American Scientist

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 digestion strategy.

Mary Beth Hall
Paul Weimer
USDA-Agricultural Research Service
Madison, WI

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.