Phenotypic characterization of βLR16
To assess the range of resistance conferred by the βLR16 clone, resistance to 15 antibiotics was
tested in minimum inhibitory concentration (MIC) assays. βLR16 conferred elevated and specific
resistance to carbenicillin on EPI300 E. coli (Table 3). When tested against additional
β-lactams, the βLR16 clone was consistently more resistant than EPI300 E. coli containing
empty vector, but the difference was never more than 2-fold.
Resistance to carbenicillin was examined in a zone-of-inhibition assay to assess whether the
mechanism of resistance conferred by βLR16 resulted from antibiotic inactivation. After 48
hours, the zone diameter resulting from the supernatant of a βLR16 culture was the same as the
zone diameter from the supernatant of carbenicillin-containing media inoculated with E. coli
containing empty vector (Table 4). This was in contrast to the diameter of the zone resulting
from the supernatant of the 48-hour βLR2 culture (Table 4). βLR2 contains a gene encoding a
metallo-β-lactamase homologue, and over time this clone inactivated the carbenicillin in the
media and reduced the size of the zone of inhibition of E. coli. The zone observed with the
βLR16 supernatant was not different from the negative control, suggesting that the resistance
does not result from inactivation of carbenicillin.