Mothers and sugars are the primary sources of caries-inducing bacteria in children:
Earlier studies suggest that infants acquire mutans streptococci (MS) from their mothers and only after the eruption of primary teeth. [49],[50] Preventive interventions for the purpose of reducing the transmission of bacteria from mothers to children improve the likelihood of better oral health for the child.
http://www.jnsbm.org/article.asp?issn=0976-9668;year=2013;volume=4;issue=1;spage=29;epage=38;aulast=%C7olak