Recent research from the DTU National Food Institute suggests that antibiotic resistance may not be a fixed trait of bacteria, but rather depends on the environment in which the bacteria are found. This challenges the traditional view that resistance is determined solely by the presence of specific resistance genes. Factors such as temperature, oxygen levels, and pH can influence whether a bacterium with a resistance gene actually behaves in a resistant way. The World Health Organization (WHO) has long warned that antimicrobial resistance, which includes antibiotic resistance, is one of the most pressing global health threats. This new understanding could have significant implications for public health, animal health, and environmental management. Traditionally, antibiotic resistance is identified through laboratory tests that determine whether a bacterium is susceptible or resistant to specific antibiotics. These tests have been vital for diagnosing infections, guiding treatment, and monitoring resistance in humans, animals, and the environment. However, the new research suggests that these tests may not fully capture the complexity of resistance, as they typically ignore the biological conditions in which bacteria actually live. For example, whether a bacterium is in the gut or in the blood, or if the host has a fever, can affect its resistance behavior. Professor Frank Møller Aarestrup from the DTU National Food Institute explains that bacteria can exhibit different resistance levels depending on their microenvironment. Factors like pH, oxygen, and temperature influence whether a bacterium is resistant to antibiotics. Standard laboratory tests do not account for these variables, which are crucial in the human body. Similarly, Professor Thomas Bjarnsholt from the University of Copenhagen and Copenhagen University Hospital notes that resistance genes do not operate in isolation. They are found in different bacteria and microenvironments, meaning that it's not enough to simply identify which resistance genes a bacterium has; researchers must also understand when and under what conditions these genes make the bacterium resistant. This shift in understanding raises new challenges for laboratory testing, as it requires considering multiple environmental factors rather than relying on standardized measurements. It also means that antibiotics may sometimes fail in real-world situations even if lab tests suggest they should work, or vice versa. Researchers propose that antibiotics could be designed to only develop resistance under specific conditions, such as high temperatures not found in the human body. This could lead to new strategies for managing resistance in humans, animals, and the environment. The study highlights the need for more targeted treatments and better monitoring of resistance in the environment. Researchers also emphasize the importance of collecting large amounts of data on how resistance behaves under various biological conditions. This could lead to new ways of categorizing resistance genes and using models or artificial intelligence to understand their interactions with different environments. While this represents a significant shift in research, it could ultimately lead to more effective strategies for controlling one of the greatest health challenges of our time.