A wide variety of bacterial pathogens are currently being studied at the CVD. Basic studies on the molecular pathogenesis of Vibrio cholerae have led to the development of a recombinant attenuated oral cholera vaccine, which is now licensed in several countries. Salmonella Typhi and Shigella strains have been attenuated by recombinant techniques and are being evaluated as live vaccines for the prevention of typhoid fever and shigellosis, and as vector vaccines for delivery of antigens of enterotoxigenic Escherichia coli and other bacterial, viral, and parasitic pathogens (see combination vaccines). Extensive work is being conducted on the molecular pathogenesis of disease due to enteropathogenic E. coli (EPEC), enterohemorrhagic E. coli O157:H7 (EHEC), and enteroaggregative E. coli (EAEC). The studies on E. coli have led to the development of a number of novel diagnostic DNA probes which have used in epidemiological studies at CVD-Chile and elsewhere throughout the world. Clinical trials of acellular pertussis vaccines have been conducted by CVD clinical investigators and laboratory investigators are currently cloning genes encoding antigens of Bordetella pertussis for use in combination vaccines. Other bacterial vaccines undergoing clinical evaluation at the CVD include vaccines for the prevention of disease due to group A Streptococcus, S. pneumoniae (at CVD-Chile), Clostridium difficile, Helicobacter pylori and Borrelia burgdorferi (Lyme Disease).