Inducible Resistance to β-Lactams in Oxacillin-Susceptible mecA1-Positive Staphylococcus sciuri Isolated From Retail Pork

Most isolated strains of Staphylococcus sciuri contain mecA1, the evolutionary origin of mecA, but are sensitive to beta-lactams (OS-MRSS, oxacillin-susceptible mecA1-positive S. sciuri). In order to improve the efficacy of antibiotic treatment, it is important to clarify whether the resistance of OS-MRSS to beta-lactams is an inducible phenotype. In this study, three OS-MRSS strains with oxacillin MIC = 1 mu g/ml were isolated from 29 retail pork samples. The resistance of OS-MRSS to beta-lactams (MIC > 256 mu g/ml) was found to be induced by oxacillin, and the induced resistance was observed to remain stable within a certain period of time. Interestingly, the induced beta-lactam resistance was not caused by mecA1, heterogeneous resistance, or any genetic mutation, but mainly due to increased wall teichoic acid (WTA) synthesis that thickened the cell wall. The induced strains also showed slower growth rate, as well as decreased adhesion ability and biofilm thickness. These phenotypes were found to be achieved through altered gene expression in associated pathways, such as the citrate cycle and pentose phosphate pathway. The results challenge the traditional antibiotic sensitivity test. In the presence of beta-lactam antibiotics, OS-MRSS that was initially sensitive to beta-lactams was observed to gradually develop beta-lactam resistance in several days. This often-neglected phenomenon in antibiotic sensitivity tests requires further research attention.

Automated summary

The study found that resistance of Staphylococcus sciuri strains to beta-lactam antibiotics was induced by oxacillin, and this induced resistance remained stable for a period of time. The induced resistance was mainly due to increased wall teichoic acid synthesis that thickened the cell wall, leading to slower growth rate, decreased adhesion ability, and biofilm thickness. The induced beta-lactam resistance challenged traditional antibiotic sensitivity tests, showing that initially sensitive strains could develop resistance in a matter of days.