Morphological classification of medical important bacteria

Morphological classification of medical important bacteria

Morphologically bacteria can resemble:

  1. Cocci (Singular: coccus)
  2. Bacilli / rods (Singular: rod, bacillus)
  3. Vibrios (Singular: vibrio)
  4. Spirilla (Singular: spirillum)
  5. Spirochaetes (Singular: spirochaet

  • Cocci: These are round or oval bacteria measuring about 0.5–1.0 m in diameter. When multiplying, cocci may form pairs, chains, or irregular groups:
    •  cocci in pairs are called diplococci, e.g. meningococci and gonococci.
    • cocci in chains are called streptococci, e.g. Streptococcus pyogenes.
    • cocci in irregular groups are called staphylococci, e.g. Staphylococcus aureus.

Gram reaction: Staphylococci and streptococci are Gram positive, whereas diplococci can be Gram positive or Gram negative.

  • Rods (bacilli): These are stick-like bacteria with rounded, tapered (fusiform), square, or swollen ends. They measure 1–10 µm in length by 0.3–1.0 µm in width. The short rods with rounded ends are often called coccobacilli. When multiplying, bacterial rods do not usually remain attached to one another, but separate. Occasionally, however, they may:
    • form chains, e.g. Streptobacillus species.
    • form branching chains, e.g. lactobacilli.
    • mass together, e.g. Mycobacterium leprae.
    • remain attached at various angles resembling Chinese letters, e.g. Corynebacterium diphtheriae.    
      • The rods of the genera Bacillus and Clostridium are able to form resistant spores when conditions for vegetative growth are unfavourable. Many rods are motile having a single flagellum, or several flagella, at one or both ends or surrounding the entire organism.
      • Gram reaction: Many rods are Gram negative such as the large group of enterobacteria. Gram positive rods include Clostridium species, Corynebacterium species, Bacillus anthracis, and Listeria monocytogenes.

Vibrios: These are small slightly curved rods measuring 3–4 µm in length by 0.5 µm in width. Most vibrios are motile with a single flagellum at one end. They show a rapid darting motility, e.g. Vibrio cholerae. 


    • Gram reaction: Vibrios are Gram negative
  • Spirilla: These are small, regularly coiled, rigid organisms measuring about 3–4 µm in length. Each coil measures about 1 µm. Spirilla are motile with groups of flagella at both ends. An example of a spirillum is Spirillum minus.
    •  Gram reaction: Spirilla are Gram negative.
  •  Spirochaetes: These are flexible, coiled, motile organisms. They progress by rapid body movements. Most are not easily stained by the Gram method. Spirochaetes are divided into three main groups:

    1. Treponemes, which are thin delicate spirochaetes with regular tight coils, measuring from 6–15 µm by 0.2 µm in width. Examples include Treponema pallidum and Treponema pertenue.
    2. Lorreliae, which are large spirochaetes with irregular open coils, measuring 10–20 m in length by about 0.5 m in width. Examples include Borrelia duttoni and Borrelia vincenti.
    3. Leptospires, which are thin spirochaetes with many tightly packed coils that are difficult to distinguish. They measure 6–20 m in length by 0.1 m in width and have hooked ends. The leptospire of medical importance is Leptospira interrogans (contains many serovars).

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