The bacterium Staphylococcus aureus does ferment mannitol. S. aureus is a common bacterium found on human skin and in nasal passages. Fermentation is an anaerobic metabolic process where the organism converts a carbohydrate, like mannitol, into organic acids to produce energy. This ability provides microbiologists with a simple tool to distinguish this species from other related bacteria in a laboratory setting.
The Process of Mannitol Fermentation
Mannitol is classified as a sugar alcohol, a type of carbohydrate that S. aureus can metabolize. When this bacterium is placed in an environment containing mannitol, it possesses the necessary enzymes to break down this compound. The process is a form of anaerobic respiration, meaning it can occur without the presence of oxygen. S. aureus converts the mannitol into pyruvate, resulting in the production of organic acids, such as lactic acid. This accumulation of acid effectively lowers the pH of the surrounding environment, which is the direct biochemical evidence that fermentation has occurred.
The Mannitol Salt Agar Test
The ability of S. aureus to ferment mannitol is used in the Mannitol Salt Agar (MSA) test. MSA is a specially formulated growth medium designed to be both selective and differential for staphylococci. The selective feature is its high concentration of sodium chloride (typically 7.5%), which inhibits the growth of most other bacteria while allowing salt-tolerant Staphylococcus species to flourish.
The differential component combines the carbohydrate mannitol and a pH indicator called phenol red. Phenol red is initially a reddish-pink color at the medium’s neutral starting pH. If S. aureus ferments the mannitol, the resulting acid production lowers the pH of the agar below 6.8. This acidic change causes the phenol red indicator to turn bright yellow. A yellow color change surrounding the bacterial growth is a positive result, indicating mannitol fermentation and presumptive identification of S. aureus. Conversely, other Staphylococcus species unable to ferment mannitol will grow but will not produce acid, leaving the medium the original reddish-pink color.
Why Identifying S. aureus is Critical
Identifying Staphylococcus aureus quickly is an important step in clinical microbiology because of its potential to cause serious human infections. This bacterium is considered pathogenic, unlike many other staphylococci that are harmless residents of the skin. The mannitol fermentation test allows for a rapid presumptive distinction between pathogenic strains and the generally less harmful Coagulase-Negative Staphylococci (CoNS).
CoNS, such as Staphylococcus epidermidis, typically do not ferment mannitol, displaying a negative result on the MSA plate. While MSA provides a strong initial indication, it is considered a presumptive identification and must often be followed by confirmatory testing. This follow-up test detects the enzyme coagulase, which is strongly associated with the most virulent strains of S. aureus.
The urgency in identifying S. aureus stems from its capacity to cause a wide range of infections, from minor skin boils to life-threatening conditions like sepsis, endocarditis, and pneumonia. The presence of antibiotic-resistant strains, such as Methicillin-Resistant S. aureus (MRSA), further emphasizes the need for rapid and accurate diagnosis. The simple yellow color change on an MSA plate thus serves as an early alert for the presence of a potentially dangerous pathogen, guiding timely clinical decisions and treatment.