What is a Gram-positive coccobacilli in rods?

Gram-Positive Coccobacilli: Definition and Clinical Significance

Gram-positive coccobacilli are bacteria that appear as short, oval-shaped rods (intermediate between cocci and bacilli) that retain the crystal violet stain in Gram staining, and include clinically important pathogens that are frequently misidentified or dismissed as contaminants in clinical practice.

Morphological Characteristics

• Gram-positive coccobacilli are pleomorphic microorganisms that appear microscopically as small, non-spore-forming rods arranged in clusters or chains 1
• These organisms form an intermediate morphology between spherical cocci and elongated bacilli, making them difficult to categorize definitively 1
• They typically form circular, smooth colonies when cultured 1

Common Pathogenic Examples

Yersinia pestis (Plague)

• Yersinia pestis is technically a gram-negative coccobacillus (not gram-positive), causing plague with high mortality if untreated 2
• This organism belongs to Enterobacteriaceae and is transmitted by flea bites or direct contact with infected tissues 2

Erysipelothrix rhusiopathiae

• Erysipelothrix rhusiopathiae is a true gram-positive coccobacillus that causes erysipeloid, a zoonotic skin infection 2
• It appears as a thin, pleomorphic, non-spore-forming gram-positive rod acquired from handling fish, marine animals, swine, or poultry 2
• Treatment requires penicillin or amoxicillin for 7-10 days 2

Arcanobacterium Species

• Arcanobacterium bernardiae and related species are gram-positive coccobacilli that can cause serious infections including necrotizing fasciitis 1
• These organisms are facultatively anaerobic, catalase-negative, non-hemolytic, and unable to reduce nitrates 1
• They are frequently misidentified as "diphtheroids" or contaminants, leading to missed diagnoses of serious infections 1

Critical Clinical Pitfall

The most important clinical consideration is that gram-positive coccobacilli are often dismissed as contaminants (particularly when labeled as "diphtheroids" or "coryneform bacteria"), but they can be significant pathogens in complicated skin and soft tissue infections, including necrotizing fasciitis 1.

• Standard laboratory techniques and enzymatic assays are frequently unable to identify these organisms to species level 1
• Genetic sequencing (16S rRNA gene analysis) is often required for accurate species identification 1
• Given their morphological similarity to non-pathogenic skin flora, these organisms may be more frequent causes of complicated infections than currently recognized in the literature 1

Diagnostic Approach

• When gram-positive coccobacilli or "diphtheroids" are isolated from deep tissue infections, surgical specimens, or in the context of severe infection, they should not be automatically dismissed as contaminants 1
• Microscopic reexamination and advanced identification methods (genetic sequencing) should be considered when clinical suspicion is high 1
• In diabetic foot infections and other polymicrobial wound infections, various gram-positive organisms including coccobacilli may be present alongside more commonly recognized pathogens 2, 3