Growth of the fish-pathogenic fungus, Ichthyophonus hoferi, measured by conductimetry and microscopy

Abstract. The influence of pH and glucose on growth of the fish pathogenic fungus Ichthyophonus hoferi was measured by indirect conductimetry and microscopy. Using indirect conductimetry, distinct resistance curves were produced within 3 days at 15 †C, Conductimetric changes were enhanced when the media were supplemented with glucose. Low pH (3–4) was essential to ensure continuous growth of the fungus, which could be further facilitated by supplementation with glucose. At pH 7, growth occurred when the fungus had been pre-cultured at low pH. Growth of the fungus ceased after three successive transfers at pH 7 and this explains why earlier attempts to subculture have failed. Only vegetative stages of I. hoferi were observed; i.e. no conidia. At pH 7, growth was mainly observed as spherical bodies of varying size, with large numbers of nuclei, and thick-walled spherical multinucleate bodies. Hyphal growth was abundant at low pH. The alternating pH simulates the natural conditions experienced by the fungus on transfer from the stomach (low pH) to the muscle (neutral pH) and reproducible sub-culturing is obtained when mimicking this pH cycle or when a low pH is maintained.