Which type of resistance occurs when bacteria are resistant to an antibiotic they once were sensitive to?

The distinction surrounding resistance types is pivotal in understanding microbial interaction with antibiotics. Acquired resistance refers specifically to the situation where bacteria develop resistance to antibiotics to which they were previously sensitive. This can occur through various mechanisms, such as mutation or horizontal gene transfer, allowing bacteria to adapt and survive in the presence of the drug.

When bacteria that were once susceptible to an antibiotic suddenly show resistance, it typically indicates that they have either acquired new genetic material that confers resistance or have undergone mutations that alter their susceptibility. This type of resistance is particularly concerning in clinical settings, as it often leads to treatment failures and challenges in managing infections effectively.

In contrast, intrinsic resistance is a natural characteristic of certain bacterial species that are inherently resistant to specific antibiotics due to their cellular structure or metabolic processes. Natural resistance is synonymous with intrinsic resistance. Cross-resistance occurs when a bacterium that is resistant to one antibiotic also exhibits resistance to another antibiotic that may have a similar mechanism of action. Each of these concepts highlights different aspects of how bacteria can resist antibiotics but does not align with the scenario of previously sensitive bacteria becoming resistant.