Last Updated on February 4, 2021 by Sagar Aryal
Morphologically bacteria can resemble:
- Cocci (Singular: coccus)
- Bacilli (rods) (Singular: rod, bacillus)
- Vibrios (Singular: vibrio)
- Spirilla (Singular: spirillum)
- Spirochaetes (Singular: spirochaete)
These are round or oval bacteria measuring about 0.5–1.0 um 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
These are stick-like bacteria with rounded, tapered (fusiform), square, or swollen ends. They measure 1–10 um in length by 0.3–1.0 um 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 unfavorable. 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.
Note: Some coccobacilli, such as Yersinia species, show bipolar staining when stained with methylene blue or Giemsa.
These are small slightly curved rods measuring 3–4 um in length by 0.5 um in width. Most vibrios are motile with a single flagellum at one end. They show rapid darting motility, e.g. Vibrio cholerae.
Gram reaction: Vibrios are Gram-negative.
These are small, regularly coiled, rigid organisms measuring about 3–4 um in length. Each coil measures about 1 um. Spirilla are motile with groups of flagella at both ends. An example of a spirillum is Spirillum minus.
Gram reaction: Spirilla are Gram negative.
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:
- treponemes, which are thin delicate spirochaetes with regular tight coils, measuring from 6–15 um by 0.2 um in width. Examples include Treponema pallidum and Treponema pertenue.
- borreliae, which are large spirochaetes with irregular open coils, measuring 10–20 um in length by about 0.5 um in width. Examples include Borrelia duttoni and Borrelia vincenti.
- leptospires, which are thin spirochaetes with many tightly packed coils that are difficult to distinguish. They measure 6–20 um in length by 0.1 um in width and have hooked ends. The leptospire of medical importance is Leptospira interrogans (contains many serovars).
These are naturally occurring stable bacteria that lack a rigid cell wall. They are among the smallest living microorganisms capable of independent existence, ranging in size from 0.1–2 um. Species of medical importance include Mycoplasma pneumoniae and Ureaplasma urealyticum.