Ultrastructure of Prototheca zopfii in bovine granulomatous mastitis

Mammary glands from cows with protothecal mastitis were examined by light and electron microscopy at 6, 13, 20, and greater than 180 days after infection. With increasing time, there were increases in severity of granulomatous inflammation, number of endospores and sporangia, and ratio of degenerate to intact algae. Algae were found in macrophages but were not seen in neutrophils, epithelial cells, or myoepithelial cells. Macrophages containing algae were markedly enlarged, chiefly from reduplication of the Golgi complex and its associated vesicles. Intracellular algae were degenerate and consisted of intact cell wall profiles which contained membrane fragments but lacked nuclei and cytoplasmic organelles. Degenerate algae in vitro had thin cell walls and did not undergo internal lysis. Cell wall material of intracellular algae stained as an acidic, nonsulfated, carboxylated glycoprotein. These findings suggest that intracellular Prototheca zopfii degenerate by progressive lysis of internal organelles with persistence of cell wall glycans and that development of aberrant cell wall forms occurs as a defective response by host macrophages.