Actinomycetales is an order of Actinobacteria

Actinobacteria

Actinobacteria are a group of Gram-positive bacteria with high guanine and cytosine content. They can be terrestrial or aquatic. Actinobacteria is one of the dominant phyla of the bacteria....

. They are very diverse and contain a variety of subdivisions as well as yet unclassified isolates. This is mainly because some genera are very difficult to classify because of a highly niche-dependent phenotype

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

. For example Nocardia

Nocardia

Nocardia is a genus of weakly staining Gram-positive, catalase-positive, rod-shaped bacteria. It forms partially acid-fast beaded branching filaments . It has a total of 85 species. Some species are non-pathogenic while others are responsible for nocardiosis. Nocardia are found worldwide in soil...

 contains several phenotypes that were first believed to be distinct species before it was proven that their differences were entirely dependent on their growth conditions.

Why is Classification difficult

Actinomycetales are Gram positive, however several species have complex cell wall

Cell wall

The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...

 structures that makes the gram staining unsuitable. Examples: Mycobacteriaceae