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The Effect of Specific Growth Rate and Stage of Growth on Nucleic Acid-Protein Values of Pure Cultures and Mixed Ruminal Bacteria¹

Michigan State University⁴, East Lansing 48824-1225

Abstract

Nucleic acid:protein ratio values were determined for six strains of ruminal bacteria cultured at different specific growth rates and harvested at mid-log and stationary phase of growth. Ribonucleic acid/protein were determined in free ruminal bacteria from steers that were fed a high-concentrate diet and in free and adherent (feed particles) ruminal bacteria from sheep fed corn silage, 50% corn silage:50% grain or high-concentrate diets. Ribonucleic acid/protein and RNA/DNA increased with specific growth rate for all bacteria in vitro. The overall RNA/protein for the bacteria grown in vitro at stationary phase was .22, while the mean RNA/protein at µ = 0 predicted from the average Y intercepts of regressions of RNA/protein vs specific growth rate for each individual strain was .20. Deoxyribonucleic acid/protein of ruminal bacteria were not affected (P>.01) by specific growth rate. A significant state (free vs bound) x diet interaction was observed in RNA/protein of ruminal microbial samples of sheep. The RNA/protein of adherent ruminal bacteria from sheep were not affected (P>.05) by time after feeding. The RNA/protein of free ruminal bacteria from steers, fed a high-concentrate diet, increased (P<.05) with time after feeding (up to 6 h), then declined back to prefeeding values. When comparing the diets as main effects, mean RNA/protein was higher (P<.05) in free bacteria from sheep fed a high-concentrate diet. When compared on a time-after-feeding basis, the RNA/protein was higher (P<.05) only at 6 h postfeeding for free ruminal bacteria from sheep fed the high-concentrate diet. Overall, RNA/protein for free bacteria were higher (P<.01) than for adherent bacteria. Because the in vitro results showed that RNA/protein were affected by growth rate and the in vivo results showed that RNA/protein can be affected by diet and time after feeding while DNA/protein were largely independent of growth rate in vitro, DNA/protein should be a better marker for microbial biomass in digesta passage studies than RNA/protein.

¹ Michigan Agr. Exp. Sta. No. 11029.

² Present address: Dept. of Anim. Sci., Univ. of Florida, Gainesville.

³ Present address: Merck, Sharp and Dohm Laboratories, Rahway, NJ.

⁴ Dept. of Anim. Sci., Ruminant Nutrition Laboratory.

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