Summary. The total number of cultivable bacteria and the number of different bacterial groups were determined at 4 levels of the digestive tract. i. e. the stomach, small intestine, caecum and colon, in 34 holoxenic (conventional) rabbits between 2 and 56 days of age. The stomach of young rabbits was almost sterile during the first postnatal week, and bacteria were established very irregularly, depending on the individual. After weaning, the total number of bacteria tended to increase with age and stabilized between 10 4 and 10 6 bacteria/g content. The colonization of the small intestine was more rapid than that of the stomach and the microflora was more abundant. The range of individual variation was wide in the young but more reduced after weaning ; the microflora was stabilized between 10 6 and 10 8 bacteria/g intestine. The caecum and colon already harbored an abundant flora (10' to 10 9 bacteria/g) from the first week ; the colony counts remained constantly high and varied little among samples. Rabbit microflora was characterized by the dominance of strictly anaerobic species, particularly non-sporulated Gram-negative bacilli (Bacteroides), distributed in the whole digestive tract. The number of sporulated anaerobic bacteria was 100 to 1 000 times lower than that of Bacteroides, and they belonged to the genera Endosporus, Clostridium and Acuformis. The stomach almost never contained any Streptococci ; in the small intestine, caecum and colon their number reached a maximum in 7 to 14-day old rabbits (10't to 10 9 ) ; it then regularly decreased with the age of the animals. After weaning, the Streptococci disappeared from the small intestine ; in the caecum and colon their number did not exceed 10 4 bacteria/g. Young rabbits generally did not contain any enterobacteria ; they appeared later and reached a maximum at the end of week 3 (10 7 / 9 ), after which their number rapidly decreased. Enterobacteria were not regularly present after weaning. Micrococci were very seldom found, except in the stomach, and the rabbit digestive tract never harbored any Lactobacilli.Introduction.

Ecological factors that control the establishment of cellulolytic bacteria and ciliate protozoa in the lamb rumen were studied in meroxenic animals. Axenic lambs received dilutions of rumen liquor from either conventional lambs and sheep (pool A) or meroxenic lambs (pool B). The total number of bacteria established in the rumen was between 10(9) and 5 x 10(10) g-1. In lambs inoculated with dilutions (10(-6), 10(-7), 10(-8)) of pool A, cellulolytic bacteria did not become established. However, subsequent inoculation with Bacteroides succinogenes, resulted in colonization in lambs that had received 10(-6) and 10(-7) dilutions of pool A. However, B. succinogenes became established in only one of three lambs that received the 10(-8) dilution. Similar results were obtained for the protozoan Entodinium sp. With pool B, lambs were inoculated earlier and cellulolytic bacteria were established directly from the 10(-6) and 10(-7) inocula. Polyplastron multivesiculatum establishment occurred readily when inoculated into the lambs which had received the 10(-6) dilution of pool B. Results obtained in this study suggest that establishment of cellulolytic bacteria and protozoa requires an abundant and complex flora and is favoured by early animal inoculation.

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Seven lambs, separated from their dam 24 h after birth, were kept in a conventional environment until transferred to sterile isolators between 1 and 9 days of age: two on day 1 (IA and IB), two on day 4 (IVA and IVB), one on day 8 (VIIIA), and two on day 9 (IXA and IXB). The lambs were reared in these isolators until 120 days of age. Lambs IA, IB, IXA, and IXB were free of cellulolytic bacteria when they were placed in the isolators. They were then inoculated with Bacteroides succinogenes S85 which became established in the four lambs. Until the age of 2 months, the population of this strain fluctuated and then stabilized at a high level (10(8)-10(9) cells/mL). Cellulolytic bacteria were present in the rumen of lambs IVA, IVB, and VIIIA when they were transferred to the isolators. In IVA, and IVB, the cellulolytic population slowly increased with the animal age. In contrast, in VIIIA, the cellulolytic bacteria disappeared within a few days. Bacteroides succinogenes S85 inoculated thereafter became established rapidly and reached a level comparable to that observed in lambs IA and IB. The total number of viable rumen bacteria in the isolated lambs was similar to that observed in conventionally raised animals, but differences were observed in the selective enumeration of bacteria utilizing specific energy sources.

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