In vitro study of the influence of virginiamycin and spiramycin on the composition and biochemical activities of the gastrointestinal flora of piglets.  II.  Influence on the biochemical activities of the microflora. 
Chemical analysis was carried out on 54 in vitro fermentations to study the influence of nutritional doses of Virginiamycin and Spiramycin on the metabolic activity of the gastro-intestinal flora of 18 in vivo treated piglets.  Changes in the decomposition of glucose and the production of NH3, volatile acids and lactic acid are recorded in function of time.  These results are in good accordance with changes noted in the composition of the bacterial flora, such as a marked decrease in the production of lactic acid, caused by Virginiamycin, combined with a decrease in the number of Lactobacilli and Enterococci.  Virginiamycin caused marked changes in the metabolic activity of certain groups of bacteria, while there was only a slight effect noted for Spiramycin, although in vivo treatments with this antibiotic still show up in the results of the in vitro incubations.  The overall results indicate that both antibiotics have marked influences on the intestinal flora of the pig, either by selective inhibiting of the flora resulting in a change of the metabolities formed, either by a change in the metabolic activity of some bacterial groups.