Burchard 9710434 When the bacterium Flexibacter maritimus is exposed to variety of antibiotics, resistant mutant colonies arise. When ampicillin or other beta-lactam antibiotics are used, a striking result is seen after four to five days of incubation. Resistant colonies appear on the plates at a frequency of between one in 10,000 and 1 in 100,000. This is 10 to 100 times more frequent than would be expected for spontaneous resistant mutants. In addition, the colonies that arise have a non-gliding appearance, unlike the parent gliding strain. The non-gliding phenotype persists with subculturing in the absence of the antibiotic. Fluctuation tests with Eschericia coli and ampicillin, or Flexibacter maritimus with other antibiotics such as tetracyline and vancomycin produce results consistent with selection of pre-existing, spontaneous mutants. Replica plating experiments with F. maritimus and ampicillin show results inconsistent with the selection of spontaneous mutants. A number of approaches are being undertaken to determine the mechanism of production of the mutations, such as the possible induction of a transposable element that would increase the frequency of mutations in the bacteria. Another possibility is that there are non-genomically encoded changes in wall structure that might be heritable. Although the causes of these mutations may eventually be ascribed to conventional mechanisms, there is the possibility that an unusual genetic or epigenetic mechanism may be operating.
