Rod-shaped Bacteria - Types and Diseases


Bacteria are of several shapes; they can be spherical (cocci), comma-shaped (vibrio), coiled (spirochaetes), etc. Rod-shaped bacteria are termed bacilli. These bacilliform bacteria are found in several taxonomic groups of bacteria. The term Bacillus (genus) includes rod-shaped gram-positive bacteria. Bacilli also comprise several other such genera. Most of the rod-shaped bacteria are soil microorganisms in which some are parasitic while some are commensals.

Let’s look at some examples of bacilli and diseases caused by them.

Types of Rod-shaped Bacteria

Bacilli are bacteria that have rod-shaped cells. In that, some of these bacilli have peculiar shapes or arrangements as follows –

  • Diplobacilli – two rod-shaped bacilli arranged side by side. Eg., Klebsiella rhinoscleromatis
  • Streptobacilli – the bacilli are arranged in chains. Eg., Streptobacillus
  • Coccobacilli – short bacilli whose length is approximately equal to their width. Eg., Brucella
  • Palisades – Chinese letter or cuneiform pattern – cells are arranged at angles to each other and have a club-shaped appearance. Eg., Corynebacterium

Gram-positive Examples of Rod-shaped Bacteria

  1. Eubacterium
  2. Lactobacillus
  3. Actinomyces
  4. Bifidobacterium
  5. Propionibacterium

Gram-negative Examples of Rod-shaped Bacteria

  1. Bacteroides
  2. Prevotella
  3. Porphyromonas
  4. Fusobacterium
  5. Leptotrichia

Also Check: Gram-negative Bacteria

Anaerobic and Aerobic Rod-shaped Bacteria

Majorly, sporogenous, rod-shaped gram-positive bacteria are grouped into two – the aerobic Bacillus and the anaerobic Clostridium. The spores of Clostridium are wider than their bacterial body. Whereas, the spores of Bacillus are of the same width as that of their bacterial body.

Non-sporing anaerobes are an important cause of animal and human infections. Most of them are found in the mouth, oropharynx, genital tracts and gastrointestinal tracts. Even in such seemingly aerobic places as skin and mouth, anaerobic bacteria are more frequent than aerobes.

Examples of anaerobic gram-positive bacilli

Eubacterium – E. brachy

Lactobacillus – L.acidophilus

Bifidobacterium – B.dentium

Propionibacterium – Cutibacterium acnes

Examples of anaerobic gram-negative bacilli

Bacteroides – B.fragilis

Fusobacterium – F.nucleatum

Leptotrichia – L.buccalis

Examples of aerobic gram-positive bacilli

Bacillus (facultative anaerobes)


Listeria (facultative anaerobes)


Examples of aerobic gram-negative bacilli

Pseudomonas aeruginosa

Enterobacter (facultative anaerobes)

Most members of the Enterobacteriaceae family can thrive well in both aerobic and anaerobic conditions.

Explore: Difference between Aerobic and Anaerobic Bacteria

Diseases Caused by Rod-shaped Bacteria

Some rod-shaped bacteria cause diseases in humans as well as animals. Most of them are soil microorganisms that can cause infections when they come in contact with animals or humans. A few such diseases are –

  1. Typhoid – It occurs when water or food contaminated with Salmonella is ingested. Salmonella typhi is the causative bacteria that is usually transmitted through the faecal-oral route. Thus typhoid can be controlled by effective sanitary measures.
  2. Diphtheria – It is an infection caused due to toxins produced by bacilli Corynebacterium diphtheriae. This bacterium is spread through direct contact and air.
  3. Tetanus – It is a bacterial infection caused by bacilli Clostridium tetani. It is typically characterised by muscle spasms. The endospores of C.tetani are found almost everywhere.
  4. Plague – It is caused by a coccobacillus bacterium, Yersinia pestis. This bacterium can be transmitted through several ways (air, faecal-oral route, vectors, etc.). The bacterium affecting lymph nodes causes bubonic plague. Likewise, pneumonic plague and septicemic plague affect the lungs and blood respectively.
  5. Anthrax – It is caused by Bacillus anthracis in animals like goats, sheep, cattle, etc. Anthrax might also occur among humans if they are in constant contact with animals. In most cases, the bacterium enters through abrasions or cuts in the skin.

Rod Bacteria vs Coccus Bacteria

Rod Bacteria

Cocci Bacteria

Rod-shaped bacteria are termed bacilli.

Round, oval or spherical shaped bacteria are termed cocci.

Gram-positive bacilli examples – Lactobacillus, Eubacterium, Actinomyces, etc.

Gram-positive cocci examples – Peptococcus, Finegoldia, Blantia, etc.

Gram-negative bacilli examples – Bacteroides, Fusobacterium, Leptotrichia, etc.

Gram-negative cocci examples – Anaeroglobus, Veillonella, Acidaminococcus, etc.

Frequently Asked Questions

What is the difference between gram-negative and gram-positive bacteria?

Gram-positive and gram-negative are two groups of bacteria based on their differentiation on gram staining. Gram-positive resist decolourisation and retain the primary stain (violet). Whereas, gram-negative is decolourised by alcohol or acetone and hence takes counterstain like safranin and appears red in colour.

What is the difference between aerobic and anaerobic bacteria?

The vital difference between aerobic and anaerobic bacteria is that the former needs oxygen to survive and the latter does not require oxygen. Example of aerobic bacilli – Nocardia. Example of anaerobic bacilli – Lactobacillus. Facultative anaerobes are another group that can survive in both aerobic and anaerobic conditions. Example – Enterobacter.

What are Enterobacteriaceae?

Enterobacteriaceae is a family that mostly includes bacteria thriving in the intestines of animals and humans. It comprises many rod-shaped bacteria like Escherichia coli, Klebsiella, Salmonella, Citrobacter, Enterobacter and Shigella.

How does Bacillus differ from bacilli?

The term ‘Bacilli’ (not italicized) includes rod-shaped bacteria like Bacillus (italicized). Bacillus is a genus of bacteria that are rod-shaped, aerobic and gram-positive. Two significant pathogenic and parasitic Bacillus species are B.cereus (food poisoning) and B.anthracis (anthrax).

Keep exploring BYJU’S Biology for more such exciting topics

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