Phenotypic Changes of Oxacillin Resistance in Methicillin-resistant Staphylococcus aureus Strains by Successive Passaging

Background and purpose: Rotational use of antibiotics can decrease antibiotic resistance in Staphylococcus aureus strains. The present study aimed to evaluate the alteration of oxacillin resistance in methicillin-resistant Staphylococcus aureus (MRSA) strain carrying SCCmec III cassette in long-term evolutionary experiments in an antibiotic-free medium. Materials and methods: In a cross-sectional study, 10 MRSA isolates were identified between September to December 2018 by conventional microbiology methods from clinical isolates of a university hospital. Then, SCCmec typing was carried out using Multiplex PCR. One strain was included in the evolutionary experiment as ancestor and was cultivated for up to 900 generations. Minimum inhibitory concentration (MIC) of oxacillin of the evolved strains was evaluated. The selected strains were compared with the ancestral strain in terms of growth characteristics such as generation time and competitive cultivation. Results: Out of ten MRSA isolates, six and four were found to harbor SCCmec type III and SCCmec type IV, respectively. One strain with SCCmec III was randomly selected as ancestral strain. The ancestor was resistant to oxacillin while the evolved strains were susceptible. Generation times in the evolved strains were 6-8 minutes shorter than those in the ancestral strain. The evolved strains, with the ratio of 1% in competition experiment, accounted for 20-48% of the final population. Conclusion: Compared to the ancestor, the evolved strains presented higher fitness in the oxacillin-free environment. The current study indicates the powerful effect of antibiotic rotation strategy on reducing methicillin resistant MRSA.