UC San Diego

Exotoxins are virulence factors produced by some bacteria and are sometimes encoded by mobile genetic elements like bacteriophage (phage). Phage are viruses of bacteria that carry genetic information and through horizontal gene transfer this genetic information is sometimes transferred to the bacteria they infect. When virulence genes are transferred to an avirulent bacterium, this bacterium can be transduced to virulence and gains the capability to cause disease. Through this process between the phage and bacterium, a whole host of novel human pathogens may emerge. This has significant implications in the area of human health. Using exotoxin-specific polymerase chain reaction (PCR), quantitative real-time PCR (QPCR), and Southern dot blot assays, over 500 environmental samples were screened for the presence of four phage-encoded exotoxin genes : cholera toxin (ctx), diphtheria toxin (dtx), Staphylococcus enterotoxin A (sea), and shiga toxin (stx). A minimum of one gene was found in 1̃5% of the samples screened. These genes were found in the phage fraction of some of the positive samples; suggesting these genes are mobile within the environment. The gene most frequently observed in the environmental samples was stx. The samples that were positive for an exotoxin gene were spread throughout environmental type and there was no correlation with presence of exotoxin genes and sample type. Identification of some bacterial environmental isolates from samples positive for an exotoxin gene, showed that the bacterium carrying the exotoxin gene was not the known pathogen and was indeed an alternative host for the gene(s). This was shown for environmental isolates carrying the sea and stx genes. The data generated from this dissertation indicate that exotoxin genes are widespread in the environment and that these genes can be carried by alternative hosts