Determination of rumen microbial growth in vitro from 32P-labelled phosphate incorporation.

1. The extracellular phosphate pool in incubations of rumen fluid or washed cell suspensions of mixed rumen bacteria (WCS) was labelled with 32P. From the constant extracellular phosphate pool specific activity and the amount of radioactivity incorporated during incubation, the amount of P incorporated in the microbial fraction was calculated. From the value for nitrogen: P determined in microbial matter, the amount of N incorporated was calculated as a measure of microbial growth. 2. Incorporation of soluble non-protein-N in incubations devoid of substrate protein was 50 and 80% of the values obtained using the isotope method for rumen fluid and WCS respectively. It is suggested that results obtained using the former method reflect 'net growth' of micro-organisms which is the result of simultaneous growth and degradation. The isotope method measures 'total growth', as isotope incorporation is not affected by degradation of non-growing cells. 3. Incorporation of 32P in P-containing microbial components (mainly nucleic acids) was compared with net synthesis of these components in incubations of WCS. The results showed different specific rates of synthesis and degradation for all components studied. It is concluded that the composition of microbial matter changed during growth. 4. When N incorporation, calculated from results obtained using the isotope method in incubations with rumen fluid, was compared with the amount of carbohydrate substrate fermented and the type of fermentation, values between 18-3 and 44-6 g N incorporated/kg of organic matter fermented were obtained. Low values were associated with large proportions of the substrate being fermented to lactate and the use of glucose instead of disaccharides as substrate. Part of the variation could also be attributed to differences in incubation period, reflected in different proportions of polysaccharide formed. 5. The use of isotopes for determination of rumen microbial growth in vitro is critically discussed.