Persistent Infections of Staphylococcus Aureus: An Insight into the Formation of Small Colony Variant (SCV)

By: Hui-min Neoh (

One of the strategies used by the gram positive bacteria Staphylococcus aureus to resist antibiotics is by the formation of small colony variants (SCV). SCVs has been linked to persistent and recurrent infections, and its slow growing trait compared to its normal-sized colony (NC) counterpart presents challenges in diagnosis and treatment. In this study, we aim to investigate factors that give rise to the vraS and graR mutation-driven colony size changes in S. aureus.

We previously reported that glycopeptide induction seems to cause the SCV phenotype of tested strains to change into NCs. Acid and alkaline induction did not cause colony phenotype changes. Oxidative stress caused some NCs to change into SCVs in all strains. Strain A (predominantly SCVs) demonstrated persistent infection in the C. elegans infection model; strain B which are predominantly NCs killed the worms in just a short duration of the assay (2 days).

In this current update, we observe that colony phenotypes are stable post storage at -800C. Scanning electron microscopy revealed that small colony variants are indeed smaller in cell size diameter compared to normal-sized colonies. We also note that propagation of SCVs from strain A will produce SCVs only, while propagation of either SCVs or NCs from strain B will all produce mixture of SCVs and NCs.