LETTERS TO THE EDITORS
Ruminating on Films
To the Editors:
We applaud Joe J. Harrison, Raymond J. Turner, Lyriam L. R. Marques
and Howard Ceri for highlighting the wide distribution and
importance of biofilms in nature ("Biofilms,"
November-December 2005), but inaccuracies in the article incorrectly
make the ruminal process in cattle appear overly similar to sewage
sludge digestion. The article gives the impression that the
protein-rich microbial mass from fermentation is the primary source
of nutrients to the animal and that methanogens play a major role in
ruminal fermentation of organic acids.
Although microbial mass is a source of nutrients, organic acids
(predominantly acetate, propionate and butyrate) produced from
carbohydrates are quantitatively the major sources of nutrients that
cattle derive from ruminal fermentation. These acids are absorbed
into the blood stream from the rumen and can provide roughly
two-thirds of an animal’s energy requirement. If organic acids
were extensively fermented by ruminal methanogens, energy derived
from the ration would be limited, and the cow would likely starve.
Ruminal methanogens readily convert hydrogen gas and carbon dioxide
(also produced by carbohydrate fermentation) to methane, but ruminal
methanogenesis from organic acids is negligible because the short
digesta retention times (typically 12 to 48 hours; compare to sewage
sludge digesters at two to three weeks) preclude establishment of
the slower growing, acetate-fermenting methanogens and their
propionate- and butyrate-fermenting symbionts.
That biofilms are important in microbial ecology is not in question.
However, the functioning of ruminal microbes portrayed in the
article would likely have led to the demise of the ruminant
Mary Beth Hall
Dr. Ceri responds:
Drs. Hall and Weimer are of course correct that organic acids, the
products of biofilm metabolism, are an integral component of the
ruminant diet. We did not intend to diminish their importance in the
digestive process, and we have shown them being made available for
metabolism in Figure 3 of our article. The intent of the description
was to use rumen metabolism to demonstrate the interactions and
commensalism between different species in the rumen.