Expression of beta-lactamase in Coxiella burnetii transformants

Coxiella burnetii can be transformed to ampicillin resistance by electroporation with plasmids encoding beta-lactamase. However, non-plasmid emergence of resistance to ampicillin also develops. To validate the usefulness of the bla gene marker for selection and detection, transformed C. burnetii were examined for beta-lactamase expression by use of immunoblotting after SDS-PAGE. The 29-kDa mature form of the beta-lactamase protein was detected in C. burnetii lysates. Quantitation of these immunoblot signals showed that C. burnetii surprisingly expressed low levels of beta-lactamase. The results validate the use of plasmid-encoded ampicillin resistance as a means for selecting C. burnetii transformants; they also suggest that levels of ampicillin used for selection pressure should be empirically determined and that detection of beta-lactamase by antibody blotting done to confirm transformants.