Reservoir of Bacterial Exotoxin Genes in the Environment

Abstract

Many bacteria produce secreted virulence factors called exotoxins. Exotoxins are often encoded by mobile genetic elements, including bacteriophage (phage). Phage can transfer genetic information to the bacteria they infect. When a phage transfers virulence genes to an avirulent bacterium, the bacterium can acquire the ability to cause disease. It is important to understand the role played by the phage that carry these genes in the evolution of pathogens. This is the first report of an environmental reservoir of a bacterial exotoxin gene in an atypical host. Screening bacterial isolates from the environment via PCR identified an isolate with a DNA sequence >95% identical to the Staphylococcus aureus enterotoxin A gene (sea). 16S DNA sequence comparisons and growth studies identified the environmental isolate as a psychrophilic Pseudomonas spp. The results indicate that the sea gene is present in an alternative bacterial host, providing the first evidence for an environmental pool of exotoxin genes in bacteria.

Evolutionary aspects of toxin-producing bacteria

“…Similar findings have been observed for other environmental bacteria [259]. The presence of particulate matter in aqueous environments was found to increase potential pathogenic bacteria-phage interactions and thereby stimulate transduction [6,259,260], the outcome of which has been proposed to supply virulence factors, such as toxins, that benefit both the bacteria and phage and contribute to their evolution as pathogens [261].…”

“…The emergence, evolution, and spread of antibiotic resistance has been firmly linked to HGT in aquatic environments [244,245]; similar findings have emerged regarding toxin genes [1,6,[82][83][84][85]. As mentioned previously, there is now strong genetic evidence that epidemic strains of V. cholerae, the etiological agent of cholera, have acquired CT, TCP, and other virulence factor genes through bacteriophage transduction and conjugal transfer in aquatic environments, possibly through V. mimicus reservoirs [7,[15][16][17]217].…”

“…Numerous studies have shown that the concentrations of bacteriophages in natural aquatic environments can be high enough to be conducive for phage transduction, but they can be especially high in contaminated water systems [6,76,[82][83][84][85]. The emergence, evolution, and spread of antibiotic resistance has been firmly linked to HGT in aquatic environments [244,245]; similar findings have emerged regarding toxin genes [1,6,[82][83][84][85].…”

“…Phages or lysogenic bacterial strains carrying toxin prophages might serve as the natural reservoir for toxin genes, with lysogenization and conversion processes taking place not only in the human or animal host (such as in the lungs or intestines), but also in the external environment, such as in food, water, soil, or other vectors or hosts (e.g., insects, amoeba, or plants) [1,[6][7][8].…”

A Perspective on the Global Pandemic of Waterborne Disease

“…This should not be surprising, given that cattle are the primary reservoir of these bacteria and that high prevalence rates of EHEC in cattle herds have been frequently documented [48][49][50]. These and similar findings of environmental reservoirs for a variety of virulence genes in other bacterial species [51,52] are worrisome.…”