Bacteria are single-celled organisms classified primarily by their shape. The two broadest categories are cocci (spherical) and bacilli (rod-shaped). Coccobacilli represent a distinct morphological group that bridges these two main types. They are an intermediate shape found in many different bacterial species, some of which commonly cause human infection.
Defining the Coccobacillus Shape
Coccobacilli are bacteria whose physical form is intermediate between the sphere of a coccus and the rod of a bacillus. The name itself combines “coccus” and “bacillus,” accurately describing the morphology. These bacteria appear as very short, plump rods, often with a width nearly equal to their length.
When viewed under a microscope, a coccobacillus may look like a slightly compressed sphere or an oval. Their small size and stoutness set them apart from longer, more slender bacilli. This unique shape can also be described as pleomorphic, meaning the bacteria may exhibit variations in shape depending on their growth environment.
In a laboratory setting, coccobacilli are identified using the Gram stain procedure, which categorizes bacteria based on their cell wall structure. Most medically significant coccobacilli are classified as Gram-negative, meaning they do not retain the crystal violet stain due to their thinner peptidoglycan layer. This classification is an important step in determining appropriate treatment.
Prominent Examples of Coccobacilli
Many medically relevant coccobacilli are found in diverse environments, from the human body to the soil. The genus Haemophilus, including species like Haemophilus influenzae, consists of small, Gram-negative coccobacilli often found in the human respiratory tract. H. influenzae does not cause the flu, but it is a frequent cause of other respiratory issues.
The genus Brucella consists of small, Gram-negative coccobacilli primarily associated with animals such as cattle, sheep, and goats. Transmission to humans occurs through contact with infected animals or consumption of unpasteurized dairy products. Bordetella pertussis is a strict aerobic, Gram-negative coccobacillus found in the human respiratory system.
The genus Acinetobacter contains species that appear as Gram-negative coccobacilli and are commonly found in the environment and hospital settings. Obligate intracellular bacteria, such as Coxiella burnetii and Chlamydia trachomatis, also exhibit this shape during certain stages of their life cycle.
Health Implications
The unique morphology of coccobacilli is linked to a wide range of infections in humans. Bordetella pertussis is the causative agent of pertussis, or whooping cough, a highly contagious respiratory infection. This illness begins with mild, cold-like symptoms but progresses to severe, characteristic coughing fits.
Haemophilus influenzae is associated with serious infections, including bacterial meningitis, which is an inflammation of the membranes surrounding the brain and spinal cord, as well as pneumonia. Chlamydia trachomatis is responsible for chlamydia, a frequently reported sexually transmitted infection. Untreated chlamydia can lead to long-term health issues like pelvic inflammatory disease and infertility.
Bacteria in the Brucella genus cause brucellosis, a systemic infection characterized by flu-like symptoms such as fever, sweating, and body aches. Acinetobacter species, particularly Acinetobacter baumannii, cause hospital-acquired infections, including pneumonia and wound infections, often showing resistance to multiple antibiotics.