Bacteria
able to shed their cell wall assume new, mostly spherical shapes. Researchers
have shown that these cells, known as L-forms, are not only viable but that
their reproductive mechanisms may even correspond to those of early life forms.
L-forms
may be created when cell wall-active antibiotics are used against Listeria,
which can cause severe cases of food infection. The drugs cause the bacteria to
shed their cell wall -- the attack point for the medication. The resulting
vesicles are enclosed only by a cytoplasmic membrane, which renders those
antibiotics ineffective.
L-forms
occur not only in Listeria, but have
been described for many others bacteria. Moreover, Mycoplasma, Rickettsiae and Chlamydiae,
all of which are human pathogens, also lack a stable cell wall. Yet, L-forms
are viable only under certain conditions; if the osmotic conditions are not
suitable or change rapidly, the cells become unstable and may burst.
Until
now, researchers have not been able to clearly demonstrate viability of the
resulting vesicles. But recent experiments by Loessner and his group have now
show that L-forms are an independent form of life that can multiply
indefinitely. "Our observations are not artefacts, but rather represent an
alternative form of bacterial life," emphasises Loessner. The results of
their latest study have just been published in the journal Nature
Communications.
In
their study, the researchers show that small membrane vesicles are formed by
invagination into larger intracellular vesicles, thereby receiving cytoplasmic
content and producing viable progeny.
When
the cytoplasmic membrane of an L-Form cell invaginates into the cytoplasm, the
mother cell produces a vesicle filled with substances from the surrounding
medium. Consequently, this primary vesicle lacks cellular components and in
particular the genetic material. However, by forming secondary vesicles through
membrane extrusion into the primary ones these are filled with cytoplasmic
material of the mother vesicle; which provides all the components of a cell
needed for viability such as chromosomes and ribosomes, the production sites
for proteins. Whether this second vesicle actually receives genetic material
is, however, unregulated and random, but as a general rule provides enough
volume and space that all vital processes can occur.
Loessner
considers it a very significant finding that in the course of their research on
Listeria L-forms, they discovered small, elastic connector tubes between the
outside vesicles, that look like "pearls on a chain." "The
vesicles form a crazy network among themselves," he says. As intact
membranes, the tubes consist of lipid molecules and they enable the vesicles to
form a continuum, similar to fungal mycelium. Until they are fully separated,
the vesicles can exchange cytoplasm via these tubes.
Similarly
unusual is that in order to multiply, the L-forms require neither a cell wall
nor the ring-forming FtsZ protein, which regular bacterial cells need to
divide. "If we consider the early cells without rigid walls in the history
of microbial evolution, then they most likely divided like the L-forms,"
explains Loessner. However, this is not a biological process but rather a
physical one that depends directly on the amount of membrane material produced.
"Multiplication follows the laws of thermodynamics." The L-forms may
thus be compared to soap bubbles, which also owe their stability (and division)
to purely physical principles.
For
further details see:
Posted by Dr. Tim Sandle
