Cholera is a severe diarrheal disease caused by specific serogroups of Vibrio cholerae that are pathogenic to humans. The disease does not persist in a chronic state in humans or animals. The pathogen is naturally present as a free-living organism in the environment. Recently, it was suggested that egg masses of the nonbiting midge Chironomus sp. (Diptera) harbor and serve as a nutritive source for V. cholerae, thereby providing a natural reservoir for the organism. Here we report that V. cholerae O9, O1, and O139 supernatants lysed the gelatinous matrix of the chironomid egg mass and inhibited eggs from hatching. The extracellular factor responsible for the degradation of chironomid egg masses (egg mass degrading factor) was purified from V. cholerae O9 and O139 and was identified as the major secreted hemagglutinin/protease (HA/P) of V. cholerae. The substrate in the egg mass was characterized as a glycoprotein. These findings show that HA/P plays an important role in the interaction of V. cholerae and chironomid egg masses.Cholera is a severe diarrheal disease that causes the death of many thousands of people each year and affects the lives of millions. This disease is caused by specific serogoups of Vibrio cholerae that are pathogenic to humans (22). Since 1991, the world has witnessed extension of the seventh pandemic into South America and South Africa, as well as the appearance of a previously unknown pathogenic serogroup of V. cholerae (O139) (23). The disease is not found in a chronic state in humans or animals, indicating that its natural reservoir is environmental (11).Chironomids (Diptera; Chironomidae), the nonbiting midges, are the most widely distributed and frequently the most abundant insects in freshwater (1). Females lay egg masses on the water's edge, and each mass contains hundreds of eggs encased in a layer of gelatinous material. The presence of several thousand egg masses at one site is not unusual. In extreme cases, gelatinous layers several centimeters thick are formed (3,18).Recently, we suggested that chironomid egg masses are an intermediate host reservoir for V. cholerae. Chironomus egg masses collected from a waste stabilization pond settled out overnight as thousands of individual eggs, most of which did not hatch. V. cholerae O9 was isolated from the degraded egg masses. When new freshly collected egg masses were reinfected with V. cholerae O9, the egg masses were destroyed. V. cholerae grew on the chironomid egg masses as a nutritive source (4).Hemagglutinin/protease (HA/P) is one of the main secreted proteases of V. cholerae, and it is usually associated with the stationary phase and starvation (2). HA/P was purified from V. cholerae O1 (7) and O139 strains (17), as well as from non-O1 strains (10, 16). All HA/P purified enzymes showed both hemagglutination and proteolytic activities. Honda et al. (10) compared the purified HA/P from V. cholerae O1 and non-O1 strains. They found that the proteases (or hemagglutinins) derived from V. cholerae O1 and non-O1 strains were immunolog...