The Intestinal Microflora During the First Weeks of Life Eugenia Bezirtzoglou Dept of Microbiology, University of Ioannina, Medical School, 45 110 - Ioannina, Greece (Received 14 October 1996, accepted in revised form 2 May 1997) Key Words: newborn, intestine, flora, microflora Bacterial colonization of the neonatal gastrointestinal tract begins during birth when the neonate comes into contact with the maternal cervical and vaginal flora. In infants delivered by Caesarean section, bacteria colonizing the neonate gastrointestinal tract are provided by the environment. The first bacteria encountered in the majority of healthy infants, are facultative anaerobes, which remain predominant during the first 2 weeks of life. Among them, Staphylococcus, Enterobacteriaceae and Streptococcus were the genera most commonly isolated from the newborn faecal flora at birth. Facultative anaerobic bacteria are followed closely by Bifidobacterium sp. Clostridium perfringens is present within 2 days with an increase incidence in newborns delivered by a Caesarean section. Clostridium perfringens seems to be the precursor for installation of other anaerobic putrefactive bacteria, as Bacteroides and other Clostridium sp. The impact of Caesarean section and the period and quality of hospitalization are mainly implicated in changes of the normal newborn flora. Feeding seems to modulate the colonization pattern. In humans, breast milk plays a role in passive immunization of the neonatal intestine, and contains factors that promote the growth of Bifidobacterium bifidum in the intestinal flora. Formula feeding seems to promote implantation and persistance of Clostridium perfringens, and clearly enhances intestinal colonization of C. difficile in newborns. © 1997 Academic Press Introduction Many published reports discuss studies on the compo- sition of the newborn intestinal flora in both health and disease. A plethora of studies effectuated concerns the anaerobic compound of this flora [1–4]. Studies on the anaerobes have always lagged behind those on aerobic organisms mainly because of technical diffi- culties and inconveniences. During the last few years significant advances have been made in techniques for the cultivation of some of the fastidious bacteria inhabiting the bowel. Recently, there is a current awareness that anaerobes are involved in human infections much more frequently than had generally been supposed [5–7]. The human newborn devoid of bacteria before birth, is particularly prone to infection during the first days of life. This risk of infection is related to the insuffi- ciently developed defense mechanisms and to expo- sure to a variety of microorganisms [2]. However, it remains a mystery how the newborn intestine selects a limited number of microbial species from the enormous diversity offered. Normal Development of the Intestinal Flora Studies performed by different researchers [5,7] have given an appreciation of the complexity of the normal Anaerobe (1997) 3, 173–177 1075-9964/97/020173 + 05 $25.00/0/an970102 © 1997 Academic Press