Here's why:

* Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, which retains the crystal violet stain used in the Gram staining technique. This makes them appear purple under a microscope.

* Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane, which allows the stain to wash away during the decolorization step. They appear pink after counterstaining with safranin.

While the mouth is home to a diverse microbial community, common Gram-positive bacteria found there include:

* Streptococcus species (like *S. mutans* and *S. sanguinis*)

* Staphylococcus species (like *S. aureus*)

* Lactobacillus species

However, some Gram-negative bacteria also exist in the oral cavity, including:

* Porphyromonas gingivalis

* Prevotella intermedia

* Treponema denticola

It's important to note that the specific types and proportions of bacteria in the mouth can vary significantly from person to person, depending on factors like age, diet, and hygiene practices.