Ultrastructure of midgut events in the pathogenesis of Bacillus sphaericus strain SSII-1 infections of Culex pipiens quinquefasciatus larvae.

The fate of Bacillus sphaericus strain SSII-1 cells ingested by Culex pipiens quinquefasciatus (= C. pipiens fatigans, C. fatigans, C. quinquefasciatus of authors; Diptera: Culicidae) larvae and the cytological events preceding death of the host were observed using electron microscopy. Bacillus sphaericus cells were digested rapidly in the anterior and central midgut. The outer cell wall layer and cytoplasmic ground substance disappeared soon after ingestion. Cytolysosomes became prominent in midgut cells as these cells gradually separated from one another. All bacteria, including B. sphaericus, were confined within the peritrophic membrane until after death of the host. Digestion by the larval host is confirmed as a possible mechanism for release of B. sphaericus toxin from the bacterial cells.