Explain how the bacteria are able to attach firmly to teeth such that the bacteria can only be removed with scraping.

The first step in the formation of the plaque is the adsorption of the bacterial molecules to the cell surface. The pellicle, which is a protein film, acts like an adhesive to the tooth surface, the pellicle attracts the bacterial molecule to get attached, hence, the colonization of the microbes.  

Due to the presence of the pellicle, the bacteria are stick to the tooth enamel at the surface. That is why; it becomes difficult to get rid of bacteria without scrapping them off.