Gram-positive bacteria retain the color of the primary stain (crystal violet) in the Gram staining procedure and appear as purple/violet under a light microscope. These bacteria have a cell wall containing a thick layer of peptidoglycan. On the basis of cell morphology, Gram-positive bacteria are divided mainly into two groups, “Gram-positive cocci” and “Gram-positive bacilli”.
Gram-positive bacilli are a diverse group of bacteria which can further be classified into various subgroups based on their characteristics such as
- Endospore forming ability: Bacillus and Clostridia are spore-forming rods while Listeria and Corynebacterium are not.
- Gaseous (oxygen requirements): Bacillus species are obligate aerobes whereas Clostridium species are obligate anaerobes.
Some of the Gram-positive rods have a unique shape based on the position of spores, presence of branches etc. Some of the typical morphological shapes of the Gram-positive bacilli are summarized in the table below:
| Organisms | Typical Shape | Remarks |
| Actinomyces | Irregular/branching | |
| Corynebacteria | Club-shaped | In Gram stain, these bacteria appear as V-in Y-shaped arrangements or in clumps that resemble Chinese letters. |
| Clostridium tetani | Drum stick or tennis racket appearance | Due to the presence of spherical and terminal spore. |
| Nocardia | Branching filaments | In Gram stain, these bacteria stain irregularly and appear beaded. |
Gram-positive bacilli are responsible for “classical” diseases such as anthrax, diphtheria, and listeriosis and also for newer syndromes, particularly in immunocompromised individuals.
Some of the diseases caused by gram-positive bacilli are as follows:
| Organism (s) | Typical agent | Disease (s) |
| Actinomyces | Actinomyces israelii | Actinomycosis |
| Corynebacteria | Corynebacterium diphtheriae | Diphtheria |
| Listeria | Listeria monocytogenes | Listeriosis, neonatal meningitis, sepsis etc. |
| Clostridium | Clostridium perfringens | Gas gangrene (clostridial myonecrosis) |
| Clostridioides difficile | Diarrhea and colitis | |
| Clostridium tetani | Tetanus | |
| Clostridium botulinum | Botulism | |
| Bacillus | Bacillus cereus | Foodborne diseases |
| Bacillus anthracis | Anthrax | |
| Erysipelothrix | Erysipelothrix rhusiopathiae | Erysipeloid |
| Nocardia | Nocardia asteroides | Nocardiosis |
| Cutibacterium | Cutibacterium (formerly Propionibacterium) acnes | Acne |
| Eubacterium | Various species | Minor clinical importance |
| Mycobacterium | Mycobacterium tuberculosis | Tuberculosis |
| Mycobacterium leprae | Leprosy |
Non-pathogenic Gram-positive rods
Lactobacillus: Lactobacillus is an important microbiota of the birth canal and helps to prevent the growth of many potential pathogens. They are mainly used in the manufacture of fermented dairy, sourdough, meat and vegetable foods or used as probiotics.
Streptomyces: Streptomyces is a Gram-positive bacteria which shape resembles filamentous fungi. Streptomyces is known for its ability to produce bioactive secondary metabolites such as antifungals, antivirals, and mainly antibiotics. Today, nearly 80% of the antibiotics are sourced from the genus Streptomyces such as penicillin, streptomycin, gentamicin, linezolid, tetracycline, etc.
