Only two beta-lactamases, TEM-1 and ROB-1, have been observed in , while four different TEM but no ROB enzymes have been found in . In order to investigate the mechanisms behind the dissemination of small beta-lactamase-encoding plasmids in and , we assessed the fitness cost of three TEM-1- (pPN223, pA1209, pA1606), one TEM-15- (pSF3) and one ROB-1-bearing (pB1000) plasmid when expressed in either bacterial species. All plasmids were stable in and except pB1000, which showed on average (sample mean) 76 % curing in after 5 days of subculture. Competition assays between isogenic strains with and without plasmid showed no competitive disadvantage of pPN223 and pA1606 in , or of pA1209 in . In contrast, pSF3 and pB1000 were associated with significant competitive disadvantages in both species. Some of the competitive disadvantages may be related to differences in plasmid copy number and mRNA expression of the beta-lactamase genes, as revealed by quantitative PCR analysis. In conclusion, plasmids encoding TEM beta-lactamases isolated from and can be stably transferred between species. The fast curing of pB1000 in observed in this study correlates to the fact that ROB-1 has never been reported for this species. TEM-1-encoding plasmids are associated with the lowest level of fitness cost, but different TEM-1 plasmids confer different levels of fitness cost on the two hosts.


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