Emergence of a small colony variant of vancomycin-intermediate Staphylococcus aureus in a patient with septic arthritis during long-term treatment with daptomycin

Abstract

Objectives: Small colony variants (SCVs) of Staphylococcus aureus are associatedwith persistent and drug-resistant infections. We demonstrated for the first time the emergence of SCVs in a patient with vancomycin-intermediate S. aureus (VISA) infection during long-term treatment with daptomycin. Methods: A 73-year-oldman with septic arthritiswas infected with VISA. The patientwas treatedwith daptomycin; however, the patient remained infected with VISA, with continuous isolation of VISA from his blood during longterm treatment. Five VISA isolates were characterized by: PFGE; genotyping including staphylococcal cassette chromosome mec (SCCmec), spa and MLST; antimicrobial susceptibility testing; and scanning and transmission electron microscopy. WGS and fatty acid analysis were also performed. Results: The five VISA isolates were from a single clone of ST239/spa3(t037) and, of these, the first three were SCCmecIII positive and daptomycin susceptible, whereas the last two were SCCmecIII negative and daptomycin resistant and exhibited the characteristics of SCVs. The first and last isolates showed 13 remarkable genetic differences in SCCmec and the mprF, cls2, clpX and fabF genes. Of these, mutation of fabF (encoding the fatty acid synthase) seemed to be partially responsible for the slow growth and ultrastructural features, including an abnormal intercellular substance, and for the daptomycin resistance of SCVs. Conclusions: For the first time, we identified SCVs of VISA in a patient with septic arthritis during long-term treatmentwith daptomycin. Daptomycin-resistant SCVs of VISAwere evolved in a stepwise manner and the mutation of fabF is likely responsible for the physical and ultrastructural characteristics and daptomycin resistance.