Trypticase soy broth and Mueller Hinton broth lacking yeast extract resulted in morphological alterations (pleomorphism) of Cardiobacterium hominis, as demonstrated by light and transmission electron microscopy. Outer membrane protein profiles were influenced by the composition of growth media, especially in the region of porin proteins. In contrast, the protein composition of inner membranes was not influenced by media composition. C. hominis isolates tested were susceptible to 16-beta-lactam antibiotics. All isolates exhibited minimal inhibitory concentrations of less-than-or-equal-to 1-mu-g/ml to 13 of the 16-beta-lactams tested. The changes in outer membrane profiles induced by media composition did not influence susceptibility to beta-lactam antibiotics. None of the isolates examined contained detectable plasmids, beta-lactamase, or resistant mutants. Characteristic beta-lactam-induced morphological defects were observed.