Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 10/12/2015
Publication Date: 10/19/2015
Citation: Brunelle, B.W., Bearson, S.M., Bearson, B.L. 2015. Duplication and amplification of antibiotic resistance genes enable increased resistance in isolates of multidrug-resistant Salmonella Typhimurium [abstract]. 75th Annual American Society of Microbiology, North Central Branch Meeting. Paper No. 194.
Technical Abstract: During normal bacterial DNA replication, gene duplication and amplification (GDA) events occur randomly at a low frequency in the genome throughout a population. In the absence of selection, GDA events that increase the number of copies of a bacterial gene (or a set of genes) are lost. Antibiotic exposure, however, is a very strong selective event. Our goal was to determine if inhibitory concentrations of tetracycline and chlortetracycline can select for rare GDA events that would lead to increased antibiotic resistance in isolates of multidrug-resistant (MDR) Salmonella enterica serovar Typhimurium. To perform these experiments, isolates of MDR S. Typhimurium DT104 and DT193 were utilized; DT104 encodes the tetracycline/chlortetracycline resistance gene tetG in the chromosome, while DT193 encodes tetA in a plasmid. Culturing these isolates on LB plates containing the minimum inhibitory levels of tetracycline or chlortetracycline resulted in very few colonies, as compared to a lawn on the no-antibiotic control plate. Subculturing these few colonies on media containing higher antibiotic concentrations revealed they had an increased level of resistance to tetracycline and chlortetracycline. Quantitative real-time PCR determined that the colonies with increased antibiotic resistance had a 5-10 fold increase in the number of their respective tet genes. Additionally, the neighboring floR gene that encodes resistance to chloramphenicol also increased 5-10 fold in DT104 and DT193 isolates grown on tetracycline, indicating that the GDA event co-selected for an increase in the floR gene independent of chloramphenicol exposure. These data demonstrate that antibiotics can select for GDA events in resistance genes that are encoded in either the chromosome or plasmid of MDR Salmonella.