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Grazing protozoa and the evolution of the Escherichia coli O157:H7 Shiga toxin-encoding prophage.

Steinberg KM, Levin BR - Proc. Biol. Sci. (2007)

Bottom Line: Why then does E. coli O157:H7 code for virulence determinants, like the Shiga toxins (Stxs), responsible for the morbidity and mortality of colonized humans?Here, we test the hypothesis that the carriage of the Stx-encoding prophage of E. coli O157:H7 increases the rate of survival of E. coli in the presence of grazing protozoa, Tetrahymena pyriformis.In the presence but not the absence of Tetrahymena, the carriage of the Stx-encoding prophage considerably augments the fitness of E. coli K-12 as well as clinical isolates of E. coli O157 by increasing the rate of survival of the bacteria in the food vacuoles of these ciliates.

View Article: PubMed Central - PubMed

Affiliation: Graduate Program in Population Biology, Ecology and Evolution, Emory University, Atlanta, GA 30322, USA. kmeltz@emory.edu

ABSTRACT
Humans play little role in the epidemiology of Escherichia coli O157:H7, a commensal bacterium of cattle. Why then does E. coli O157:H7 code for virulence determinants, like the Shiga toxins (Stxs), responsible for the morbidity and mortality of colonized humans? One possibility is that the virulence of these bacteria to humans is coincidental and these virulence factors evolved for and are maintained for other roles they play in the ecology of these bacteria. Here, we test the hypothesis that the carriage of the Stx-encoding prophage of E. coli O157:H7 increases the rate of survival of E. coli in the presence of grazing protozoa, Tetrahymena pyriformis. In the presence but not the absence of Tetrahymena, the carriage of the Stx-encoding prophage considerably augments the fitness of E. coli K-12 as well as clinical isolates of E. coli O157 by increasing the rate of survival of the bacteria in the food vacuoles of these ciliates. Grazing protozoa in the environment or natural host are likely to play a significant role in the ecology and maintenance of the Stx-encoding prophage of E. coli O157:H7 and may well contribute to the evolution of the virulence of these bacteria to colonize humans.

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Simulation result changes in the total density of bacteria and protozoa and the ratio of prophage-bearing (Tox+) and prophage-free (Tox−) E. coli 0157:H7. Parameters γB=2×10−6, γM=2×10−6 and γT=5×10−7; e=10−4, k=10−7. The initial densities of the populations were B=109, M=T=5×104 and P=2×102 cells ml−1. Open squares, density of Tetrahymena; filled squares, density of bacteria; filled triangles, ratio (Tox+/Tox−).
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fig6: Simulation result changes in the total density of bacteria and protozoa and the ratio of prophage-bearing (Tox+) and prophage-free (Tox−) E. coli 0157:H7. Parameters γB=2×10−6, γM=2×10−6 and γT=5×10−7; e=10−4, k=10−7. The initial densities of the populations were B=109, M=T=5×104 and P=2×102 cells ml−1. Open squares, density of Tetrahymena; filled squares, density of bacteria; filled triangles, ratio (Tox+/Tox−).

Mentions: With the chosen parameter values and initial densities of bacteria and protozoa, as with the real data in figure 1a,b in our article, the ratio of Tox+/Tox− increases when the density of protozoa increases. As the total density of the bacteria declines, the rate of ascent of the protozoa population declines and, within a few days, the density of both bacteria and protozoa levels off (figure 6).


Grazing protozoa and the evolution of the Escherichia coli O157:H7 Shiga toxin-encoding prophage.

Steinberg KM, Levin BR - Proc. Biol. Sci. (2007)

Simulation result changes in the total density of bacteria and protozoa and the ratio of prophage-bearing (Tox+) and prophage-free (Tox−) E. coli 0157:H7. Parameters γB=2×10−6, γM=2×10−6 and γT=5×10−7; e=10−4, k=10−7. The initial densities of the populations were B=109, M=T=5×104 and P=2×102 cells ml−1. Open squares, density of Tetrahymena; filled squares, density of bacteria; filled triangles, ratio (Tox+/Tox−).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2211389&req=5

fig6: Simulation result changes in the total density of bacteria and protozoa and the ratio of prophage-bearing (Tox+) and prophage-free (Tox−) E. coli 0157:H7. Parameters γB=2×10−6, γM=2×10−6 and γT=5×10−7; e=10−4, k=10−7. The initial densities of the populations were B=109, M=T=5×104 and P=2×102 cells ml−1. Open squares, density of Tetrahymena; filled squares, density of bacteria; filled triangles, ratio (Tox+/Tox−).
Mentions: With the chosen parameter values and initial densities of bacteria and protozoa, as with the real data in figure 1a,b in our article, the ratio of Tox+/Tox− increases when the density of protozoa increases. As the total density of the bacteria declines, the rate of ascent of the protozoa population declines and, within a few days, the density of both bacteria and protozoa levels off (figure 6).

Bottom Line: Why then does E. coli O157:H7 code for virulence determinants, like the Shiga toxins (Stxs), responsible for the morbidity and mortality of colonized humans?Here, we test the hypothesis that the carriage of the Stx-encoding prophage of E. coli O157:H7 increases the rate of survival of E. coli in the presence of grazing protozoa, Tetrahymena pyriformis.In the presence but not the absence of Tetrahymena, the carriage of the Stx-encoding prophage considerably augments the fitness of E. coli K-12 as well as clinical isolates of E. coli O157 by increasing the rate of survival of the bacteria in the food vacuoles of these ciliates.

View Article: PubMed Central - PubMed

Affiliation: Graduate Program in Population Biology, Ecology and Evolution, Emory University, Atlanta, GA 30322, USA. kmeltz@emory.edu

ABSTRACT
Humans play little role in the epidemiology of Escherichia coli O157:H7, a commensal bacterium of cattle. Why then does E. coli O157:H7 code for virulence determinants, like the Shiga toxins (Stxs), responsible for the morbidity and mortality of colonized humans? One possibility is that the virulence of these bacteria to humans is coincidental and these virulence factors evolved for and are maintained for other roles they play in the ecology of these bacteria. Here, we test the hypothesis that the carriage of the Stx-encoding prophage of E. coli O157:H7 increases the rate of survival of E. coli in the presence of grazing protozoa, Tetrahymena pyriformis. In the presence but not the absence of Tetrahymena, the carriage of the Stx-encoding prophage considerably augments the fitness of E. coli K-12 as well as clinical isolates of E. coli O157 by increasing the rate of survival of the bacteria in the food vacuoles of these ciliates. Grazing protozoa in the environment or natural host are likely to play a significant role in the ecology and maintenance of the Stx-encoding prophage of E. coli O157:H7 and may well contribute to the evolution of the virulence of these bacteria to colonize humans.

Show MeSH
Related in: MedlinePlus