It might be expected that bacterial cells, being small and relatively simple, would be uniform in shape and size. This is not the case, as the microbial world offers considerable variety in terms of morphology. However, the two most common shapes are cocci and rods.
Cocci
Cocci (s., coccus) are roughly spherical cells. They can exist singly or can be associated in characteristic arrangements that can be useful in their identification. Diplococci (s., diplococcus) arise when cocci divide and remain together to form pairs.
Long chains of cocci result when cells adhere after repeated divisions in one plane; this pattern is seen in the genera Streptococcus, Enterococcus, and Lactococcus.
Members of the genus Staphylococcus divide in random planes to generate irregular, grapelike clumps. Divisions in two or three planes can produce symmetrical clusters of cocci. Bacteria in the genus Micrococcus often divide in two planes to form square groups of four cells called tetrads.
In the genus Sarcina, cocci divide in three planes, producing cubical packets of eight cells.
Bacillus
Bacillus megaterium is an example of a bacterium with a rod shape. Rods, sometimes called bacilli (s., bacillus), differ considerably in their length-to-width ratio, the coccobacilli being so short and wide that they resemble cocci. The shape of the rod’s end often varies between species and may be flat, rounded, cigar-shaped, or bifurcated. Although many rods occur singly, some remain together after division to form pairs or chains (e.g., Bacillus megaterium is found in long chains).
Other cell shapes and arrangements are also observed.
Vibrios are comma-shaped.
Spirilla are rigid, spiralshaped cells. Many have tufts of flagella at one or both ends.
Spirochetes are flexible, spiral-shaped bacteria that have a unique, internal flagellar arrangement. These bacteria are distinctive in other ways, and all belong to a single phylum, Spirochaetes. The oval- to pear-shaped members of the genus Hyphomicrobium produce a bud at the end of a long hypha.
Other bacteria are pleomorphic, being variable in shape and lacking a single, characteristic form. Phylum Spirochaetes; The Caulobacteraceae and Hyphomicrobiaceae (section 22.1)
Some bacteria can be thought of as multicellular. Actinomycetes typically form long filaments called hyphae. The hyphae may branch to produce a network called a mycelium, and in this sense, they are similar to eukaryotic filamentous fungi.
Many cyanobacteria, a group of photosynthetic bacteria, are also filamentous. Being filamentous allows some degree of differentiation among cells in the filament. For instance, some filamentous cyanobacteria form heterocysts within the filament; these are specialized cells that carry out nitrogen fixation. The myxobacteria are of particular note. These bacteria sometimes aggregate to form complex structures called fruiting bodies. Suborder Streptomycineae; Phylum Cyanobacteria; Order Myxococcales
Escherichia coli is an excellent representative of the average size of bacteria. This rod-shaped bacterium is 1.1 to 1.5 11m wide by 2.0 to 6.0 11m long. However, the size range of bacterial cells extends far beyond this average. Near the small end of the size continuum are members of the genus Mycoplasma (0.3 11m in diameter).
At the other end of the continuum are bacteria such as some spirochetes, which can reach 500 11m in length, and the cyanobacterium Oscillatoria, which is about 7 11m in diameter (the same diameter as a red blood cell). The huge bacterium Epulopiscium fishelsoni lives in the intestine of the brown surgeonfish (Acanthurus nigrofuscus). E. fishelsoni grows as large as 600 by 80 11m, a little smaller than a printed hyphen and clearly larger than the well-known eukaryote Paramecium. An even larger bacterium, Thiomargarita namibiensis, has been discovered in ocean sediment. Thus a few bacteria are much larger than the average eukaryotic cell (typical plant and animal cells are around 10 to 50 11m in diameter).