Examples of Gram-Positive Bacilli
The most clinically significant gram-positive bacilli include Bacillus species, Listeria monocytogenes, Corynebacterium species, and Actinomyces species, which are important pathogens in various clinical settings.
Common Gram-Positive Bacilli
Clinically Significant Species
- Bacillus species - Often found as contaminants but can cause serious infections, particularly in immunocompromised patients 1
- Listeria monocytogenes - A short, non-spore-forming gram-positive bacillus that causes a narrow zone of hemolysis on blood agar and is responsible for listeriosis 2
- Corynebacterium species - Commonly found as part of normal flora but can cause catheter-related infections 1, 3
- Actinomyces species - Comprise approximately two-thirds of aerobically growing gram-positive bacilli isolates in soft tissue infections, particularly in the perianal region 3
Other Important Gram-Positive Bacilli
- Lactobacillus species - Part of normal oral and gastrointestinal flora but can cause bacteremia in immunocompromised patients 4
- Eggerthella species - Associated with clinically significant bacteremia, particularly in patients with gastrointestinal tract disease 4
- Clostridium species - Anaerobic, spore-forming bacilli that can cause various infections 1
Clinical Significance and Identification
Laboratory Identification
- Gram-positive bacilli are often difficult to differentiate without genetic testing, leading to potential misidentification as "diphtheroids" or contaminants 5
- 16S rRNA gene sequencing may be required for accurate species identification of many gram-positive bacilli 5, 4
- Standard phenotypic tests like API Coryne have poor agreement with genotypic identification, correctly identifying only about one-third of isolates to species level 3
Clinical Relevance
- Unidentified gram-positive bacilli growing on agar should not be automatically dismissed as contaminants, particularly in suspicious clinical settings (e.g., febrile illness in previously healthy person) 1
- Gram-positive bacilli can be categorized as part of the oral bacterial flora along with other groups based on Gram staining and morphology 6
- In bacteremia cases, isolation of certain gram-positive bacilli (Eggerthella and Lactobacillus) is more likely to represent clinically significant infection compared to others like Propionibacterium acnes 4
Antibiotic Susceptibility
- Actinomyces species are uniformly susceptible to penicillin and amoxicillin/clavulanate 3
- Antibiotic susceptibilities are more varied for other genera of gram-positive bacilli 3
- For Listeria monocytogenes infections, the treatment of choice includes intravenous ampicillin alone or in combination with gentamicin 2
Special Considerations
- Catheter removal is often recommended for infections caused by Bacillus species 1
- Atypical mycobacterial infections (which can appear as gram-positive bacilli) often require catheter removal and debridement of infected tissue 1
- The pathogenicity of gram-positive bacilli varies significantly between genera and species, with some being highly virulent and others typically considered contaminants 5
Gram-positive bacilli represent a diverse group of organisms with varying clinical significance. Due to difficulties in identification using conventional methods, they may be underreported causes of infection. When isolated from clinical specimens, particularly in sterile sites or from immunocompromised patients, their potential pathogenicity should be carefully evaluated rather than automatically dismissing them as contaminants.