Coccobacilli: Bacteria, Not Fungi
Coccobacilli are definitively bacteria, not fungi. Coccobacilli represent a morphological category of bacteria that are intermediate between cocci (spherical) and bacilli (rod-shaped) in appearance 1.
Characteristics of Coccobacilli
Coccobacilli are characterized by:
- Morphology: Short, oval-shaped bacteria that appear as intermediate forms between spherical cocci and rod-shaped bacilli
- Size: Typically 0.4 × 1-1.5 microns in average size 2
- Staining properties: Can be either Gram-positive or Gram-negative depending on the specific species
Important Coccobacilli Pathogens
Several clinically significant bacterial pathogens present as coccobacilli:
Gram-Negative Coccobacilli
- Francisella tularensis: The causative agent of tularemia, described as "a small, aerobic, pleiomorphic, catalase-positive, Gram-negative, facultative-intracellular coccobacillus" 1
- Yersinia pestis: The causative agent of plague, described as "a gram-negative coccobacillus" 1, 3
- Acinetobacter baumannii-calcoaceticus complex: Gram-negative coccobacilli that typically cause nosocomial infections 4
- Pasteurella species: Non-motile, facultative anaerobic, Gram-negative coccobacilli found in the oral cavity of animals 5
- Gardnerella vaginalis: Gram-variable coccobacilli associated with bacterial vaginosis 2
Gram-Positive Coccobacilli
- Propionibacteriaceae family members: Several genera including Enemella and Parenemella are described as "Gram-stain-positive cocci, coccobacilli or short, rod-shaped strains" 6
Differentiation from Fungi
Fungi differ from coccobacilli in several fundamental ways:
- Cell wall composition: Bacterial cell walls contain peptidoglycan, while fungal cell walls contain chitin and glucans
- Cellular organization: Bacteria are prokaryotes (no membrane-bound nucleus), while fungi are eukaryotes
- Morphology: Fungi typically appear as yeasts (oval cells) or molds (filamentous hyphae), not as coccobacilli
- Size: Fungi are generally larger than bacteria
Clinical Significance
The identification of an organism as a coccobacillus has important clinical implications:
- Diagnostic approach: Bacterial culture methods and Gram staining are appropriate (not fungal culture methods)
- Treatment: Antibacterial agents (not antifungal agents) are required for treatment
- Epidemiology: Understanding transmission patterns specific to bacterial pathogens
Conclusion
When encountering coccobacilli in clinical specimens, clinicians should recognize these as bacteria and pursue appropriate bacterial identification and susceptibility testing. The morphological description as "coccobacilli" provides an important clue to the bacterial nature of the organism, guiding proper diagnostic and therapeutic approaches.