I wonder that where do these bacterias come from?

I'll answer this question since your post revolves around this question and the rest are follow-ups.

The infant gets its first bacteria (commensal or mutualist) from the mother's vagina as the amniotic membrane ruptures during parturition and gradually the diversity increases from its contact with other organisms and environment.

Nolte (1977) suggested that the body acquires its first microflora of the skin, nose, mouth and conjuctiva from the mother’s birth canal during normal vaginal delivery. The local environment is also important as a source from which neonates acquire bacteria (Mortimer et al., 1966; Nolte, 1977; Harvey, 1979; Shinebaum, Cooke and Brayson, 1979). The colonisation of the skin surface starts at the umbilical stump (Mortimer et al., 1966)

The normal commensal flora of the oral cavity is more complex. The mouth is sterile at birth but is rapidly colonised by bacteria derived from attendants and the local environment. By the second day of life, aerobes and facultative species can be detected, but strict anaerobes do not appear until after the eruption of the deciduous teeth (Hurst, 1957; McCarthy, Snyder and Parker, 1965).

Reference: THE DEVELOPMENT OF THE BACTERIAL FLORA IN NORMAL NEONATES

Also (updated)

A research published in 2013 has found that colostrum contain more than 700 species of bacteria! It being the only source of food for an infant also contributes (may be largely) to the infant's normal flora.

I wonder that where do these bacterias come from?

I'll answer this question since your post revolves around this question and the rest are follow-ups.

The infant gets its first bacteria (commensal or mutualist) from the mother's vagina as the amniotic membrane ruptures during parturition and gradually the diversity increases from its contact with other organisms and environment.

Nolte (1977) suggested that the body acquires its first microflora of the skin, nose, mouth and conjuctiva from the mother’s birth canal during normal vaginal delivery. The local environment is also important as a source from which neonates acquire bacteria (Mortimer et al., 1966; Nolte, 1977; Harvey, 1979; Shinebaum, Cooke and Brayson, 1979). The colonisation of the skin surface starts at the umbilical stump (Mortimer et al., 1966)

The normal commensal flora of the oral cavity is more complex. The mouth is sterile at birth but is rapidly colonised by bacteria derived from attendants and the local environment. By the second day of life, aerobes and facultative species can be detected, but strict anaerobes do not appear until after the eruption of the deciduous teeth (Hurst, 1957; McCarthy, Snyder and Parker, 1965).

Reference: THE DEVELOPMENT OF THE BACTERIAL FLORA IN NORMAL NEONATES

Also (updated)

A research published in 2013 has found that colostrum contain more than 700 species of bacteria! It being the only source of food for an infant also contributes to the infant's normal flora, though the composition of the bacteria colonising an infant's oral cavity is inconsistent with the composition of the colostrum.

I wonder that where do these bacterias come from?

I'll answer this question since your post revolves around this question and the rest are follow-ups.

The infant gets its first bacteria (commensal or mutualist) from the mother's vagina as the amniotic membrane ruptures during parturition and gradually the diversity increases from its contact with other organisms and environment.

Nolte (1977) suggested that the body acquires its first microflora of the skin, nose, mouth and conjuctiva from the mother’s birth canal during normal vaginal delivery. The local environment is also important as a source from which neonates acquire bacteria (Mortimer et al., 1966; Nolte, 1977; Harvey, 1979; Shinebaum, Cooke and Brayson, 1979). The colonisation of the skin surface starts at the umbilical stump (Mortimer et al., 1966)

The normal commensal flora of the oral cavity is more complex. The mouth is sterile at birth but is rapidly colonised by bacteria derived from attendants and the local environment. By the second day of life, aerobes and facultative species can be detected, but strict anaerobes do not appear until after the eruption of the deciduous teeth (Hurst, 1957; McCarthy, Snyder and Parker, 1965).

Reference: THE DEVELOPMENT OF THE BACTERIAL FLORA IN NORMAL NEONATES

I wonder that where do these bacterias come from?

I'll answer this question since your post revolves around this question and the rest are follow-ups.

The infant gets its first bacteria (commensal or mutualist) from the mother's vagina as the amniotic membrane ruptures during parturition and gradually the diversity increases from its contact with other organisms and environment.

Nolte (1977) suggested that the body acquires its first microflora of the skin, nose, mouth and conjuctiva from the mother’s birth canal during normal vaginal delivery. The local environment is also important as a source from which neonates acquire bacteria (Mortimer et al., 1966; Nolte, 1977; Harvey, 1979; Shinebaum, Cooke and Brayson, 1979). The colonisation of the skin surface starts at the umbilical stump (Mortimer et al., 1966)

The normal commensal flora of the oral cavity is more complex. The mouth is sterile at birth but is rapidly colonised by bacteria derived from attendants and the local environment. By the second day of life, aerobes and facultative species can be detected, but strict anaerobes do not appear until after the eruption of the deciduous teeth (Hurst, 1957; McCarthy, Snyder and Parker, 1965).

Reference: THE DEVELOPMENT OF THE BACTERIAL FLORA IN NORMAL NEONATES

Also (updated)

A research published in 2013 has found that colostrum contain more than 700 species of bacteria! It being the only source of food for an infant also contributes (may be largely) to the infant's normal flora.