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Concept
Bacterial cellular morphologies
Summary
Bacterial cellular morphologies are morphologies that are characteristic of various types bacteria and often a key factor in identifying bacteria species. Their direct examination under the light microscope enables the classification of these bacteria and archaea.
Generally, the basic morphologies are spheres (coccus) and round-ended cylinders or rod shaped (bacillus). But, there are also other morphologies such as helically twisted cylinders (example Spirochetes), cylinders curved in one plane (selenomonads) and unusual morphologies (the square, flat box-shaped cells of the Archaean genus Haloquadratum). Other arrangements include pairs, tetrads, clusters, chains and palisades.
A coccus (plural cocci, from the Latin coccinus (scarlet) and derived from the Greek kokkos (berry)) is any microorganism (usually bacteria) whose overall shape is spherical or nearly spherical. Describing a bacterium as a coccus, or sphere, distinguishes it from bacillus, or rod. This is the first of many taxonomic traits for identifying and classifying a bacterium according to binomial nomenclature.
Important human diseases caused by coccoid bacteria include staphylococcal infections, some types of food poisoning, some urinary tract infections, toxic shock syndrome, gonorrhea, as well as some forms of meningitis, throat infections, pneumonias, and sinusitis.
Coccoid bacteria often occur in characteristic arrangements and these forms have specific names as well; listed here are the basic forms as well as representative bacterial genera:
pairs or diplococci (e.g. Neisseria spp.)
groups of four or eight known respectively as tetrads and sarcina (e.g. Micrococcus spp.)
bead-like chains (e.g. Streptococcus spp.)
grapelike clusters (e.g. Staphylococcus spp.)
A bacillus (plural bacilli) is a rod-shaped bacterium. Although Bacillus, capitalized and italicized, specifically refers to the genus, the word bacillus (plural bacilli) may also be used to describe any rod-shaped bacterium, and in this sense, bacilli are found in many different taxonomic groups of bacteria.
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