Bacteria: A Brief Exploration


Size: The length of bacteria ranges from 0.5 to 5 µm.

Shape: Bacteria come in a variety of forms, such as spherical cocci, rod-shaped bacilli, distorted rod-shaped or comma -shaped vibrio, and spiral-shaped spirilla.

Inside the cell: Organelles and a membrane-bound nucleus are absent from bacterial cells. Within the nucleoid area, there is a single circular DNA molecule containing the genetic material. Sometimes they have extranuclear DNA called plasmids.

Discovery: Using a crude microscope, Anton van Leeuwenhoek made the first observation of bacteria in the 17th century. Christian Gottfried Ehrenberg coined the term “bacteria.”

Reproduction: The main method of bacterial reproduction is binary fission, in which a single parent cell divides into two identical daughter cells, enabling exponential population expansion.

Importance: Produces many antimicrobial agents (like pmathiapeptide, destotamide, Marfomycins, spirotetronates etc.), and products like vinegar (Acetobacter, Gluconacetobacter, Gluconobacter and Komagataeibacter).

Agents of Nitrogen fixation: Nitrogen fixation is a vital process that involves some bacteria, such as Rhizobium species, transforming atmospheric nitrogen into forms that plants can use.

Applications in Biotechnology: In biotechnology, bacteria are often employed in procedures including genetic editing, insulin synthesis, and environmental pollution removal.

Bacterial groups

Classification based on shape: Bacteria are of different shapes and can be classified into the following groups

1. Bacilli


  • Escherichia coli (commensal intestinal flora)
  • Lactobacillus (produces lactic acid as a by-product of glucose metabolism)
  • Coxiella burnetiid (causes Q fever in humans)
  • Klebsiella rhinoscleromatis (causes rhinoscleroma, a disease of nasal mucosa)
  • Streptobacillus moniliformis (present in the oropharynx of rodents)
  • Corynebacterium diphtheriae (the cause of diphtheria)

2. Cocci

Spherical shaped

  • Occur as a single sphere
  • Neisseria gonorrhoeae (causes STD gonorrhoea), can also exist as diplococci
  • Neisseria meningitidis (Causes meningococcal meningitis and fulminant meningococcemia)
  • Grows in chain
  • Streptococcus pneumoniae (causes pneumonia, meningitis), Streptococcus pyogenes (causes Pharyngitis, glomerulonephritis)
  • Streptococcus aureus (toxin- and non-toxin-mediated infections, surgical wound infections, primary bacteremia)

3. Vibrio

Comma or distorted rod-shaped

  • Gram-negative
  • Highly motile (1-3 flagella at one end)
  • Curved rods 0.5 μm
  • Vibrio cholerae (causes cholera), Vibrio fischeri (a bioluminescent marine bacterium)

4. Spirillum

Coiled morphology, like a metal spring

  • Gram-negative
  • Spirochetes
  • Motile (lack external flagella, they have endogenous flagella)
  • 3.0 μm by 5 to 250 μm
  • Ex: Treponema pallidum (causes Syphilis), Serpulina (present in the mammalian intestine), Borrelia cause relapsing fever and Lyme disease, Leptospira causes  Leptospirosis
Classification based on flagellation: Flagella are the locomotory organelle of bacteria generally present on the cell surface

1. Atrichous

“thrix” or “trichos,” = “hair”

  • They lack flagella
  • Ex: Lactobacillus (convert milk into curd)
2. Monotrichous 
  • A single flagellum at one pole
  • Ex: Vibrio alginolyticus (causes wound and ear infections)
3. Lophotrichous 
  • A tuft of flagellum at one pole
  • Ex: Vibrio fischeri (a bioluminescent marine bacterium)
4. Amphitrichous 
  • A single or a tuft of flagella at both poles
  • Ex: Campylobacter jejuni (causes acute gastroenteritis with diarrhoea, abdominal pain, fever, nausea, and vomiting)
5. Peritrichous 
  • Flagella distributed all over the cell surface
  • Escherichia coli (an entric bacteria that lives in the intestine as a commensal in humans and other warm-blooded animals), Salmonella enterica (sv. Typhi and sv. Paratyphi are transmitted through drinking water and cause typhoid fever in humans; “sv” stands for “serovar” or “serotype”)
Classification based on Gram-staining: Gram-staining is a technique developed by the Danish bacteriologist Hans Christian Gram in 1884. This staining method provides a rapid means of differentiating bacterial cells into two groups based on the colour difference produced after staining. The colour differences arise due to different structures of the cell wall.
  • They have a thick peptidoglycan layer present in the cell wall that retains the primary stain crystal violet
  • Appear purple
  • Ex: Staphylococcus, Streptococcus, Clostridia, Listeria, Corynebacterium etc.
  • They have a thin peptidoglycan layer present in the cell wall and an outer membrane that prevents the retention of primary stain but retains the counterstain safranin
  • Appear pink
  • Generally pathogenic
  • Ex: Enterobacteriaceae, Escherichia, Proteus, Enterobacter, Klebsiella, Citrobacter, Yersinia, Shigella, and Salmonella

Bacteria can also be classified on other parameters such as:

  • Metabolism (Photoautotrophs, chemoautotrophs, and heterotrophs).
  • Pathogenicity (Pathogenic and non-pathogenic)
  • Oxygen requirement (Aerobic and Anaerobic)
Bacterial classification


Among bacteria, a few lack the cell wall or have little wall material. They are mycoplasma or related organisms. They are the smallest cells that can grow outside the living cells. They can pass through bacterial filters. Their plasma membrane has sterols which protect them from lysis. They are pathogenic to humans.


The tiny cells of the living world, bacteria, are remarkably diverse in terms of size, form, and function. Their sizes range from 0.5 to 5 µm, and their diverse shapes—such as spiral-shaped spirilla, rod-shaped bacilli, and spherical cocci—affect how they interact with their surroundings.

Bacterial cells are dependent on a single circular DNA molecule located within the nucleoid area since they lack membrane-bound nuclei and organelles. Binary fission is the main method of reproduction, which enables quick population expansion. Since their discovery in the 17th century by Anton van Leeuwenhoek, bacteria have played a major role in influencing our knowledge of the microbial world.

Their importance goes well beyond their diminutive stature. The vital activities that are necessary for plant nourishment, such as nitrogen fixation, are facilitated by bacteria. By categorizing bacteria according to their morphology, flagellation, and Gram staining, we can learn more about their taxonomy and the intricate web of microbial life.

Bacteria are excellent biotechnological instruments that help in genetic editing, the synthesis of insulin, and the reduction of environmental pollutants. As we learn more about these microscopic marvels, we also learn about their significant effects on human health, industry, and the fragile environmental balance.


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