Siwak: 

Dear brothers, I would like to discuss the scientific topic, and let me start with the following Hadith: 

("Siwak (tooth-stick) cleanses and purifies the mouth and pleases the Lord.")

[Al-Bukhari, on the authority of A'ishah]

A scientist specialized in the bacteriology in Germany wrote an article for a famous Arab magazine saying: I read about the siwak that Arabs use as a tooth brush.

A German scientist specialized in bacteriology and epidemiology wrote an article in an Arab magazine about siwak. The writer had read that Arabs use as a tooth brush, in a book written by a traveler who visited Arab countries. The writer was sarcastic and he used this as a proof on the ignorance of those people who clean their teeth with plant sticks.

The German scientist said, "The western traveler made fun of Muslims for using siwak to clean their teeth, but I had a different point of view and I thought that there might be a scientific truth behind this stick that Arabs use as a toothbrush.  I hoped to perform some experiments about it." He went on saying, "A colleague of mine traveled to Sudan, and when he came back, he brought with him some sticks of siwak. I started my experiments immediately by crushing the sticks till they became like powder. I mixed the powder with some water and put it on bacteria cultures.

Surprisingly enough, the effects of the siwak on the bacteria were the same effects of penicillin, which is effective in killing germs." That is what the German scientist who is a specialist in bacteriology and epidemiology said. 
This article make us believe beyond any doubts that the health instructions of the Prophet, peace be upon him, have nothing to do with his culture, experience, opinion or facts of his era. They are rather an inspiration inspired to him. 

Benefits of Siwak: 

The great scholar, Imam Ibn al-Qayyim, may Allah have mercy upon him, said, "There are several benefits of siwak. It strengthens the gum, cures the phlegm,

sharpens the sight, heals the stomachach, softens the voice, helps in digesting food, makes articulation easier, activate the brain to read,

remember Allah and pray without feeling asleep, pleases the Lord, pleases the angles and increases man's good deeds." Accordingly, using siwak is an act of worship, since the Prophet, peace be upon him, used to use it.  However, one should not exaggerate in using it. 

While I was in an Islamic country, I attended Friday Khutbah. A man sitting next to me kept cleaning his teeth from the beginning till the end of khutba. That was really unbearable. Some exaggerations are so annoying.
Then he threw it on the ground, picked it up again and started to clean his teeth. Well, you should use siwak in a proper way and at suitable times. 

Dear brothers, when you believe that revelation has two kinds; one is recited in the exact words and vowels, which is the Noble Quran, and the other one is not recited, but it is inspired from Allah as Allah, Exalted and Sublime be He, says:   

(Nor does he speak of (his own) desire* It is only an Inspiration that is inspired.)

[An-Najm, 3-4]

Hence, all the instructions of the Prophet, peace be upon him, are an inspiration from Allah, but in the prophet’s own words. However, the meaning is inspired to him from Allah.