No abstract

The effect of glycerol on the growth, adhesion, and cellulolytic activity of two rumen cellulolytic bacterial species, Ruminococcus flavefaciens and Fibrobacter succinogenes subsp. succinogenes, and of an anaerobic fungal species, Neocallimastix frontalis, was studied. At low concentrations (0.1-1%), glycerol had no effect on the growth, adhesion, and cellulolytic activity of the two bacterial species. However, at a concentration of 5%, it greatly inhibited their growth and cellulolytic activity. Glycerol did not affect the adhesion of bacteria to cellulose. The growth and cellulolytic activity of N. frontalis were inhibited by glycerol, increasingly so at higher concentrations. At a concentration of 5%, glycerol totally inhibited the cellulolytic activity of the fungus. Thus, glycerol can be added to animal feed at low concentrations.

The effect of Levucell SC, a strain of Saccharomyces cerevisiae marked as a feed additive for ruminants, was investigated in vitro on lactate metabolism by the ruminal bacteria Streptococcus bovis and Megasphaera elsdenii. The coculture between 10(7) live cells x mL(-1) of SC and a Streptococcus bovis strain in the presence of glucose reduced lactate production by the bacterial strain. Live yeast cells were able to compete with Streptococcus bovis for glucose utilization in strictly anaerobic conditions, so less glucose was available for the bacterium. SC also stimulated L-lactate utilization by a strain of M. elsdenii. The effect depended on the concentration of yeast cells added. Bacterial growth and fermentation end-product concentrations were also increased in the presence of SC. Some amino acids and vitamins, but not dicarboxylic acids, stimulated the bacterial specific activity of L-lactate uptake. SC was able to provide amino acids to M. elsdenii. In a coculture of Streptococcus bovis and M. elsdenii on glucose, the reduction of lactate concentration was improved by SC, the same trend being observed when maltose or soluble starch were used as carbon and energy source. These results indicate that SC can be a very useful tool to reduce lactate accumulation in vitro during fermentation of soluble sugars.

The development of hydrogenotrophic bacteria in the rumen of lambs was investigated by culture and labeling experiments. 14CO2 and 13CO2 incorporation by the rumen microflora of a 24-h-old lamb showed that while there was no labeled methane, double-labeled acetate was formed indicating the presence of hydrogen-dependent acetogenesis. In vitro counts from rumen fluid of 20-h-old lambs confirmed an extensive colonization of acetogenic bacteria while methanogens were absent. Methanogens appeared in the rumen of 30-h-old lambs, and as they developed there was a proportional decrease in the numbers of acetogens, indicating a competition for hydrogen between these two groups. Hydrogen-utilizing sulfate-reducing bacteria, which were established by the 3rd day after birth, did not seem to be affected by this competition.

Ruminococcus flavefaciens adhered instantly to cellulose, while Fibrobacter succinogenes had the highest percentage of adherent cells after about 25 min of contact between bacteria and cellulose. Adhesion of R. flavefaciens was unaffected by high concentrations of sugars (5%), temperature, pH, oxygen, metabolic inhibitors, and lack of Na+. In contrast, the attachment was affected by the removal of divalent cations (Mg2+ and Ca2+), the presence of cellulose derivatives (methylcellulose and hydroxyethylcellulose), and cystine. Adhesion of F. succinogenes was sensitive to low and high temperatures, high concentrations of glucose and cellobiose (5%), hydroxyethylcellulose (0.1%), redox potential, pH, lack of monovalent cations, and the presence of an inhibitor of membrane ATPases or lasalocid and monensin. Cells of F. succinogenes heated at 100°C no longer were adherent. On the other hand, adhesion was insensitive to the lack of divalent cations (Mg2+ and Ca2+), the presence of 2,4-dinitrophenol, tetrachlorosalicylanilide, or inhibitors of the electron transfer chains. Adhesion of F. succinogenes seems to be related to the metabolic functions of the cell. External proteins and/or cellulases themselves might play a part in the attachment process. Several mechanisms are probably involved in the adhesion of R. flavefaciens, the main one being the interaction between the large glycocalyx and the divalent cations Ca2' and Mg2+. Hydrophobic bonds and enzymes may also be involved.