The Size, Shape, and Arrangement of Bacterial Cells

The Size, Shape, and Arrangement of Bacterial Cells

  • Bacteria come in a many sizes and several shapes.
  • Most bacteria range from 0.2 to 2.0 μm in diameter and from 2 to 8 μm in length.
  • They have a few basic shapes: spherical coccus (plural: cocci), rod-shaped bacillus (plural: bacilli), and spiral.
  • Cocci are usually round but can be oval, elongated, or flattened on one side.
  • When cocci divide to reproduce, the cells can remain attached to one another.
  • Cocci that remain in pairs after dividing are called diplococci; those that divide and remain attached in chain like patterns are called streptococci .

Arrangements of cocci and Bacilli

  • Those that divide in two planes and remain in groups of four are known as tetrads.
  • Those that divide in three planes and remain attached in cubelike groups of eight are called sarcinae.
  • Those that divide in multiple planes and form grape like clusters or broad sheets are called staphylococci.
  • Bacilli divide only across their short axis, so there are fewer groupings of bacilli than of cocci.
  • Most bacilli appear as single rods.
  • Diplobacilli appear in pairs after division and streptobacilli occur in chains. Some bacilli look like straws.
  • Others have tapered ends, like cigars. Still others are oval and look so much like cocci that they are called coccobacilli.
  • “Bacillus” has two meanings in microbiology. As we have justn used it, bacillus refers to a bacterial shape. When capitalized and italicized, it refers to a specific genus.
  • For example, the bacterium Bacillus allthracis is the causative agent of anthrax.
  • Bacillus cells often form long, twisted chains of cells.

Spiral bacteria.

  • Spiral bacteria have one or more twists; they are never straight.
  • Bacteria that look like curved rods are called vibrios. Others, called spirilla, have a helical shape, like a corkscrew, and fairly rigid bodies.
  • Another group of spirals are helical and flexible; they are called spirochetes.
  • Unlike the spirilla, which use propeller-like external appendages called flagella to move, spirochetes move by means of axial filaments, which resemble flagella but are contained within a flexible external sheath.
  • In addition to the three basic shapes, there are star-shaped cells rectangular, flat cells and triangular cells.

Star-shaped and rectangular

  • The shape of a bacterium is determined by heredity.
  • Genetically, most bacteria are monomorphic; that is, they maintain a single shape. However, a number of environmental
    conditions can alter that shape.
  • If the shape is altered, identification becomes difficult.
  • some bacteria, such as Rhizobium and Corynebacterium are genetically pleomorphic. which means they can have many shapes, not just one.

The Size, Shape, and Arrangement of Bacterial Cells

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