Gram positive bacteria have thick peptidoglycan layer and no outer lipid membrane thus referred to as monoderms. When subjected to Gram staining, the Gram positive bacteria have a distinctive purple appearance when observed under light microscope. This is due to retention of purple crystal violet stain in the thick peptidoglycan layer of the cell wall. Examples of Gram positive bacteria include:- all staphylococci, all streptococci and some listeria species.
Gram negative bacteria on the other hand have a thin peptidoglycan layer and have an outer lipid membrane. Thus they are referred to as diderms. Following Gram staining , Gram negative bacteria have a distinctive pale reddish color when observed under light microscope. This is due to the fact that the structure of their cell wall is unable to retain the crystal violet stain so are colored by safranin counterstain. Examples of Gram negative bacteria include:- enterococci , salmonella and pseudomonas species.
Spore forming bacteria have the ability to undergo complex developmental cell differentiation process that allows them to adapt to harsh environmental conditions by producing highly resistant endospores. They mainly belong to two genera, that is, the aerobic or facultative Bacilli and the strictly anaerobic Clostridia.