We analyze acousto-optic (AO) diffraction occurring at the light wavelength of a so-called "isotropic point" in AgGaS2 crystals. It is shown that a natural optical activity, which manifests itself as circular birefringence at the isotropic point, can lead to two different types of AO Bragg diffraction, isotropic and anisotropic. The first type concerns the interaction of circularly polarized optical eigenwaves with the same sign of rotations of their electric-field vectors. In the case of the second diffraction type, the AO interaction takes place between the circularly polarized waves having opposite signs of electric-field rotations. Relations for effective elasto-optic (EO) coefficients are derived for different types of AO diffraction, including a collinear one. The main peculiarities of the latter diffraction type are revealed. The modules of all the EO coefficients are determined experimentally for the AgGaS2 crystals, using a Dixon-Cohen method. Finally, we evaluate AO figures of merit referring to the AO interactions of circularly polarized optical eigenwaves at the optical wavelength of the isotropic point.