Identification of the genetic basis of a CO₂-dependent Staphylococcus aureus small-colony variant isolated from...

Small-colony variants (SCVs) are a slow-growing subset of bacteria that exhibit unusual colony morphology and unique biochemical characteristics. They are associated with chronic and persistent infections. CO₂-dependent SCVs of Staphylococcus aureus have been rarely isolated from clinical specimens. This study aimed to characterize the CO₂-dependent phenotype of S. aureus SCV isolated from pacemaker leads and to determine the genetic basis underlying this trait.

CO₂-dependent S. aureus SCV-5700 isolated from pacemaker leads of a patient with a pacemaker infection was used in this study. Phenotypic testing, antimicrobial susceptibility testing, and mecA polymerase chain reaction of the isolate were performed. Moreover, whole-genome sequencing was conducted for multilocus sequence typing and comparative genomic analysis.

CO₂-dependent S. aureus SCV-5700 grew poorly under ambient air conditions; however, tiny colonies formed after 48 h incubation. The isolate was sequence type 45 and did not harbor the mecA. The isolate was identified as S. aureus by biochemical characterization in a 5 % CO₂ atmosphere. Comparative genomic analysis revealed that the isolate had a nonsense mutation (c.565C>T) in the mpsB; however, the revertant strain, Rev-5700, had no such mutation.

The CO₂-dependent phenotype of clinical S. aureus isolates can be attributed, in part, to loss of MpsB function.