Domains Bacteria and Archaea
 
Organisms known as archaea (archaebacteria) belong to the Domain Archaea, and are typically chemosynthetic. They share the characteristics of prokaryotes (no membrane-bound organelles; genes on a ring of DNA rather than real chromosomes; usually quite small) with the bacteria, which belong to the Domain Bacteria. In lab, we are only concerned with the Domain Bacteria. For these organisms, cell shape (bacillus, coccus or spirillum) and gram-stain (positive or negative) are among the more practical means of classification.
 

Eubacteria

For many species of "true" bacteria, the known biology still does not allow a reasonable means of classifying them into groups that are both closely related and recognizable. Thus, we consider them Eubacteria. These are typically heterotrophic cells with walls containing peptidoglycan. There are numerous (not always widely agreed upon) phylum-level categories and evolutionary lineages. They may be motile or not. Examples of common types of Eubacteria include the following:
            gram negative bacilli
                    Note the color (pink is negative) and shape (bacillus is rod-shaped).

            gram positive bacilli
                    Note the color (purple is positive) and shape (bacillus is rod-shaped).

            gram negative cocci
                    Note the color (pink is negative) and shape (coccus is spherical).

            gram positive cocci
                    Note the color (purple is positive) and shape (coccus is spherical).

            some of these bacteria form endospores, which resist harsh environments.
 

Phylum Spirochetes

The spirillum (helical) shape distinguishes this group of heterotrophic bacteria, which tend to be large for prokaryotes and often possess flagella. 

    Spirillum
            Note shape and flagella (at ends).

 Phylum Cyanobacteria

These autotrophic, algal-like organisms appear plantlike, but their chlorophyll a appears in photosynthetic lamellae (not true chloroplasts). They are, however, prokaryotic, and some species fix nitrogen in specialized cells called heterocysts (see Anabaena). In general, their size is intermediate between other bacteria and the unicellular protists. 
            Oscillatoria
                    This apparently multi-cellular form consists of ribbon-like chains of cells.

            Anabaena
                    This form also has chains of cells, but these are smaller and embedded in a gelatinous matrix; note the heterocysts.

            Nostoc
                    Similar in form to Anabaena.

Biological Diversity index