Definition
Aerotolerant organisms are microorganisms that do not use oxygen for energy production but can survive and grow in its presence. They rely on fermentation or anaerobic respiration for ATP synthesis and possess enzymes to detoxify reactive oxygen species, allowing them to tolerate exposure to air.
Explanation
Unlike obligate anaerobes that are harmed by oxygen and facultative anaerobes that can switch between respiration and fermentation, aerotolerant microorganisms are indifferent to oxygen. They lack a functional electron transport chain that uses oxygen as the terminal electron acceptor, so they derive all their energy from fermentation pathways that oxidize carbohydrates and reduce organic compounds. However, aerotolerant bacteria produce protective enzymes, such as superoxide dismutase and peroxidase, that neutralize reactive oxygen species generated by exposure to air. Many members of the lactic acid bacteria group, including Streptococcus, Enterococcus and Lactobacillus species, are aerotolerant. They colonize mucosal surfaces in humans, contribute to the normal microbiota of the mouth, gastrointestinal tract and vagina, and play roles in food fermentation. In the oral cavity, Streptococcus mutans and Streptococcus sobrinus ferment sugars to lactic acid, contributing to dental caries. In fermented foods, Lactobacillus species acidify the environment, inhibit pathogens and enhance flavor and texture. Aerotolerant bacteria typically lack catalase activity, so they cannot break down hydrogen peroxide into water and oxygen; instead they rely on peroxidase or manganese-dependent systems. Their ability to persist in both oxygen-rich and oxygen-depleted niches allows them to inhabit diverse environments such as soil, dairy products and animal mucosa.
Examples and Roles
Common aerotolerant bacteria include Streptococcus mutans, Streptococcus pyogenes, Enterococcus faecalis and Lactobacillus acidophilus. These organisms ferment sugars to lactic acid and are important in dental plaque formation, probiotic preparations and fermented foods like yogurt, cheese, kimchi and sauerkraut. Some pathogenic species, such as Streptococcus pyogenes, cause pharyngitis and soft tissue infections. Knowledge of their oxygen tolerance aids in culturing these organisms and designing control strategies in food production and medicine.
Aerotolerant bacteria showcase the diversity of microbial metabolism. By relying on fermentation yet tolerating oxygen, they occupy ecological niches between strictly anaerobic and aerobic life. Their metabolic products have significant impacts on health, industry and the environment.
Related Terms: Anaerobe, Facultative anaerobe, Microaerophile, Aerobes, Fermentation