Failure of cephalosporins to cure experimental listeriosis.

In vitro, 15 strains of various serotypes of Listeria monocytogenes were found to be susceptible to cefazedone and cephalotin. Other cephalosporins, such as cefotaxime, were much less active. In spite of their relatively good in vitro activities, however, neither cefazedone nor cephalotin reduced the bacterial counts in mice infected with a virulent strain of L. monocytogenes. In contrast, ampicillin was active both in vitro and in vivo. Cephalosporins should not be used clinically for treating listeriosis.