© THOM GRAVES
T
he cultures reminded him of Swiss cheese, says Egbert
Hoiczyk. It was 2009 and the microbiologist and his col-
leagues at Johns Hopkins University were growing the
motile bacterium Myxococcus xanthus on agar plates. “What we
observed was something quite bizarre,” recalls Hoiczyk, now at
the University of Sheffield in the UK.
Bacteria had grown out in a yellowish smear coating the plates,
but then holes appeared in the microbial film. The empty spaces
looked like plaques created by bacteria-killing viruses, so Hoic-
zyk’s team concluded there must be a viral invader in their cul-
tures. When the researchers scraped the cleared areas and exam-
ined the material under an electron microscope, they observed
32-nanometer-wide structures resembling 12-sided dice—the
right size and shape for a bacteriophage.
Hoiczyk, undergraduate Alan Lam, and graduate student Col-
leen McHugh set out to isolate the virus. But something was off.
When the researchers attempted to purify the phages by centri-
fuging them in standard cesium chloride gradients, where viruses
usually show up in the middle, the particles always dropped to thebottom of the columns. The investigators eventually realized they
weren’t purifying viruses, but tiny, iron-laden nanocompartments
native to the bacteria. (They never did figure out why the holes
appeared in the cultures.)
The structures were relatively new to science, having been
described for the first time just a year earlier in the bacterium
Thermotoga maritima.^1 At 25–66 nm across, they are typically too
small to notice unless you’re looking for them, Hoiczyk explains.
But in recent years, thanks to modern genomics and bioinfor-
matics, scientists have found nanocompartments across a range
of bacterial phyla.
It’s not the first time researchers have identified what is essen-
tially an organelle in Bacteria, a kingdom traditionally thought to
lack subcellular compartments. Decades earlier, scientists had first
described larger structures called microcompartments—100–600
nm in diameter—that appeared in micrographs of cyanobacteria,
though researchers are only building a detailed understanding of
those compartments today. “It’s becoming accepted, a lot, in the
last 10 or 20 years that the prokaryotic cytoplasm is highly orga-Bacteria, traditionally thought to lack organelles, get a metabolic boost
from geometric compartments that act as cauldrons for chemical reactions.BY AMBER DANCEShell Games
12.2018 | THE SCIENTIST 37