from Jeanette N. Pham writing in Yersinia: Systems Biology and Control:
Yersinia enterocolitica and Yersinia pseudotuberculosis, the two enteropathogenic species of the genus Yersinia, are poles apart in their natural resistance to β-lactam antibiotics. While Yersinia pseudotuberculosis, as a species, is susceptible to all antibiotics used in the treatment of Gram-negative infections, Y. enterocolitica susceptibility to β-lactam antibiotics varies. Due to the presence of chromosomal β-lactamases, Y. enterocolitica are inherently resistant to ampicillin and cephalothin. The consistent pattern of β-lactam antibiotic susceptibility of each bioserotype and subgroup within a bioserotype is explained by the production or the lack of production of two chromosomally encoded β-lactamases. One is the non inducible broad spectrum enzyme A, a β-lactamase of molecular class A, and the other one is enzyme B, an inducible cephalosporinase of molecular class C. Fluoroquinolones alone or in combination with a third generation cephalosporin are very effective at treating severe infections caused by Y. pseudotuberculosis or Y. enteterocolitica.
Further reading: Yersinia: Systems Biology and Control