Ampicillin (and amoxicillin and aminopenicillins) are penicillinase-sensitive penicilins. Resistance occurs when bacteria produce penicillinase (a type of beta lactamase) which cleaves the B-lactam ring. Resistance to cephalosporins occurs when bacteria produce cephalosporins (a type of beta lactamase) OR through structural changes in penicillin-binding proteins (transpeptidases). FA2018 188-189

Though ampicillin and ceftriaxone are both considered beta lactam antibiotics, Most cephalosporins have intrinsic resistance to the beta-lactamases that inactivate other beta-lactams (FA2020 p189). Cephalospoin resistance is caused by structural changes in the transpeptidases (also called penicillin binding protein), or inactivation by cephalosporinases (a specific type of beta-lactamase)