Which group of bacteria is primarily responsible for nitrification?

The group of bacteria primarily responsible for nitrification is indeed ammonia-oxidizing and nitrite-oxidizing bacteria. Nitrification is a two-step process crucial for the nitrogen cycle, where ammonia is first oxidized to nitrite by ammonia-oxidizing bacteria. Following this, nitrite-oxidizing bacteria then convert the nitrite into nitrate. This process is essential for wastewater treatment, as it helps in the removal of ammonia, which can be toxic to aquatic life and is a significant pollutant in wastewater.

In contrast, the other groups mentioned are not directly involved in the nitrification process. Aerobic bacteria do play a role in various metabolic processes in wastewater treatment, but they encompass a broader range of functions beyond just nitrification. Pathogenic bacteria and microbes are primarily associated with causing diseases, not with the beneficial nitrification process. Fermenting bacteria and fungi are involved in decomposition and the breakdown of organic matter, but they do not participate in the conversion of ammonia and nitrite in the nitrification pathway. Hence, the specific roles of ammonia-oxidizing and nitrite-oxidizing bacteria in the nitrification process make this the correct answer.