Table 3.

Major bacterial taxa found in newborns at each colonization site, according to the studies discussed in the review.

	Newborns
Oral cavity	-Streptococcus spp. appears early after birth [5,11,180] -one week after birth, neonatal oral biofilm resembles that of the mother. The presence of anaerobic Gram negative Fusobacterium nucleatum is associated with maternal chronic periodontitis, and also with bacterial vaginosis and preterm delivery [10,181]
Gut	-most represented species in newborns < 33 weeks of gestational age: Lactobacillus spp., Staphylococcus spp., Enterobacter spp. and Enterobacteriaceae [122,141] -lower percentage of Bacteroides in the offspring of mothers following a high-fat diet -in the offspring of overweight mothers, increase in Bacteroides and reduction in Enterococcus spp., Acinetobacter spp., Pseudomonas spp. [20,144,145], and Bifidobacterium spp. [83,182] -following intra-partum antibiotics’ administration, decrease in bacterial diversity, reduction in Actinobacteria and Bacteriodetes and increase in Proteobacteria [13] -abnormal colonization by Pseudomonas spp. and E. coli was detected in necrotizing enterocolitis [183,184,185,186,187,188,189,190,191,192] -higher levels of Clostridium difficile, Clostridium innocuum and Bacteroides thetaiotaomicron in sudden infant death syndrome cases [193] -neonates born by vaginal delivery acquire microbial communities similar to maternal gut and vagina, while those born by cesarean section acquire environment-like bacteria, such as Staphylococcus spp., Corynebacterium spp. and Propionibacterium spp., are associated with lower microbial diversity and delayed colonization of Bacteroides spp. and Bifidobacteri [14]
Meconium	-most represented species: Enterobacteriaceae, Enterococcus spp., Lactobacillus spp., Bifidobacterium spp. [117,120] -very similar to the amniotic fluid [122] -in preterm infants, different microbiota than term infants [122,140,141], with lower bacterial diversity with the decrease in gestational age [122,140] -In preterm infants born by mothers with chorioamniotitis: large quantities of pathogenic bacteria, such as Ureaplasma parvum, Fusobacterium nucleatum and Streptococcus agalactiae [30] -in infants born by vaginal delivery, the abundance of Bifidobacterium spp. (7° and 14° day of life), Bacteroides spp. (7° and 14° day of life) and Lachnospiraceae spp. (7° day of life) was significantly greater than those born by cesarean section, with a lower abundance of Enterobacteriaceae spp. [194]