Resistance Profiles and Biological Characteristics of Rifampicin-Resistant Staphylococcus aureus Small-Colony Variants

Abstract

Background

Staphylococcus aureus (S. aureus) is a major contributor to nosocomial and community-acquired infections. S. aureus small colony variants (SCVs) which changed in relevant phenotype have made more limited and difficult for therapeutic options against S. aureus infections increasingly. Rifampicin is considered as the “last-resort” antibiotic against S. aureus. Our study investigated resistance profiles and biological characteristics of rifampicin-resistant S. aureus SCVs.

Methods

We collected S. aureus SCVs that were selected from 41 rifampicin-resistant clinical isolates. Then, biological characteristics, resistance spectrum, and rifampicin resistance mechanisms of tested S. aureus SCVs and corresponding parental strains were investigated by classic microbiological methods, agar dilution method, polymerase chain reaction (PCR). Moreover, the fitness cost of S. aureus SCVs, including growth, biofilm formation ability, and virulence profile, was also determined by bacterial growth curve assay, biofilm formation assay, and Galleria mellonella infection model.

Results

There were three S. aureus SCVs (JP310 SCVs, JP1450 SCVs, JP1486 SCVs) that were selected from 41 rifampicin-resistant S. aureus. S. aureus SCVs colonies were tiny, with decreased pigmentation, and the hemolysis circle was not obvious compared with corresponding parental strains. And SCVs could not be restored to normal-colony phenotype after hemin, menaquinone, or thymidine supplementation. Different rpoB mutations occurred in JP1486 SCVs. Antimicrobial susceptibility testing revealed MICs of SCVs were higher than corresponding parental strains. Besides, the growth ability and virulence of SCVs were lower, and biofilm formation ability of which increased compared with parental strains.

Conclusion

S. aureus SCVs share the rifampicin resistance mechanisms with parental strains, although there were some differences in the position of rpoB mutations. Moreover, we found that the biological characteristics of SCVs were significantly different from corresponding parental strains. In contrast, decreased susceptibility to other antibiotics of SCVs was observed during phenotype switch. Furthermore, SCVs incur the fitness cost.

Keywords:

• Staphylococcus aureus
• small-colony variant
• rifampicin resistance
• resistance profile
• biological characteristic

Acknowledgments

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data Sharing Statement

All data generated or analyzed during this study are included in this published article.

Ethical Statement

The whole investigation protocols in this study were approved by The Ethics Committee of the First Affiliated Hospital of Wenzhou Medical University. There are no studies with humans or animals performed by any of the authors in this article. Informed consent was waived because this study with observational nature mainly focused on bacteria and did no interventions to patients.

Author Contributions

All authors contributed to data analysis, drafting or revising the article, have agreed on the journal to which the article will be submitted, gave final approval of the version to be published, and agree to be accountable for all aspects of the work. Xiangkuo Zheng and Renchi Fang contributed equally to this work and are the co-first authors of this article. Tieli Zhou and Chunquan Xu are joint corresponding authors.

Disclosure

The authors report no conflicts of interest in this work.