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Kinetics of the establishment of gastrointestinal microflora in the conventional new-born rat.

The kinetics of the establishment of gastrointestinal flora in the conventional new-born rat aged 5 h to 8 days was studied. Anaerobic species were counted by the method of Hungate in an anaerobic glove box, in the stomach and the whole of the intestine. In both organs, staphylococci and micrococci were the first bacteria to settle, along with a few unidentified aerobic bacilli. Counts levelled off after the first day. Streptococci and enterococci settled somewhat later, and their numbers too levelled off after the first day. The appearance of Escherichia coli was sometimes early and always very irregular. Permanent implantation of this species only began to occur after the first day. Actinobacilli were observed at 20 h, but irregularly. Lactobacilli (mainly Lactobacillus murinus and L. leichmanii) were initially very low in number, but later multiplied rapidly towards the end of the first day to become the dominant genus. No strictly anaerobic bacteria, sporing or not, were ever found in animals younger than 4 days. Inoculation of axenic mice with gut contents from rats aged up to 1 day confirmed the above results. Analysis of superficial mammary flora of suckling parent females consistently showed staphylococci and micrococci during the first day post partum. By the 15th day post partum, lactobacilli had progressively become implanted alongside these two other genera. On the other hand, the vaginal flora of parent females immediately post partum was found to be quite different from that observed in the gut of the new-born rat. Possible origins for the early colonizing genera, including suckling, are discussed.