Escherichia coli (E. coli) is a common bacterium found in the environment and the intestines of warm-blooded animals. Bacteria utilize fermentation to break down organic compounds, such as sugars, to generate energy in the absence of oxygen. Mannitol, a sugar alcohol, can serve as an energy source for many bacteria.
E. coli’s Ability to Ferment Mannitol
E. coli can ferment mannitol, utilizing this sugar alcohol as a source of carbon and energy. This metabolic capability is a distinguishing characteristic. When E. coli ferments mannitol, it produces acidic byproducts, which lowers the pH of the environment. This acidification is a key indicator in laboratory tests.
Some E. coli strains also produce gas during mannitol fermentation. These results are detected in specific laboratory media. For instance, in phenol red mannitol broth, a pH indicator changes color from red to yellow in the presence of acid, indicating successful fermentation. Gas bubbles in an inverted Durham tube further confirm gas production.
Significance in Bacterial Identification
The ability of E. coli to ferment mannitol is significant for bacterial identification. This characteristic is often used as a diagnostic tool to differentiate E. coli from other bacterial species that may or may not ferment mannitol. Microbiologists rely on such biochemical reactions to characterize unknown bacterial isolates.
While E. coli can ferment mannitol, certain differential media, like Mannitol Salt Agar (MSA), are primarily used to distinguish other bacteria, such as Staphylococcus aureus, which ferments mannitol, from non-fermenting species like Staphylococcus epidermidis. E. coli does not grow well on MSA due to its high salt concentration, highlighting how different media contribute to bacterial differentiation. The patterns of sugar fermentation, including mannitol, are integral to biochemical tests that help identify bacterial strains. This identification is crucial for public health, clinical diagnostics, and research.
The Biochemical Process
The fermentation of mannitol by E. coli involves enzymatic reactions beginning with mannitol transport into the bacterial cell. Mannitol enters the cell via the phosphoenolpyruvate-dependent phosphotransferase system (PTS). This system transports mannitol across the cell membrane and phosphorylates it, converting it into mannitol-1-phosphate.
Inside the cell, mannitol-1-phosphate dehydrogenase (MtlD) converts mannitol-1-phosphate into fructose-6-phosphate. Fructose-6-phosphate then enters the glycolysis pathway. Through glycolysis, the bacterium extracts energy as adenosine triphosphate (ATP), producing organic acids and, in some cases, gases as metabolic byproducts.