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Expression of Cholera Toxin under Non-AKI Conditions in Vibrio cholerae El Tor Induced by Increasing the Exposed Surface of Cultures

Joaquín Sánchez,^1* Gerardo Medina,^2, Thomas Buhse,³ Jan Holmgren,⁴ and Gloria Soberón-Chavez⁵

Facultad de Medicina,¹ Centro de Investigaciones Químicas, UAEM,³ Departamento de Microbiología Molecular, Instituto de Biotecnología, Universidad Nacional Autónoma de México, Cuernavaca,² Instituto de Investigaciones Biomédicas, UNAM, Circuito Escolar, Ciudad Universitaria, México D. F., México,⁵ Department of Medical Microbiology and Immunology, Göteborg University, Göteborg, Sweden⁴

Received 26 August 2003/ Accepted 18 November 2003

The regulatory systems controlling expression of the ctxAB genes encoding cholera toxin (CT) in the classical and El Tor biotypes of pathogenic Vibrio cholerae have been characterized and found to be almost identical. Notwithstanding this, special in vitro conditions, called AKI conditions, are required for El Tor bacteria to produce CT. The AKI conditions involve biphasic cultures. In phase 1 the organism is grown in a still tube for 4 h. In phase 2 the medium is poured into a flask to continue growth with shaking. Virtually no expression of CT occurs if this protocol is not followed. Here we demonstrated that CT expression takes place in single-phase still cultures if the volume-to-surface-area ratio is decreased, both under air and under an inert atmosphere. The expression of key genes involved in the regulation of CT production was analyzed, and we found that the expression pattern closely resembles the in vivo expression pattern.

Present address: Universidad Autónoma de Baja California, Mexicali, México.