In vitro predation of pure bacterial species by rumen protozoa from monofaunated sheep, determined by qPCR
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Predation of bacteria has important implications on rumen microbial ecology and metabolism. The predation rate of bacteria by protozoa in vitro was studied by bacterial DNA quantification. Entodinium caudatum (EC), Diplodinium dentatum (DD) and Metadinium medium (MM) from monofaunated sheep rumen contents were incubated with two pure cultures of bacteria: the fibrolytic Ruminococcus albus (Ra) and the amylolytic Streptococcus bovis (Sb). Total bacterial and protozoal numbers were counted in the media by microscopy, and concentration of Ra, Sb and total bacterial DNA was measured in the protozoal fraction as an index of predation. No differences (P>0.10) were observed in the proportion of DNA from Ra in the different protozoal fractions, but MM predated a higher amount of total bacteria than EC (P=0.045). Proportion of Ra and Sb in the protozoal fraction agrees with that in the medium, precluding selective predation. Protozoal fraction from EC showed a higher (P=0.018) proportion of Sb DNA and a lower amount of total bacterial DNA than MM (P<0.001). Total DNA in MM cultures either per number of cells or per cell volume was highest, indicating a higher engulfment ability of this protozoal species. No differences were observed among incubation times in any studied parameter (P>0.10), suggesting that the predation rate of each protozoa was similar to the intracellular digestion rate.