Studies on beta-lactamase production in Aeromonas species



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Aeromonas species are ubiquitous environmental bacteria that occasionally cause major fish and human diseases. Although many of the aeromonads never get exposed to beta-lactam antibiotics in their natural habitats, they carry up to three inducible, beta-lactamase-encoding genes in their chromosomes. The exact reason why these environmental bacteria carry these antibiotic resistance genes is debatable. However, scientists believe that there might be secondary roles of beta-lactamase-encoding genes in bacteria, affecting the normal cellular function and relative fitness in organisms. The current study investigates the relationship between expression of beta-lactamase genes in Aeromonas and changes in bacterial fitness and cell wall composition in order to understand the possible secondary roles of beta-lactamase production in Aeromonas. To investigate whether carrying beta-lactamase-encoding genes has an advantage to the environmental Aeromonas spp., ampicillin-sensitive and resistant Aeromonas isolates were investigated for relative fitness with respect to bacterial growth and biofilm formation. According to the results, there was no significant effect on bacterial fitness under non-selective conditions in the tested isolates. However, when the media were supplemented with antibiotics, the relative fitness of the ampicillin-sensitive isolates was reduced when compared to the resistant aeromonads. Three beta-lactamase-hyperproducing mutants were isolated from the reference strain Aeromonas hydrophila ATCC 7966T, and relative fitness and possible changes in cell wall composition were investigated. The blrB gene was cloned and sequenced from the mutants to determine the mutations responsible for beta-lactamase hyperproduction. In the 7966M1 and 7966M2 mutants, a point mutation caused a single amino acid substitution in the BlrB polypeptide. Two of the mutants (7966M2 and 7966M3) demonstrated a significantly decreased level of fitness when compared to the wild type, whereas the third (7966M1) mutant did not show a significant difference in fitness. Transmission electron microscopy of the wild-type and mutant cells did not reveal any observable differences in the ultrastucture of the bacterial cell walls.



Aeromonas, Beta-lactamases