Scientists pinpoint when harmless bacteria became flesh-eating monsters

April 18, 2014 1:59 AM

Bacterial diseases cause millions of deaths every year. Most of these bacteria were benign at some point in their evolutionary past, and we don’t always understand what turned them into disease-causing pathogens. In a new study, researchers have tracked down when this switch happened in one flesh-eating bacteria. They think the knowledge might help predict future epidemics.

Nassera W, Beresa SB, Olsen RJ et al. 2014. Evolutionary pathway to increased virulence and epidemic group A Streptococcus disease derived from 3,615 genome sequences. PNAS. doi: 10.1073/pnas.1403138111
We sequenced the genomes of 3,615 strains of serotype Emm protein 1 (M1) group A Streptococcus to unravel the nature and timing of molecular events contributing to the emergence, dissemination, and genetic diversification of an unusually virulent clone that now causes epidemic human infections worldwide. We discovered that the contemporary epidemic clone emerged in stepwise fashion from a precursor cell that first contained the phage encoding an extracellular DNase virulence factor (streptococcal DNase D2, SdaD2) and subsequently acquired the phage encoding the SpeA1 variant of the streptococcal pyrogenic exotoxin A superantigen. The SpeA2 toxin variant evolved from SpeA1 by a single-nucleotide change in the M1 progenitor strain before acquisition by horizontal gene transfer of a large chromosomal region encoding secreted toxins NAD+-glycohydrolase and streptolysin O. Acquisition of this 36-kb region in the early 1980s into just one cell containing the phage-encoded sdaD2 and speA2 genes was the final major molecular event preceding the emergence and rapid intercontinental spread of the contemporary epidemic clone. Thus, we resolve a decades-old controversy about the type and sequence of genomic alterations that produced this explosive epidemic. Analysis of comprehensive, population-based contemporary invasive strains from seven countries identified strong patterns of temporal population structure. Compared with a preepidemic reference strain, the contemporary clone is significantly more virulent in nonhuman primate models of pharyngitis and necrotizing fasciitis. A key finding is that the molecular evolutionary events transpiring in just one bacterial cell ultimately have produced millions of human infections worldwide.

Significance
Epidemics of microbial infections are a considerable threat to human and animal health. Analysis of 3,615 genome sequences, coupled with virulence studies in animals, permitted us to delineate the nature and timing of molecular events that contributed to an ongoing global human epidemic of infections caused by group A Streptococcus, the “flesh-eating” pathogen. We clarified decades-long uncertainty about the timing and sequence of genomic alterations that underpinned the global epidemic. Analyses of this type are crucial for developing better strategies to predict and monitor strain emergence and epidemics, formulate effective protective public health maneuvers, and develop or modify vaccines.

posted by Blasdelb (14 comments total)

I almost fell asleep reading Nassera W, Beresa SB, Olsen RJ et al. 2014. Evolutionary pathway to increased virulence and epidemic group A Streptococcus disease derived from 3,615 genome sequences but then I remembered that it involved flesh-eating monsters.
posted by twoleftfeet at 2:43 AM on April 18, 2014 [2 favorites]

Horizontal gene transfer is amazing.
posted by ocherdraco at 3:07 AM on April 18, 2014 [1 favorite]

. . . of Ulm.
 
posted by Herodios at 4:07 AM on April 18, 2014 [2 favorites]

I was sure it had something to do with the breakdown of the nucleus family.
posted by GenjiandProust at 5:10 AM on April 18, 2014 [5 favorites]

Also interesting, and somewhat related - tar balls from spilled oil teeming with flesh-eating bacteria.
posted by Slap*Happy at 5:47 AM on April 18, 2014 [5 favorites]

Now if this can only be applied to humans we can prevent the next zombie apocalypse.

I love that phrasing, "The NEXT zombie apocalypse."

Yep, we barely survived the last three, it was touch and go there for a while in the Uprising of '02...

;)
posted by misha at 7:19 AM on April 18, 2014 [1 favorite]

Well, this puts paid to my theory, which involved mysterious glowing meteorites.
posted by happyroach at 8:30 AM on April 18, 2014

Compared with a preepidemic reference strain, the contemporary clone is significantly more virulent. . . the molecular evolutionary events transpiring in just one bacterial cell ultimately have produced millions of human infections worldwide. . . the last genetic change, which made GAS a highly virulent bacteria, must have occurred in 1983.

That's just about the time the Science FUD industry began to turn its attention from pollution denial to climate change denial.

Perhaps this is Earth's immune system at work.



"Antibodies, Bones. . . . Ann (slap) Tee (slap) Bod (slap) Deez!" (slap)
posted by Herodios at 8:43 AM on April 18, 2014

This is neat, and good example of the Rube Golbergian nature of biology. The GAS strain in question had to acquire a few genes via phages, have a nucleotides change, AND pick up a chunk of DNA from some other bacteria, only THEN did it have the advantage over other strains, leading to that dramatic flip-flop of prevalence in the 80s. Maybe I'm missing it though, but the paper seems to skirt around the question of where the all the pieces finally came together, noting only that intercontinental spread was extremely rapid. They do use a lot of samples from Sweden and Finland, though it was unclear to me as to whether that was simply convenient or a deliberate choice.

Also,

Comparative Strain Virulence in Infected Nonhuman Primates

Science cannot progress without heaps!
posted by Panjandrum at 9:27 AM on April 18, 2014

Now if this can only be applied to humans we can prevent cause the next zombie apocalypse.
posted by Renoroc at 9:34 AM on April 18, 2014 [1 favorite]

So, my takeaway is that we live in an immense sea of bacteria, any of which could evolve to eat us at any moment. This line of thought is not making for a cheerful weekend.
posted by GenjiandProust at 9:51 AM on April 18, 2014 [3 favorites]

We have always lived in this sea. That's why we have evolved defenses, like skin and the immune system. They save us from getting eaten, most of the time.
posted by hat_eater at 1:02 PM on April 18, 2014 [2 favorites]

Yep. That's why we're doughnuts. We let the sea of bacteria flow through us, through the hole in our middle, taking advantage of the helpful things it can do while not letting it get to our precious, precious insides. (The GI tract is not our insides; it's a special kind of outside. Same with the lungs, the vagina, the ear canal, etc.)
posted by ocherdraco at 2:28 PM on April 18, 2014 [1 favorite]

In other news: Concrete-Dissolving Bacteria Are Destroying Our Nation's Sewers
posted by homunculus at 12:27 AM on April 20, 2014