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A plasmid marker rescue system based on restoration of the nptII gene was established in Streptococcus gordonii to study the transfer of bacterial and transgenic plant DNA by transformation. In vitro studies revealed that the marker rescue efficiency depends on the type of donor DNA. Plasmid and chromosomal DNA of bacteria as well as DNA of transgenic potatoes were transferred with efficiencies ranging from 8.1 ؋ 10 ؊6 to 5.8 ؋ 10 ؊7 transformants per nptII gene. Using a 792-bp amplification product of nptII the efficiency was strongly decreased (9.8 ؋ 10 ؊9 ). In blood sausage, marker rescue using plasmid DNA was detectable (7.9 ؋ 10 ؊10 ), whereas in milk heat-inactivated horse serum (HHS) had to be added to obtain an efficiency of 2.7 ؋ 10 ؊11 . No marker rescue was detected in extracts of transgenic potatoes despite addition of HHS. In vivo transformation of S. gordonii LTH 5597 was studied in monoassociated rats by using plasmid DNA. No marker rescue could be detected in vivo, although transformation was detected in the presence of saliva and fecal samples supplemented with HHS. It was also shown that plasmid DNA persists in rat saliva permitting transformation for up to 6 h of incubation. It is suggested that the lack of marker rescue is due to the absence of competence-stimulating factors such as serum proteins in rat saliva.

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