The pathogenicity of Staphylococcus aureus strains varies tremendously (as seen with animals). It is largely dependent on global regulators, which control the production of toxins, virulence, and fitness factors. Despite the vast knowledge of staphylococcal molecular genetics, there is still widespread dispute over what factors must come together to make a strain highly virulent. S. aureus NCTC8325 (RN1 and derivatives) is a widely used model strain for which an incomparable wealth of knowledge has accumulated in the almost 50 years since its isolation. Although RN1 has functional agr, sarA, and sae global regulators, it is defective in two regulatory genes, rsbU (a positive activator of SigB) and tcaR (an activator of protein A transcription), and is therefore considered by many to be a poor model for studies of regulation and virulence. Here, we repaired these genes and compared the resulting RN1 derivatives with other widely used strains, Newman, USA300, UAMS-1, and COL, plus the parental RN1, with respect to growth, extracellular protein pattern, hemolytic activity, protein A production, pigmentation, biofilm formation, and mouse lethality. The tcaR-repaired strain, showed little alteration in these properties. However, the rsbU-repaired strain was profoundly altered. Hemolytic activity was largely decreased, the exoprotein pattern became much more similar to that of typical wild-type (wt) S. aureus, and there was a surprising increase in mouse lethality. We note that each of the strains tested has a mutational alteration in one or more other regulatory functions, and we conclude that the repaired RN1 is a good model strain for studies of staphylococcal regulation and pathobiology; although strain Newman has been used extensively for such studies in recent years, it has a missense mutation in saeS, the histidine kinase component of the sae signaling module, which profoundly alters its regulatory phenotype. If this mutation were repaired, Newman would be considerably improved as a model strain.
