Chapter 3 · Chapter 4

Shape and arrangement

Two questions, asked in one breath: what shape is the cell, and how do the cells sit together after they divide? The name of an organism often answers both before you have seen it.

The six shapes

Shape comes from the cell wall, which is why a cell without one — Mycoplasma — has no fixed shape at all.

Fig. 1Drawn at one scale. A coccobacillus is genuinely ambiguous at the microscope, which is why it has its own name.

Singular	Plural	Description	Example
Coccus	Cocci	Spherical	Staphylococcus aureus
Bacillus	Bacilli	Rod shaped	Escherichia coli
Coccobacillus	Coccobacilli	A short, stubby rod, close to round	Haemophilus influenzae
Vibrio	Vibrios	A curved rod, like a comma	Vibrio cholerae
Spirillum	Spirilla	A rigid spiral	Spirillum volutans
Spirochete	Spirochetes	A flexible corkscrew	Treponema pallidum

The six arrangements

Arrangement is a record of division: which plane the cell split in, and whether the daughters let go afterwards.

Fig. 2Cocci divide in one, two or three planes, which is where pairs, tetrads and cubes come from. Rods only divide across the short axis.

Prefix	Means	Applies to	Example
Diplo-	Pairs	cocci and bacilli	Neisseria gonorrhoeae
Strepto-	Chains	cocci and bacilli	Streptococcus pyogenes
Staphylo-	Irregular clusters, like grapes	cocci	Staphylococcus aureus
Tetrad	Squares of four	cocci	Micrococcus luteus
Sarcina	Cubes of eight	cocci	Sarcina ventriculi
Palisade	Side by side, like a fence	bacilli	Corynebacterium diphtheriae

Build the name

Pick a shape and an arrangement. Not every pair is possible — the ones that are not tell you something about how the cell divides.

Shape

Arrangement

The name you would write

Staphylococci

For example, Staphylococcus aureus.

Staphylo- means irregular clusters, like grapes, and applies to cocci. Coccus means spherical.

Fig. 3The combination as it would appear on a slide. Arrangement comes from the plane the cell divides in and whether the daughters stay attached.