Recovery of cell wall deficient forms (L-forms) of the fish pathogens Aeromonas salmonicida and Yersinia ruckeri

Cell wall deficient/defective forms (L-forms) of Aeromonas salmonicida and Yersinia ruckeri were induced by means of antibiotics, namely benzylpenicillin and carbenicillin, polyclonal antisera and lysozyme in combination with a nutrient rich medium containing sucrose (10% w/v) and horse serum (10% v/v) as osmotic support and detoxifying agent, respectively. L-forms produced characteristic minute colonies with distinctive globose or diffuse edges. These colonies contained cells of varying morphologies, including spheres of 0.3 to 3.0 µm in diameter, and small rods. The cells could pass through the pores of 0.45 µm filters. L-forms of A. salmonicida and Y. ruckeri were recovered in low numbers from clinically diseased salmonids. Moreover, L-forms of Y. ruckeri contributed to disease processes.