Composition of fractions of the rumen population of different cell size

Abstract

Proc. Aust. Sot. Anim. Prod. I994 Vol. 20 COMPOSITION OF FRACTIONS OF THE RUMEN POPULATION OF DIFFERENT CELL SIZE K.A. MUNKARDA and S.K BAKER AB *Faculty of Agriculture, The University of Western Australia, Nedlands, W.A. 6009 BCSIRO Division of Animal Production, Private Mail Bag, PO Wembley, W.A. 6014 The rumen microbial population has a structure that is related to cell size (Baker 1990). This size structure has been determined using a Coulter counter where cell size is expressed as the diameter of a sphere of the same volume as the cell, regardless of morphology. In this study centrifugal elutriation was used to separate the rumen population into fractions containing microorganisms of different cell size, in order to determine the microbial composition of each fraction. Rumen contents from a mature wether fed a diet of oaten hay, lupins, and minerals (75:20:5) were collected 6 hours after feeding, and formaldehyde was added to a final concentration of 1% (v/v). The sample of rumen contents was strained and the filtrate was separated at 5OC into 5 fractions using an elutriation system and rotor (Beckman Instruments) under the following conditions; fraction 1, 1000 rpm and 50 ml/minute; fraction 2, 2700 rpm and 50 ml/minute; fraction 3, 5800 rpm and 15 ml/minute and fraction 4, 6000 rpm and 3 mL/minute. The material that was not retained in fraction 4 was designated fraction 5. The diluent used was 0.5% (v/v) formaldehyde in 0.9% (w/v) NaCl. Representative samples of each fraction were examined using bright field microscopy and microorganisms were identified using the schemes of Moir and Masson (1952) and Ogimoto and Imai (1981) in either unstained or brilliantgreen stained preparations. The cell size of microorganisms in each fraction was measured using a Coulter counter and is expressed as the diameter of a sphere of the same volume. There was very little cross-contamination between fractions and there were clear differences between the cell sizes and morphologies of organisms present in different fractions (Table 1). Where an organism was represented in 2 successive fractions the size of the organism was distinct between the 2 fractions, for example Entodinium in fracions 1 and 2. Zoospores were found only in fraction 3. The use of centrifugal elutriation to separate fractions of the rumen microbial population is a novel approach to the study of microbial ecology of the rumen in relation to the nutrition of the animal. Table 1. The range of cell size km) in and composition of fractions of the rumen microbial population separated by centrifugal elutriation BAKER, SK. (1990). In 'Microbial and Plant Opportunities to Improve Lignocellulose Utilization by Ruminants', (Eds D.E. Akin, LG. Ljungdahl, J.R. Wilson and P.J. Harris) pp. 253-64 (Elsevier Science: New York). MOW R.J. and MASSON, M.J. (1952). J. Pathol. Bacterial. 2: 343-50: OGIMTQ K. and IMAI, S. (1981). 'Atlas of Rumen Microbiology' (Japan Scientific Societies Press: Tokyo). 388