Microbes
to the Rescue
How bacteria are used to restore delicate works of art
A
By Kate Mitchell
May, 2015
Kate Mitchell is a Technical
Support Specialist at Hardy
Diagnostics. She earned her
bachelor’s degree in
Anthropology and a certificate
in Archeology at Southern
Oregon University in Ashland,
Oregon where she studied the
use of mycology to characterize
archaeological excavations.
HardyDiagnostics.com
rt restoration experts
spend countless hours
in awkward positions,
breathing in harsh chemicals
while meticulously cleaning,
varnishing, and stippling
irreplaceable works of art. A
shaky hand or a rough touch
could result in the destruction
of a priceless work of art.
Consequently, students at the
Polytechnic University of
Valencia’s Institute of
Heritage Restoration may
have an unlikely ally. In their
quest to restore the 17th
century frescoes of the
Church of Santos Juanes, they
have implemented a strain of
Pseudomonas stutzeri to
replace the toxic chemicals
normally used to remove
years of grime.
The frescoes at the Church of
Santos Juanes, like many
historical artifacts, have
suffered through the years: the
Spanish Civil War brought
fire, a botched restoration in
the ‘60s added glue, and some
irreverent pigeons making
nests caused salt blooms.
Needless to say, the beauty of
these frescoes was hidden
under the unforgiving
elements of time.
Using bacteria as a means of
environmental remediation is
not new, but when dealing
with sensitive historical works
of art, it may seem like risky
business.
Figure 1: Pseudomonas stuzeri
is a denitrifying bacteria that
can be useful for
bioremediation.
To prep Pseudomonas stutzeri
for this task, students raise
bacteria in a culture
containing some of the very
substrates they want to
eliminate, such as salt and
glue. Once the bacteria are
“trained” to process these
materials, they are brushed on
to frescoes like a tempera
wash. The bacterial wash is
then covered by an agar gel
that is heated to create a cozy
humid environment that
promotes bacterial growth.
After the bacteria are allowed
sufficient time (several hours
to several days) for their
enzymes to munch away at
the layers of grime, the
surface is then rinsed with
water to remove the
microorganism and dried. The
entire removal process can
take as little as 90 minutes
and does not require the use
of any harsh toxic chemicals.
Pilar Bosch working on the
paintings in the Church of Santos
Juanes in Valencia.
Credit: Image courtesy of Asociación
RUVID
Helping Spanish frescoes
show their true color isn’t
bacteria’s only cleaning
talent. Archeologists in the
EU are also using them to
clean the surfaces of stone
buildings damaged by
atmospheric pollution and
weathering. Sites exposed to
harsh coastal weather
typically exhibit crusts of salt,
sulfate or gypsum. A lot of
back-breaking work goes into
scraping off these crusts, and
the process often discolors or
damages surfaces.
The growing field of
bioremediation offers a safer,
often more effective
alternative to chemical
treatment. Much like in the
treatment of frescoes, these
bacteria are applied directly to
the stone surface where they
deposit a calcite layer. The
calcite layer consolidates
mineral surfaces, which are
then much easier to remove.
Researchers at the Universita
degli Studi di Milano have
done similar work on the
removal of black gypsum
crusts on marble in the Milan
Cathedral.(1) This black crust
forms from the interaction
between a calcareous
substrate and pollution in the
atmosphere, particularly in
humid environments sheltered
from rainfall. The chemical
transformation of calcite into
gypsum combines with
mineral and smog deposits
and forms a darkened crust,
obstructing the marble
underneath.
Figure 2: The darker central part
of this photo is where bacteria were
applied.
In 2007 Dr. Cappitelli et al.
published findings comparing
two cleaning methods. Two
samples were cleaned, one
using an ammonium
carbonate-EDTA mixture, and
the other using Desulfovibrio
vulgaris ATCC 29579, a
sulfate reducing bacterium.
Not only did the procedure
involving the microorganism
result in a more homogeneous
removal of surface deposits, it
also did not produce the
sodium sulfate residue that
chemical procedures can
cause. (1)
Using bioremediation may
also have some additional
conservation benefits, as the
presence of calcite implies
microbial calcification
activity. Atlas, Gauri, and
Chowdhury used
Desulfovibrio desulfuricans in
a similar manner in their 1988
study. They proposed that
calcium ions released by
gypsum dissolution react with
carbonate from bacteria
produced CO2 to form calcite.
This metabolic conversion is a
significant benefit to the
structure, making the
biological treatment not only
a cleaning procedure but also
a consolidation treatment.(4)
Chemical agents can also
create this calcite layer;
however, there is some
evidence that carbonates
formed by bacteria are more
resistant to mechanical stress
and less soluble than
inorganic calcite.(2,3)
Using the unique abilities of
bacteria to improve not only
the safety of those who restore
art, but also to preserve these
works for longer periods for
future generations to enjoy
may shift the way we think
about the benefits of
microorganisms.
When understood and used
with care, microorganisms
can be powerful allies in the
fight to reduce our
dependence on chemicals. In
addition, unlocking the
potential of microbial
metabolism may yield even
more powerful and
environmentally friendly tools
in our arsenal against the
elements of time.
Referencs:
1. Cappitelli, F., L. Toniolo, A.
Sansonetti, D. Gulotta, G. Ranalli,
E. Zanardini, C. Sorlini. 2007.
Advantages of using microbial
technology over traditional chemical
technology in removal of black
crusts from stone surfaces of
historical monuments, Appl.
Environ. Microbiol. 73
2. Morse, J. W. 1983. The kinetics
of calcium carbonate dissolution
and precipitation. Rev. Mineral.
11:227–264.
3. Rodriguez-Navarro, C., M.
Rodriguez-Gallego, K. B.
Chekroun, and M. T. GonzalezMun˜oz. 2003. Conservation of
ornamental stone by Myxococcus
xanthus-induced carbonate
biomineralization. Appl. Environ.
Microbiol. 69: 2182–2193.
4. Atlas, R. M., A. N. Chowdhury,
and L. K. Gauri. 1988. Microbial
calcification of gypsum-rock and
sulfated marble. Stud. Conserv.
33:149–153.
The Church of Santos Juanes in Valencia, Spain showing frescoes being
treated with bacteria to digest away centuries of pollution.
Photo: Ivan Jurado