Abstract: Objective To investigate the in vitro susceptibilities of HSV-2 clinical strains isolated in Nanjing to acyclovir(ACV)and valaciclovir(VCV),and to develop a plaque assay to purify ACV-resistant HSV isolates.Methods HSV-2 strains were isolated in Vero cells from genital herpes lesions of patients attending our STD clinic from May 2005 to November 2005, and typed with monoclonal antibodies by direct immunofluorescence assay.The susceptibilities to ACV and VCV of HSV-2 isolates were determined by MTT assay.To induce drug-resistance, HSV reference and clinical strains were cultured in the presence of ACV(5 μg/mL).The ACV resistant strains were isolated by plaque purification.Results A total of 15 HSV-2 strains were isolated from clinical specimens.Their ICS to ACV and VCV ranged 0.0215-0.2799 μg/mL(mean 0.0766 μg/mL)and 0.0549-0.4575 μg/mL(mean 0.1605 μg/mL),respectively.No significant differences were found regarding the antiviral susceptibilities between HSV-2 strains isolated from patients with primary infection and those recurrent infection, or between those from patients with previous antiviral treatment and those without antiviral treatment(both P>0.05).No drug-resistant plaque was isolated from the HSV-2 clinical strains.Only one drug-resistant plaque was isolated from HSV-1 Sam strain with an IC₅₀ of 13.36 μg/mL, and nine from HSV-2 333 strain with an IC₅₀ of 1.04-5.08μg/mL.Conclusions All the 15 HSV-2 strains isolated in Nanjing are highly sensitive to acyclovir and valaciclovir, and no drug-resistant strains are found.Antiviral resistant HSV-2 strains can be rapidly obtained by in vitro drug-inducing and plaque purification.

Key words: Herpesvirus 2, human, Acyclovir, Valaciclovir, Microbial sensitivity tests