Physiological characteristics of two acetogenic bacteria isolated from ruminal contents
Abstract
Interest exists in replacing methanogenesis with acetogenesis in ruxminal contents. However, acetogenesis (and the number of acetogens) in ruminal fluid is low. Unfortunately, a reasonable explanation for the low rates of acetogenesis in ruminal contents has not been set forth. The characteristics of two acetogens isolated from ruminal contents (isolates A10 and H3HH) were examined to gain insight into the relatively low rate of acetogenesis in ruminal contents. Although no plasmids were detected in isolate A10, five cryptic plasmids were identified in isolate H3HH. A plasmid-free derivative (H3HP) of isolate H3HH displayed increased sensitivity to several antibiotics. A physical map of the smallest plasmid of isolate H3HH was constructed using restriction enzyme analysis. Next, utilization of glucose and H$\sb2$/CO$\sb2$ by isolate A10 was investigated. Diauxic growth curves and differing patterns of glucose and H$\sb2$/CO$\sb2$ consumption suggest that this organism is incapable of mixotrophy. Changes in intracellular hydrogenase activity and NAD(H) concentration as cells shifted from glucose to H$\sb2$/CO$\sb2$ utilization reinforce these conclusions. Because carbohydrates repress H$\sb2$/CO$\sb2$ utilization by isolate A10, the concentration of soluble carbohydrates in ruminal fluid was examined. While maltose was not detectable, glucose, cellobiose and unidentified pentose(s) were detected. The minimum concentration of glucose required for growth was approximately 10 times greater than the range of glucose detected in ruminal fluid. However, both isolates appear capable of utilizing the cellobiose present in ruminal fluid. Volatile fatty acids and low pH inhibit growth of some bacterial species, Escherichia coli, in ruminal fluid. The growth rates of both isolates were negatively affected by low pH and/or high acetate concentrations. We conclude that the abiliity of isolates A10 and H3HH to grow and utilize H$\sb2$/CO$\sb2$ are reduced by the presence of low pH, acetate and organic energy sources. These factors may contribute to the low numbers of acetogens generally found in the rumen.
Degree
Ph.D.
Advisors
Patterson, Purdue University.
Subject Area
Livestock|Microbiology|Molecular biology
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