were identified using MALDI-TOF mass spectrometry scores ≥ 2 and confirmed as MRSA using the cefoxitin (30 µg) disc diffusion method. Mupirocin-resistant MRSA was detected by the Epsilometer test (E-test). All the resistant strains were tested for Panton-Valentine-Leucocidin (PVL) toxin by PCR. Out of 481 strains in our database, 474 pure non-duplicate MRSA isolates were included in our study. Mupirocin resistance was found in 15 (3.2%) MRSA strains by E-test, whereby three (0.6%) isolates were HLMuR and 12 (2.5%) isolates were LLMuR. The prevalence of mupirocin-resistant MRSA was similar in 2016 (10.6%) and 2021 (12.9%) and decreased to 6.5% in 2022. Even though the decrease in the proportion of resistant isolates from 2021 to 2022 did not reach statistical significance (<i>p</i> = 0.103), the mean resistance level among resistant isolates decreased significantly over the study period (<i>p</i> ≤ 0.001), dropping from 94.3 µg/ml in 2016 to 46.4 µg/ml in 2021 with a further decrease to 0.7 µg/ml in 2022. Although the mupirocin resistance decreased in 2022 compared with 2016, continuous monitoring of mupirocin resistance development in MRSA and surveillance of excessive use is of utmost importance in order to rule out possible failure of mupirocin decolonization treatment with subsequent increased mortality due to infections, prolonged hospital stays and more difficult treatment requirements at an early stage.