Ecole Polytechnique de Lausanne. Switzerland: “Living-dead” bacteria exist in limbo: biologically
active but not proliferating. Buried in this zombie state,
disease-causing bacteria could come back from the dead to re-infect
patients. Researchers at EPFL have produced the first evidence of this
strange phenomenon in tuberculosis, suggesting new avenues for
treatment.
Tuberculosis affects over 12 million people
globally, and is usually treated with a course of four drugs over
several months. However, even after completing the treatment, many
patients suffer relapses. Based on studies of harmless environmental
bacteria, scientists think that the tuberculosis bacterium, Mycobacterium tuberculosis,
retreats into a bizarre “zombie” state in the patient’s body, and comes
back to life when the conditions permit. Scientists at EPFL have now
made the first experimental observation of M. tuberculosis in
this zombie state, which seems to be amplified by stressful conditions
such as attacks from the host’s immune system. The work, which points to
entirely new pathways for treating tuberculosis, is published in Cell Host & Microbe.
Living
bacteria divide and proliferate; if not, they are considered dead.
However, some bacteria can go into a strange, in-between state where
they are biologically active – producing energy and making proteins –
but do not divide. “It’s a kind of living-dead, zombie existence,” says
John McKinney, whose postdoc, Giulia Manina, led the study on M. tuberculosis.
“The bacteria are somewhat active, but they’re neither growing nor
dividing. We refer to this state as ‘Non-Growing but Metabolically
Active’ or ‘NGMA’”.
This state is thought to underlie the relapse of tuberculosis: when the population of M. tuberculosis
infecting a patient is faced with an aggressive antibiotic regimen, a
part of it falls into this zombie state as a defense mechanism. However,
there has been little evidence to support this theory, partly because
experimental techniques for studying bacterial populations usually
depend on the bacteria actually growing in the first place.
There
is, however, a way in: the zombie bacteria are still metabolically
active, which means that they keep making new proteins. Manina exploited
this with a technique that can tag and track a gene that turns on when M. tuberculosis
makes new proteins. The technique, developed by the lab’s senior
scientist, Neeraj Dhar, tags the gene with a fluorescent molecule that
can be tracked with a microscope over time.
Manina grew the
gene-tagged bacteria under different stressful conditions such as
limited nutrients, antibiotics, and also conditions that simulate an
attack by the patient’s immune system. In addition, bacteria were taken
from the lungs of infected mice at different stages of infection.
Tracked with the fluorescent gene-tag, the bacteria were tested to see
how the different conditions affected them over time.
The researchers found that M. tuberculosis
responds to stressful conditions like immune attacks or lack of
nutrients by diversifying its population and pushing some it into the
zombie state. “This means that this state could be a defensive response
to the patient’s immunity,” says Manina. “What is unclear at the moment
is whether it is an active bacterial response, or just a consequence of
an immune attack.”
In addition, the researchers found that the
lungs of mice with tuberculosis contained an unexpectedly large
subpopulation of zombie cells. Surprisingly, these cells were not found
in mice that were genetically modified to lack an immune system, further
suggesting that the immune system and the zombie state are somehow
related.
The scientists think that this phenomenon could
contribute to the bacterium’s antibiotic resistance, and that immunity
and drug pressure may contribute to the persistence and relapse of
tuberculosis. Consequently, these findings could be used to develop
novel drug strategies against M. tuberculosis, especially
targeting the non-replicating, zombie cells. The group is currently
interested in optimizing their gene-tracking techniques and getting a
clearer picture of the underlying physiology of the zombie state.
Reference
Manina G, Dhar N, McKinney JD. Stress
and Host Immunity Amplify Mycobacterium tuberculosis Phenotypic
Heterogeneity and Induce Nongrowing Metabolically Active Forms. Cell Host & Microbe (2015), http://dx.doi.org/10.1016/j.chom.2014.11.016