Screening of herpes simplex virus type 1 isolates for acyclovir resistance using DiviTum® assay

Rapid alternative methods are required to evaluate easily acyclovir (ACV) sensitivity of clinical herpes simplex virus (HSV) isolates. The objective of this study was to screen 54 ACV-sensitive and 41 ACV-resistant clinical HSV-1 isolates, well characterized by phenotypic and genotypic methods, for the phosphorylation activity of the viral thymidine kinase (TK) using a commercially available and modified non-radioactive DiviTum® test on the basis of an indirect enzyme linked immunosorbent assay. The ACV-sensitive HSV-1 isolates had high TK activity values between 31.5±6.4 DiviTum® Units per liter (DU/L) and 487.4±60.1 DU/L. The mean activity of all ACV-sensitive isolates was calculated as 212.3±15.7 DU/L. By contrast, the mean activity of all ACV-resistant HSV-1 isolates was significantly lower at 5.5±1.3 DU/L. Out of the 41 ACV-resistant HSV-1 isolates, 38 had no or very low phosphorylation activities of the viral TK between 0 DU/L and 9.3±3.2 DU/L. The remaining three ACV-resistant viral isolates had TK activities between 44.6±5.1 DU/L and 80.9±13.3D U/L. In conclusion, the modified DiviTum® test can be used to screen HSV-1 isolates for their sensitivity to ACV. Acyclovir-sensitive HSV-1 isolates show TK activities >30 DU/L and ACV-resistant isolates have activity values <10 DU/L. However, single ACV-resistant HSV-1 isolates can have TK activity values >30 DU/L. These strains are most likely ACV-resistant TK-altered mutants, but no evidence was provided for an alteration of the TK.