Coccobacilli · What Are They, Treatment, and More

Published: Nov 17, 2025
Author: Alyssa Haag, MD
Editor: Ahaana Singh
Editor: Ian Mannarino, MD, MBA
Editor: Kelsey LaFayette, DNP, ARNP, FNP-C
Editor: Arianna Succi, MD
Illustrator: Aileen Lin
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What are coccobacilli?

Coccobacilli are a type of bacteria that are shaped like short rods or ovals. Their shape is intermediate between cocci (i.e., round bacteria) and bacilli (i.e., rod-shaped bacteria).  Coccobacilli are thus usually described as short rods and can sometimes be mistaken for cocci due to their similar, rounded shape.

One common method for classifying bacteria involves examining their staining properties under a microscope. The Gram stain is the most widely used technique for this purpose. Depending on their cell wall structure, bacteria are classified as either Gram-positive, which appear purple because they are able to retain the stain, or Gram-negative, which cannot retain the stain and appear pink after counterstaining. 

Classifying bacteria based on their shape and Gram stain characteristics is important for identifying the infectious agent and guiding appropriate treatment. 

Many different species of Gram-positive and Gram-negative coccobacilli can cause disease in humans. Among Gram-negative coccobacilli, Gardnerella vaginalis is the agent responsible for bacterial vaginosis, a condition characterized by white or yellow vaginal discharged and a distinctive fishy vaginal odor. Haemophilus influenzae, also Gram-negative, can cause various infections including meningitis, an inflammation of the membranes surrounding the brain and spinal cord, and pneumonia, or lung infection. Yersinia pestis is the Gram-negative coccobacillus that caused the devastating “Balck Plague” of the 14th century. Bordetella pertussis is responsible for whooping cough, a childhood infection characterized by severe coughing followed by the distinctive “whoop” sound during inhalation. Chlamydia trachomatis is the Gram-negative coccobacillus causing chlamydia, a common sexually transmitted infectionFinally, Brucella spp. are coccobacilli responsible for brucellosis, a zoonotic infection that can be transmitted from animals to humans. Brucellosis may affect multiple organ systems and typically presents with non-specific symptoms such as fever. 

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How is a coccobacillus infection treated?

Coccobacilli are responsible for a variety of infections, and treatment is tailored to the specific pathogen and the type of illness it causes. Antibiotics are typically the first step, and their choice depends on several factors including the coccobacillus species involved and its antibiotic susceptibility profile.  

Moreover, vaccines are available against some coccobacilli species to help prevent infections. For instance, immunization against Haemophilus influenzae and Bordetella pertussis are widely used and has lead to a significant reduction in the incidence of these infections.  

What are the most important facts to know about coccobacilli?

The term coccobacilli refers to bacteria with a shape that is intermediate between cocci (round) and bacilli (rod-shaped). Coccobacilli can be classified as either Gram-positive or Gram-negative and can cause various types of infections in humans. Coccobacilli infections are usually treated with antibiotics. Vaccines are available for specific coccobacilli bacteria and have greatly reduced the incidence of certain infections. 

Key Takeaways

Definition 
 

Coccobacilli are a type of bacteria shaped like short rods or ovals. Their shape is intermediate between cocci (i.e., round bacteria) and bacilli (i.e., rod-shaped bacteria). They can be classified as Gram-positive (purple after counterstaining) or Gram-negative (pink after counterstaining).  

Examples 
 

- Gram-negative (pink): 

     - Gardnerella vaginalis 

          - Bacterial vaginosis  

     - Hemophilus influenzae  

          - Meningitis, pneumonia  

     - Yersinia pestis  

          - “Black plague”  

     - Bordetella pertussis  

          - Whooping cough  

     - Chlamydia trachomatis  

          - Chlamydia  

     - Brucella spp.  

          - Brucellosis  

Treatment 

- Antibiotics  

     - Choice depends on the species and susceptibility  

 - Prevention: vaccinations  

     - Hemophilus influenzae  

     - Bordetella pertussis  

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References


Abou Chacra L, Fenollar F, Diop K. Bacterial vaginosis: what do we currently know? Front Cell Infect Microbiol. 2022;11:672429. doi:10.3389/fcimb.2021.672429


 

Cooke FJ, Slack MPE. Gram-negative coccobacilli. In: Cohen J, Powderly WG, Opal SM, eds. Infectious Diseases. 4th ed. Elsevier; 2017:1611–1627.e1. doi:10.1016/B978-0-7020-6285-8.00183-0


 

Mishori R, McClaskey EL, WinklerPrins VJ. Chlamydia trachomatis infections: screening, diagnosis, and management. Am Fam Physician. 2012;86(12):1127–1132. Accessed December 4, 2020. https://pubmed.ncbi.nlm.nih.gov/23316985/


 

Tille PM. Bailey & Scott’s Diagnostic Microbiology. 16th ed. Elsevier; 2025. Accessed July 15, 2025. https://pageburstls.elsevier.com/reader/books/9780443118937


 

Yang DC, Blair KM, Salama NR. Staying in shape: the impact of cell shape on bacterial survival in diverse environments. Microbiol Mol Biol Rev. 2016;80(1):187–203.