Difference Between Cocci and Bacilli

Main Difference – Cocci vs Bacilli

Cocci and bacilli are two shapes of bacteria. The main three shapes of bacteria are cocci, bacilli and spiral. Different spatial arrangements like diplococci, streptococci, staphylococci, tetrads and sarcina are found in cocci. In bacilli, diplobacilli, streptobacilli and coccobacillus are found. Both cocci and bacilli can cause infectious diseases in plants and animals. Anthrax is one of the most dangerous infectious diseases caused by bacilli. Prevention of infections by both cocci and bacilli can be achieved using antibiotics. Both bacteria contains a rigid cell wall, which is made up of murin. The average diameter of a bacterium in both cocci and bacilli is around 0.5 µm. The main difference between cocci and bacilli is that cocci are spherical or oval-shaped bacteria whereas bacilli are rod-shaped bacteria.

This article examines,

1. What is Cocci
     – Characteristics, Structure, Examples
2. What is Bacilli
      – Characteristics, Structure, Examples
3. What is the difference between Cocci and Bacilli

What is Cocci

Any bacterium, which has either a spherical, ovoid or round-shaped is a coccus. That means, one axis of the bacterium is almost the same as the other. Cocci are found either as a single cell or attached to a surface. During multiplication, different arrangements of cocci can be found: diplococci, streptococci, staphylococci, tetrads and sarcina. A pair of cocci is called diplococci. Streptococci is a chain of cocci. Irregular clusters of cocci are called staphylococci. Four cocci arrangements in the same plane are called a tetrad. Cuboidal arrangements of eight cocci are called sarcina. Examples of each arrangement of cocci are given in table 1.

Table 1: Examples of cocci arrangements

Most of the cocci are anaerobic. Several types of infections like bacteremia, cellulitis , pneumonia, gangrene, peritonitis, abscesses, bite wounds and pelvic inflammatory disease are caused by cocci. Cocci can be either gram positive or gram negative. P eptostreptococcus, Streptococcus and Gemella are gram positive cocci. Veillonella is a gram negative cocci. The gram staining of Staphylococcus aureus is shown in figure 1.

Figure 1: Staphylococcus aureus

What are Bacilli

Any bacterium which is in rod shape is classified as a bacillus. That means, one axis of the bacterium is longer than the other. Bacillus is mostly arranged as a single bacteria. A single curved rod is called vibrio. Other different shapes of bacilli are filaments, club-shaped rods, spirochetes, spindle-shaped rods, and spirillum. Examples for each bacilli shapes are given in table 2. During multiplication, different arrangements of bacilli are found: diplobacilli, streptobacilli and coccobacillus. Diplobacilli is the side by side arrangement of two bacteria. Streptobacilli are bacterial chains. Coccobacillus is short rod-shaped bacteria.

Table 2: Different shapes of bacilli

Bacilli are either endospore-forming aerobic or facultatively anaerobic. They are associated with bacteremia meningitis, endocarditis and infectious wounds in the eye, ear, urinary tract and respiratory tract. Two orders of bacilli is found: Bacillales and lactobacillus. Almost all bacilli are gram positive. Gram staining of the Bacillus subtilis is shown in figure 2.

Figure 2: Bacillus subtilis

Difference Between Cocci and Bacilli

Shape

Cocci: Cocci are either spherical, oval, bean or kidney-shaped.

Bacilli: Bacilli are either rod, vibrio, filamentous, spirochetes, spindle or spirillum-shaped.

Symmetry

Cocci:  One axis of the bacterium is almost the same as the other.

Bacilli: One axis of the bacterium is longer than the other. Bacillus is mostly arranged as single bacteria in bacillus.

Arrangements

Cocci: Cocci are arranged either in diplococci, streptococci, staphylococci, tetrads or sarcina.

Bacilli: Bacilli are arranged either in diplobacilli, streptobacilli or coccobacillus.

Metabolism

Cocci: Most cocci are anaerobic.

Bacilli: Bacilli are either endospore-forming aerobic or facultatively anaerobic.

Gram Positive/Gram Negative

Cocci: Both gram positive and gram negative bacteria are found in cocci.

Bacilli: Almost all of the bacilli are gram positive.

Diseases

Cocci: Several types of infections like bacteremia, cellulitis , pneumonia, gangrene, peritonitis, abscesses, bite wounds and pelvic inflammatory disease are caused by cocci.

Bacilli: Bacilli are associated with anthrax, bacteremia meningitis, endocarditis and infectious wounds in eye, ear, urinary tract and respiratory tract.

Examples

Cocci: Neisseria gonorrhoeae, Streptococcus pneumoniae, Staphylococcus aureus and Micrococcus sp. are the examples of cocci.

Bacilli: Clostridium, Escherichia, Corynebacterium, Actinomyces and Nocardia are the examples of bacilli.

Conclusion

Cocci and bacilli are two shapes of bacteria. Cocci is basically spherical-shaped and bacilli is rod-shaped. Cocci are arranged either in diplococci, streptococci, staphylococci, tetrads or sarcina during multiplication. Bacilli are arranged either in diplobacilli, streptobacilli and coccobacilli. Cocci can be either gram positive or gram negative bacteria. In contrast, bacilli are gram positive bacteria. Most of the cocci are anaerobic and cause infections in both plants and animals. They cause infections in wounds and pneumonia. Bacilli are either endospore-forming aerobic or facultatively anaerobic. Anthrax is caused by bacilli. Both bacterial infection types can be treated with antibiotics like penicillin and tetracycline. However, the main difference between cocci and bacilli is their basic shape of the bacteria.  

Reference:
1. Wells, Carol L. “Anaerobic Cocci.” Medical Microbiology. 4th edition. U.S. National Library of Medicine, 01 Jan. 1996. Web. 25 Apr. 2017.
2. Turnbull, Peter C. B. “Bacillus.” Medical Microbiology. 4th edition. U.S. National Library of Medicine, 01 Jan. 1996. Web. 25 Apr. 2017.

Image Courtesy:
1. “Staphylococcus aureus Gram” By Y Tambe – Y Tambe (CC BY-SA 3.0) via Commons Wikimedia
2. “Bacillus subtilis Gram” By Y tambe (original uploader) – Own work (CC BY-SA 3.0) via Commons Wikimedia