Shiga Toxin and Shiga Toxin-Encoding Phage Do Not Facilitate Escherichia coli O157:H7 Colonization in Sheep
Nancy A. Cornick, Amy F. Helgerson, and Vijay Sharma
Applied and Environmental Microbiology, 73(1), Jan. 2007 344-346
In this paper, the big question of the prevalence of shiga toxin bearing Escherichia coli in the ruminant population is addressed. Shiga Toxin genes are generally carried by a phage (bacterial virus) that integrates into the E. coli genome. These genes are a major factor in creating a serious human pathogen from an otherwise benign intestinal commensal. In other words, bad bacteria from good bacteria.
Basically, these shiga toxin carrying bacteria don't cause any noticeable problem for ruminants - fore-gut fermenters - as opposed to what they do to humans. However, they are carried at relatively high rates by ruminants. This brings them into the food supply both as contaminants of meat, and then, as manure, as contaminants of organic produce.
So, why are they so prevalent? What about ruminants and shiga toxin creates such a high occurrence, by which many cattle and sheep carry these human pathogens?
In this paper, there is an attempt to see whether the shiga toxin creates higher rates/longer periods of shedding by sheep. It is a proper and elegant study, in that the parent strain (a human pathogen, taken from a clinical case) carries the phage and is then subjected to mutagenesis, to remove the toxin alone, or also the phage as a whole. They inoculated different sheep with different strains, and even tried co-inoculations.
They did a power study, and determined that they had an 80% expectation of finding a real effect (if there was one) using 8 sheep per group. So, that's what they did. This feels like a serious footnote - there is a 20% chance that their conclusions are wrong. However, this sort of analysis really belongs in many more studies than have it.
First, we should highlight that _all the inoculated strains_ disappeared over time from the animals. They were using antibiotic resistance as markers and running the experiment for several months under non-sterile conditions (but BL2 to prevent escape of the pathogen). The animals already had living flora. So, we are really looking at differential loss among losers, not loss or success among something that would be a stable floral component. The experiment only went a couple months; a stable strain should really last longer than that.
They also only sampled shedding - not whether the animals were carrying the bacteria internally - and they acknowledge this weakness in the text. They didn't look at colonization dosage, either, or movement of the toxin from strain to strain, or at animals who were coinfected with BLV, testing a variety of hypotheses about the effects of the shiga toxin. They also didn't look at survival in feces or anything about that part of the bacterial ecology cycle.
However, the one hypothesis they sought to test, they found no particular effect. There was lots of variability among individuals and no distinguishable bias towards or away from the shiga carrying strains. Certainly, it would have been interesting to see an effect.
This doesn't necessarily mean that the toxin is 'selfish' and not helping the bacteria; however, it may mean that the effect is really found elsewhere in the life cycle, or requires other genes to be coordinated with it. It might be that there is a population of bacteria that have some genes where shiga helps them colonize ruminants, and that the phage is shed by these bacteria and infects O157:H7 type strains, producing a human pathogen, while the human pathogen itself isn't favored in the animals. Then again, it might be something completely different. We need more good little experiments - hopefully, with a slightly higher power than 80%, but we'll take what we can get - to find out how this all works.
Nancy A. Cornick, Amy F. Helgerson, and Vijay Sharma
Applied and Environmental Microbiology, 73(1), Jan. 2007 344-346
In this paper, the big question of the prevalence of shiga toxin bearing Escherichia coli in the ruminant population is addressed. Shiga Toxin genes are generally carried by a phage (bacterial virus) that integrates into the E. coli genome. These genes are a major factor in creating a serious human pathogen from an otherwise benign intestinal commensal. In other words, bad bacteria from good bacteria.
Basically, these shiga toxin carrying bacteria don't cause any noticeable problem for ruminants - fore-gut fermenters - as opposed to what they do to humans. However, they are carried at relatively high rates by ruminants. This brings them into the food supply both as contaminants of meat, and then, as manure, as contaminants of organic produce.
So, why are they so prevalent? What about ruminants and shiga toxin creates such a high occurrence, by which many cattle and sheep carry these human pathogens?
In this paper, there is an attempt to see whether the shiga toxin creates higher rates/longer periods of shedding by sheep. It is a proper and elegant study, in that the parent strain (a human pathogen, taken from a clinical case) carries the phage and is then subjected to mutagenesis, to remove the toxin alone, or also the phage as a whole. They inoculated different sheep with different strains, and even tried co-inoculations.
They did a power study, and determined that they had an 80% expectation of finding a real effect (if there was one) using 8 sheep per group. So, that's what they did. This feels like a serious footnote - there is a 20% chance that their conclusions are wrong. However, this sort of analysis really belongs in many more studies than have it.
First, we should highlight that _all the inoculated strains_ disappeared over time from the animals. They were using antibiotic resistance as markers and running the experiment for several months under non-sterile conditions (but BL2 to prevent escape of the pathogen). The animals already had living flora. So, we are really looking at differential loss among losers, not loss or success among something that would be a stable floral component. The experiment only went a couple months; a stable strain should really last longer than that.
They also only sampled shedding - not whether the animals were carrying the bacteria internally - and they acknowledge this weakness in the text. They didn't look at colonization dosage, either, or movement of the toxin from strain to strain, or at animals who were coinfected with BLV, testing a variety of hypotheses about the effects of the shiga toxin. They also didn't look at survival in feces or anything about that part of the bacterial ecology cycle.
However, the one hypothesis they sought to test, they found no particular effect. There was lots of variability among individuals and no distinguishable bias towards or away from the shiga carrying strains. Certainly, it would have been interesting to see an effect.
This doesn't necessarily mean that the toxin is 'selfish' and not helping the bacteria; however, it may mean that the effect is really found elsewhere in the life cycle, or requires other genes to be coordinated with it. It might be that there is a population of bacteria that have some genes where shiga helps them colonize ruminants, and that the phage is shed by these bacteria and infects O157:H7 type strains, producing a human pathogen, while the human pathogen itself isn't favored in the animals. Then again, it might be something completely different. We need more good little experiments - hopefully, with a slightly higher power than 80%, but we'll take what we can get - to find out how this all works.
posted by BenK at 2:46 PM
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