Molecular typing of methicillin resistant Staphylococcus aureus (MRSA) in patients with mutual exposure reveals low proportion of transmission events.

Question: If patients with newly detected MRSA during their hospital stay had epidemiological exposure in the same room or on the same ward to other patients with hitherto unknown MRSA colonization, it is tempting to assume a transmission event. However, a previous unknown MRSA colonization could as well explain the above constellation, especially in a setting without universal screening on admission.

Methods: Data on patient admission, transfer and discharge were linked with microbiological lab data in a Jupyter Notebook using common Python packages to determine pairs of patients with MRSA colonization that could possibly be explained by mutual exposure either in the same patient room or on the same ward. Molecular typing by whole genome sequencing and cgMLST analysis was used to determine clonal relatedness.

Results: Between 2017 and 2020 five pairs of MRSA patients with first detection later than two days after admission and overlapping stay in the same room were evaluable, of whom two had the same clone according to cgMLST typing. The respective numbers for patient pairs with overlapping stay on the same ward and available isolates were six out of 47 pairs. Of the latter patients, 2.1% had a negative MRSA screening within the previous 14 days. Complex types accounting for transmissions were CT1426, CT1422, CT32066, CT30303, CT31532.

Conclusion: Even in MRSA patient pairs with a plausible epidemiological link the majority was not due to a transmission event between them in a setting without universal screening on admission. Our results help to estimate the pretest probability for transmission events, which might be useful when typing results are lacking or not yet available.