First generation cephalosporins cannot be used to treat which of the following?

First-generation cephalosporins are primarily effective against gram-positive bacteria and are often used to treat infections caused by organisms like Streptococcus and Staphylococcus. They are also effective against certain gram-negative bacteria, particularly those like E. coli.

However, significant limitations exist in their effectiveness against methicillin-resistant Staphylococcus aureus (MRSA). MRSA has acquired resistance mechanisms that render first-generation cephalosporins ineffective. This resistance is primarily due to the alteration of penicillin-binding proteins, which are the target sites for beta-lactam antibiotics, including cephalosporins. Because of this mechanism, first-generation cephalosporins cannot successfully treat infections caused by MRSA, making this correct option the one that cannot be treated with these antibiotics.

In contrast, Streptococcus and Staphylococcus (non-MRSA) infections can be managed with first-generation cephalosporins, and E. coli strains that are sensitive to these antibiotics are also treatable, showcasing the specific limitations in treating MRSA with this class of drugs.