Which one of the following organisms is anaerobic?

Actinomyces israelii is classified as an anaerobic organism, which means it thrives in environments devoid of oxygen. This characteristic is essential for its growth and metabolism. Actinomyces species are part of the normal flora found in the human mouth and can be involved in opportunistic infections, particularly in conditions where oxygen levels are limited, such as in deep tissue infections or abscesses.

In contrast, Escherichia coli is a facultative anaerobe, meaning it can grow in both the presence and absence of oxygen, but prefers aerobic conditions. Saccharomyces cerevisiae, commonly known as baker's yeast, can tolerate anaerobic environments but primarily functions under aerobic conditions, especially for efficient fermentation processes. Staphylococcus aureus is also a facultative anaerobe that can grow in various oxygen levels, but it does not require anaerobic conditions for survival.

Understanding the oxygen requirements of these microorganisms is crucial for microbiological practices, as it influences the culture methods, selection of media, and treatment of infections.