Morphology and Staining Characteristics
Bacteria of the genus Clostridium are motile rods. Their primary morphological feature is spore formation. Clostridial spores are oval or round and, upon maturation, visibly deform the mother cell. This gives the bacterium a characteristic spindle shape (the genus name originates from the Greek word kloster, meaning "spindle").
When evaluating staining properties, the age of the culture must be considered. In fresh, recently grown populations, clostridia reliably stain Gram-positive. However, as the culture ages, the cell wall undergoes changes, causing older cells to stain Gram-negative.
Physiology, Respiration, and Metabolism
Based on respiration, the vast majority of clostridia are obligate anaerobes, meaning oxygen is lethal to them. Nevertheless, some aerotolerant species exist that can survive in the presence of oxygen, although they do not use it for growth.
Nutritionally, these microorganisms are chemoorganoheterotrophs. They possess a robust enzymatic machinery, demonstrating high saccharolytic and proteolytic activity.
Their primary pathway for carbohydrate breakdown under anaerobic conditions is butyric acid fermentation. This process yields specific end products:
- Butyric acid;
- Carbon dioxide ($CO_2$);
- Hydrogen ($H_2$);
- Methane in some cases.
A key biochemical marker used for laboratory identification is their ability to reduce sulfites to sulfides.
Ecological Niche
Clostridia are widely distributed in the environment. Their natural reservoir is soil, as well as sediments in both freshwater and marine bodies of water.
Additionally, members of the genus Clostridium are normal inhabitants of the digestive tract. They colonize the intestines of humans and many animal species. While many clostridia are entirely harmless or even beneficial as part of the normal microbiota, certain species possess marked pathogenic potential and cause severe human infections.