Principles of viable preservation of parasitic protozoa

Abstract

Viable preservation is fundamentally different from serial passage as a way of maintaining Protozoa in the laboratory. In serial passage there is continuous selection of a reproducing population so that biological characteristics deviate from those representative of the original population towards those of a population adapted to the special conditions of the passage. Viable preservation arrests reproduction and so, this selection. Viably preserved material, therefore, deserves to be defined by a special term, ‘stabilate’, a stabilate being defined as a population of organisms viably preserved on a unique occasion. Materials maintained by serial passage are appropriately called ‘strains’. Stabilates of Protozoa may be set up by cryopreservation. Given good technique, successive samples of a stabilate are consistent in their behaviour in any given experimental situation. The proportion of organisms surviving cryopreservation may be only small but, given suspensions of high initial potency, Stabilates of adequate potency are usually obtainable. Biological characteristics do not appear to be altered. Inconsistencies in behaviour of protozoal populations in experiment may be due to their being heterogeneous, several different populations of diverse capability having been transmitted together in the serial passage. Clones of known homogeneity have advantages as experimental materials and are conveniently preserved as stabilates.