Gram-positive bacteria are bacteria that give a positive result in the Gram stain test. Gram-positive bacteria take up the crystal violet stain used in the test and then appear purple-colored when seen through a microscope. Six gram-positive genera are typically pathogenic in humans. Two of these, Streptococcus and Staphylococcus, are cocci (sphere-shaped). The remaining organisms are bacilli (rod-shaped) and can be subdivided based on their ability to form spores. The non-spore formers are Corynebacterium and Listeria (a coccobacillus), whereas Bacillus and Clostridium produce spores.

Gram-Positive Endospore-Forming Rods

Gram-positive bacteria retain the crystal violet stain when treated by Gram’s method. Gram-positive endospore-forming rods are rod-shaped bacteria that form endospores. Representative genera include Bacillus and Clostridium. They include:

• Nocardia
• Bacillaceae
• Clostridia
• Clostridium botulinum
• Micromonosporaceae
• Paenibacillus
• Paenibacillus
• Staphylococcus
• Streptococcus
• Thermoactinomyces

Gram-Positive Asporogenous Rods, Regular

Gram-positive not spore-bearing rods, regular, are a group of regular rod-shaped bacteria that stain gram-positive and do not produce endospores. They include:

• Brochothrix
• Erysipelothrix
• Lactobacillaceae
• Listeria
• Mycobacteriaceae

Gram-Positive Asporogenous Rods, Irregular

Gram-positive not spore-bearing rods, irregular, are a group of irregular rod-shaped bacteria that stain gram-positive and do not produce endospores. They include:

• Acetobacterium
• Actinobacteria
• Butyrivibrio
• Corynebacterium
• Eubacterium
• Propionibacteriaceae
• Thermoanaerobacter

Gram-Positive Cocci

• Sarcina