Which of the following is not a characteristic of fourth-generation cephalosporins?

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Fourth-generation cephalosporins are characterized by their enhanced antibacterial activity and resistance profiles compared to earlier generations. They possess increased resistance to beta-lactamases, enabling them to withstand enzymatic breakdown by certain bacteria. Their broad-spectrum activity allows them to target a wide range of bacteria, making them useful in treating serious infections that are not effectively addressed by prior generations.

One of the defining features of fourth-generation cephalosporins is their effectiveness against gram-negative bacteria, which are often more challenging to treat due to their outer membrane structure that impedes antibiotic penetration. While they do retain some activity against gram-positive bacteria, they are particularly noted for their potency against a variety of gram-negative organisms.

The assertion that they are "only effective against gram-positive bacteria" is inaccurate, as fourth-generation cephalosporins actually have significant efficacy against gram-negative pathogens as well. This makes it clear why this is not a characteristic of this antibiotic category. Thus, the choice correctly identifies a statement that does not align with the known properties of fourth-generation cephalosporins.