Morphology of Bacteria


Bacteria (Gk. bakterion – small stick) are predominantly unicellular organisms. The size of bacteria varies from 0.1 to 20 μm or more. Most of pathogenic bacteria are within the range 0.2 to 10 μm.

The shapes and sizes of microbes are not strictly stable. They get adaptation to the surroundings and environmental conditions. But in constant situations bacteria maintain their sizes and shapes that are specific characteristics for certain microbial groups.

Bacteria demonstrate several basic morphological forms. Among them are spherical round-shaped cells (or cocci); rod-shaped (bacteria, bacilli, and clostridia); coiled or spiral forms (vibrios, spirilla, spirochetes); filamentous and branched bacteria.

  • Spherical Bacteria or Cocci

Cocci (Gk. kokkos – berry) are round-shaped bacterial forms. They can be spherical, ellipse-, bean- or lancet-like. Cocci are further arranged into six main groups according to number of cells in clusters, planes of cellular division, their common biological features, etc.

  1. Micrococci. These round cells are placed separately, singly and irregularly.
  2. Diplococci (Gk. diplos – double) are divided within one plane being attached in pairs. Among them are pathogenic Neisseriae: meningococci – causative agents of cerebrospinal meningitis; and gonococci – causative agents of gonorrhoea and ophthalmia neonatorum (or blennorrhoea).
  3. Streptococci (Gk. streptos – curved) are divided in one plane making long or short chains. Many of them are pathogenic for humans causing suppurative infections, pneumonia, caries. 4. Tetracocci (Gk. tetra – four) are reproduced within two planes at right angles making clusters of four cocci. They are non-pathogenic.
  4. Sarcinae are divided in three perpendicular planes at right angles producing packages of 8-16 or more cells. They can be present as normal habitants in air.
  5. Staphylococci (Gk. staphyle – grape) are irregularly divided within several planes producing variable cell clusters; the latter in most cases resemble grapes. Various species of staphylococci cause suppurative diseases in humans.
  • Rod-shaped Bacteria

Rod-like bacterial forms comprise bacteria, bacilli, and clostridia.

Bacteria are rod-like microorganisms that don’t produce spores. Among them are all enterobacteria, corynebacteria, bacteroids, fusobacteria and many others.

Bacilli and clostridia are the bacteria that produce spores.

Bacilli cells contain spore that don’t exceed the width of microbial cell (e.g., causative agent of anthrax), whereas clostridiae spores protrude out of microbial body (causative agents of tetanus, botulism, gas gangrene, etc.).

Rod-shaped bacteria vary by their forms and sizes. There are short or middle-size rods (eg, enterobacteria, bordetella or whooping cough agent and others); some bacteria are long (such as anthrax bacilli). Most of bacteria demonstrate blunt ends, certain species (eg, fusobacteria) carry tapered ends.

Bacteria that occur in pairs are diplobacteria (eg, Klebsiella pneumoniae) or diplobacilli (if they contain spore).

Streptobacteria and streptobacilli make chains of various length (B. anthracis).

Nevertheless, vast number of bacteria, bacilli or clostridia are separated one from another without regular pattern.

  • Coiled or Spiral Bacterial Forms

Vibrios, spirilla and spirochetes pertain to this group of bacteria.

  1. Vibrios (Lat. vibrio – to vibrate) are the curved rods that make half a coil and look like comma. Typical representative here is Vibrio cholerae – causitive agent of cholera.
  2. Spirilla (Lat. spira – coil) are cork-screw-like coiled bacteria with twists of one or more turns. Pathogenic Helicobacter pylori belongs to spirilla.
  3. Spirochetes are twisted forms of bacteria exhibiting multiple compacted twists with many turns. Pathogenic Treponemas, Borreliae, and Leptospirae pertain to spirochetes.
  • Filamentous Bacteria

Filamentous and branched bacteria exhibit long and thin thread-like microbial cells. Typical representatives of thread-like bacteria are actinomycetes. Tangled mixture of their cells produce complex interwoven structure known as mycelium.