In the second part of this work on the effect of dietary fat on rumen microbiota, we review experimental results from in-vivo assays and discuss in more detail the process of ruminal biohydrogenation of fats. As we have already mentioned in part 1, biohydrogenation is the step following ruminal lipolysis, which is the first process the fats undergo after entering the rumen.
Biohydrogenation process is a detoxifying adaptation (Kemp et al., 1984). It marginally contributes to the elimination of reducing equivalents produced during ruminal fermentation (Lourenço, et al. 2010). It comprises various steps, depending on the types of UFAs, and several metabolic pathways, depending on the diet and the ruminal environment (Fig. 1) (Griinari et al., 1998).
Figure 1. Ruminal biohydrogenation (UFA: Unsaturated Fatty Acids).
Protozoa engulf bacteria and bacterial biohydrogenation may occur inside them (Jenkins et al., 2008). This may explain the high concentration of intermediate products inside the protozoa (Devillard et al., 2006).
Besides the experimental studies performed on selected microbiological isolates, there have been attempts to evaluate in-vivo the relationship between different ruminal bacteria and biohydrogenation. Those studies were based on adding bacteria in the rumen and measuring their products, or on adding feed supplements known to affect BH and measuring the effec on bacteria abundance.
Figure 2. Methanobrevibacter ruminantium
Studies with pure archaea strains showed that, when adding organic acids or saturated fatty acids, there was an inhibition of the production of methane by Methanobrevibacter ruminantium (Fig. 2).
Altogether, these first studies on saturated and monounsaturated FA emphasized that the effect of fats in ruminal bacteria depend on bacterial metabolism, instauration of FA, and geometric configuration of double bonds.
The negative effects of FA on B. fibrisolvens are:
Stronger (+) for ALA than for AL
Stronger (++) for FA of long chain eicosapentaenoic (EPA; cis-5, cis-8, cis-11, cis-14, cis-17-C20: 5 ) and docosahexaenoic (DHA; cis-4, cis-7, cis-10, cis-13, cis-16, cis-19-C22: 6).
Similarly, ALA strongly increases the latent phase and reduces the growth rate of Propionobacterium acnes (Maia et al., 2016).
The effects of fat supplements were investigated in vivo. Bacteria were classified at species level by means of quantitative PCR (Martin et al., 2016; Vargas-Bello-Perez, et al., 2016) or at genus level using 16S rDNA pyrosequencing (Zened et al., 2013a; Huws et al., 2014). The latter authors found significant effects of fat on bacteria not yet sequenced or classified.
Figure 4. Bacteria were classified at species level by means of quantitative PCR
These experiments added mainly oil, differently from most of the studies on pure cultures, in which free FA were used.
Altogether, the observed effects were lower than in experiments with pure cultures. This could be due to the type of fat added or to the fact that the ffects on the latency phase cannot be observed in vivo.
Changes in ruminal microbiota due to higher proportion of concentrates were much higher than the ones observed after the addition of fat. Furthermore, some genera were affected differently after the addition of oil to diets of low and high concentrate, especially Acetitomaculum, Lachnospira y Prevotella (Zened et al. 2011).
Figure 5. Prevotella spp.
Among the genera or species of bacteria studied in several experiments, Fibrobacter and Ruminococcus were negatively affected in most cases. The effects on genera Butyrivibio y Prevotella were very variable. These latter genera comprise many species with somehow diverse metabolic functions, different metabolic pathways, and different sensitivities to FA in cultures (Maia et al., 2007).
The drop in abundance of a bacterium genum in vivo after a change in the diet cannot be univocally interpreted as a direct consequence of such change. However, it could reflect a more general modification in nutrient degradation and in the relations between different ruminal microorganisms.
There are several hypotheses to explain the inhibiting mechanism of FA on bacterial growth:
The relationship between dietary lipids and ruminal microbiota is dominated by the toxicity that UFA produce in many microorganisms, especially fibrolytic bacteria.
Many recent studies suggest that the biochemical pathways are more complex and that the involved bacteria could be more diverse than it was thought several decades ago.
Practical applications involve both sides of the relationship:
The most adequate options to shape the rumen microbiota and its activity depend on many factors:
However, in order to apply in the field all the discussed manipulations, it is necessary to run further in vivo experiments, in diverse dietetic conditions, with prolonged experimental periods. The resilience of the ruminal microbiota, as well as its adaptation to the degradation of vegetal compounds, could alter the effects throughout time (Weimer, 2015)
Furthermore, directing new applied research on ruminal metabolism of fats makes necessary get to know better the types of microorganisms, the enzymatic mechanisms, as well as the interactions between microbiota and the host.
This article was originally publised in nutriNews Spain, under the title Influencia de la grasa de la dieta en la microbiota ruminal-Parte 1
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