In a search for alternatives of in-feed antibiotics, the antimicrobial activity of caprylic (C8:0), capric (CI 0:0) and oleic (C 18:1) acid was investigated in pure cultures of 19 strains of rumen and rabbit caecal bacteria, and in incubations of the rumen and rabbit caecal contents. In glucose-grown bacterial cultures the minimum inhibitory concentration (MIC) of caprylic acid ranged from 1 to 3 uTml 1. Two strains of Bacteroides ovatus were less susceptible to capric than to caprylic acid. In other strains, the MIC of capric acid was 0.25-0.50 uTml~!. The growth of most strains was not much affected by oleic acid. An exception to this were rumen bacteria Butyrivibrio fibrisolvens (MIC from < 0.05 to 1 uTml" 1) and Lachnospira multiparus (MIC of 0.25 to 1 uTml" 1). In incubations of the rumen and caecal contents caprylic and capric acid decreased the production of volatile fatty acids and gas, and increased production of lactate. In latter incubations the inhibitory effects of caprylic and capric acid were similar. In incubations of the rumen contents, capric acid was more efficient than caprylic acid when supplied at low concentrations (<1.25 uTml 1), but less efficient when supplied at 2.5 and 5 uTml" 1. Effects of oleic acid in rumen and caecal cultures were not significant, except the increase in production of lactate by rumen microorganisms. It can be concluded that microorganisms of the animal digestive tract are susceptible to inhibition by caprylic and capric acid added to microbial cultures at fairly low concentrations. Oleic acid was far less effective.

The susceptibility of Salmonella spp. to 15 fatty acids was determined in vitro in cultures grown on glucose. Antimicrobial activity was expressed as IC50 (a concentration at which only 50% of the initial glucose in cultures was utilized). Caprylic acid was the only acid inhibiting glucose utilization. In cultures of S. enteritidis, S. infantis and S. typhimurium, IC50 of caprylic acid ranged from 0.75 to 1.17 mg/mL. A moderate adaptation effect was observed as these values increased 1.5-1.8 times when bacteria were subcultured 10 times in media containing a low concentration of caprylic acid (1/3 IC50). No effect of calcium ions added in excess on antimicrobial activity of caprylic acid was observed. Incubation of salmonellas with caprylic acid (1 mg/mL; 30 min) at pH 5.2-5.3 led to a reduction in the concentration of viable cells below the detection limit; 2-6% of Salmonella cells survived at pH 6.3-6.6.

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Summary The effect of salinomycin was investigated in anaerobic incubations of the pig caecal contents collected at slaughter, diluted with buffer and either supplied with starch or not. Salinomycin was present in cultures at 0, 2, 5 and 10 mg/l. In the whole range of concentrations that were tested, salinomycin significantly increased the production of propionate at the expense of acetate and butyrate. At 5 and 10 mg/l, these fermentation shifts were accompanied by a significant decrease of the total VFA, gas and methane production. The formation of molecular hydrogen which was low in control incubations without substrate (0.20 mmol/l), was increased in the salinomycin‐treated cultures with starch (1.30 mmol/l). Salinomycin tended to decrease the synthesis of microbial protein, but its effect was not significant. The metabolic hydrogen recovery was significantly increased in salinomycin‐treated cultures, indicating a partial inhibition of acetate formation from CO2 and H2. Zusammenfassung Der Einfluß von Salinomycin auf die caecale Fermentation in vitro bei Schweinen. Es wurde der Einfluß von Salinomycin auf einige Metabolite und die mikrobielle Proteinsynthese mit Hilfe der anaeroben Inkubation von Blinddarminhalt bei Mastschweinen untersucht. Die Schweine erhielten vor der Schlachtung Kraftfutter. Der Blinddarminhalt wurde mit Puffer verdünnt und unter Zusatz von 0, 2, 5 und 10 mg/l Salinomycin mit oder ohne Stärke als Substrat inkubiert. Der Zusatz von Salinomycin führte zu einer signifikanten Erhöhung der Propionat‐Konzentration auf Kosten von Acetat und Butyrat. Bei höherer Salinomycin‐Konzentration (5 und 10 mg/l) waren die unterschiedlichen Verhältnisse in den flüchtigen Fettsäuren von einer Verminderung der gesamten Produktion an flüchtigen Fettsäuren, Methan und Gärgasen begleitet. Die Entstehung von molekularem Wasserstoff, die bei der Inkubation ohne Stärke als Substrat niedriger (0, 20 mmol/l) lag, betrug mit Stärke und Salinomycin 1, 30 mmol/l). Der Zusatz von Salinomycin erniedrigte die Synthese von mikrobiellem Protein. Allerdings war dieser Effekt nicht signifikant.

Prevotella ruminicola is a fibrolytic rumen bacterium that degrades complex carbohydrates and ferments the resulting hexoses and pentoses. Cultures of strain AR29 grew more rapidly on glucose than on xylose, but produced almost the same metabolite profiles on both carbon sources. Glucose in glucose-plus-xylose medium was used preferentially. The production of cell dry matter, growth yields of dry matter and protein, and cell composition (cell carbohydrate and protein) were not significantly different in glucose-and xylose-grown cultures. High aldolase (EC 4.1.2.13) activity was found both in glucose-and xylose-grown cells of the strain AR29. A very low phosphoketolase (EC 4.1.2.9) activity was detected in cells cultured on xylose. In contrast, enzymes of the EntnerDoudoroff pathway (6-phosphogluconate dehydrase and 2-keto-3-deoxy-6-phosphogluconate aldolase) could not be evidenced. KEY WORDS: Prevotella ruminicola, metabolism, carbohydrates INTRODUCTIONAnaerobic bacteria belonging to the species Prevotella ruminicola represent one of the most numerous bacterial groups inhabiting the rumen (Stewart and Bryant, 1988). The predominance of this organism can be explained by its ability to use a variety of substrates and also by its efficient energy metabolism. Strains that have been isolated from the rumen ferment soluble sugars including soluble cellodextrins, polysaccharides, and decompose protein, peptides and amino acids. Several studies attempted to elucidate carbohydrate metabolism of P. ruminicola. Experiments by Joyner and Baldwin (1966) and Mountfort and Roberton (1978)