Effects of supplemental zinc concentration on cellulose digestion and cellulolytic and total bacterial numbers in vitro

Abstract

Effects of various zinc (Zn) concentrations on cellulose digestion and cellulolytic and total bacterial numbers were studied in vitro. All fermentation tubes were inoculated with a 1:10 dilution of whole rumen contents and used either 0.0075 or 0.015 g/ml purified cellulose as substrate. When Zn was added at six different concentrations: 5, 10, 15, 20, 25 and 50 μg/ml, using either 0.0075 or 0.015 g/ml cellulose, the 50 μg/ml concentration reduced cellulose digestion ( P<0.01) at 24 h of incubation. However, cellulose digestion was similar to the control tubes at 48 h. At 24 h, addition of Zn at 50 μg/ml to the medium containing 0.0075 g/ml cellulose lowered cellulose digestion ( P<0.016), but did not affect cellulolytic and total bacterial concentrations. By 48 h, cellulolytic bacterial concentrations in the control tubes were lower ( P<0.001) than in the Zn supplemented tubes. Total bacterial concentrations were also lower ( P=0.076) for control tubes. Using 0.015 g/ml cellulose and Zn concentrations of 0, 50, 100 and 150 μg/ml, cellulose digestion after 24 h was lower than the control for all tubes with added Zn ( P<0.01). By 48 h, cellulose digestion was similar in tubes containing 0 and 50 μg/ml concentrations of Zn, while digestion remained lower with concentrations of 100 and 150 μg/ml ( P<0.01). After 72 h, only the 150 μg/ml Zn concentration had less cellulose digested ( P<0.05). Final pH values followed a similar pattern: the higher the Zn concentration, the less decrease in pH. Concentrations of cellulolytic bacteria were lower ( P<0.001) in all tubes with Zn after 24 h incubation, but their concentration in the control tubes decreased markedly between 24 and 48 h, presumably due to low pH and substrate limitation. No differences were observed in total bacterial concentrations at any fermentation time period. The decreased cellulolytic bacterial concentration in the tubes with high Zn levels may be due to decreased utilization of cellulose by the bacteria, rather than a toxic effect of high Zn on the bacteria. Possibly Zn has an inhibitory effect on the cellulolytic enzymes produced by the bacteria.