With the mouth:[1] The Metzitza is an integral part of the Mitzvah of circumcision and must be performed specifically with the mouth. It may not be performed using a tube. A Mohel who refuses to perform the Metzitza with his mouth and desires to use a tube to do so, is not to be allowed to circumcise.[2] [Nonetheless, if it is not possible to perform the Metzitza with his mouth, such as due to refusal of the parents to permit a circumcision to take place with it, then it is better for the Metzitza to be performed using a tube then for it to not be performed at all. In such an event, one is to use a glass tube which contains a small ball of cotton that has a disinfection agent absorbed in it.[3]]

[1] See Binyan Tziyon 23;24; Maharam Shick Y.D. 244; Mahariy Asad Y.D. 258; Tzafnas Paneiach 2:152; Sdei Chemed Kuntrus Hametzitza; Igros Kodesh Rashab 1:387; Igros Kodesh 3:176; 15:92; Shulchan Menachem 5:132; Shevach Baris 18:6 ; Pesakim Uteshuvos 264:15 in length

No known health risk: Experience from the past thousands of years shows that there is no worry of danger involved in performing the Metzitza directly with one’s mouth. [Igros Kodesh Rashab 1:387 based on Meishiv Nefesh; Maharam Shick ibid]

[2] Igros Kodesh Rashab 1:387

[3] Igros Kodesh 15:341; Kefar Chabad vol. 708 p. 166; ; Sdei Chemed; Chasam Sofer; See Pesakim Uteshuvos 264:15 footnotes 122-124