Inhibition of Horizontal Gene Transfers under Antibiotic Stress Switches Multi-Drug Resistance to Multi-Drug Hyper-susceptibility in Pseudomonas aeruginosa.

AIMS: To study the effect of spatial distancing between bacteria on horizontal gene transfers, and on the evolution of multi-drug resistance and multi-drug susceptibility to antibiotics.

METHODS AND

RESULTS

We controlled the plating density of bacteria on agar, and showed that, by repeatedly stressing P. aeruginosa with aztreonam, spatial distancing lowers the rate of minimum inhibitory concentration increase and the number of resistant strains. More importantly, resistant PAO1 that evolved without horizontal gene transfers exhibited 2- to 64-fold increases in susceptibility as that of the wild-type PAO1 to multiple unrelated antibiotics, including ciprofloxacin, imipenem, and tetracycline, with which the bacteria were not treated a priori. While biofilm formation by the wild-type PAO1 is generally promoted by antibiotics, biofilm formation by the hypersusceptible PAO1 was inhibited by the sub-minimum inhibitory concentration of antibiotics.

CONCLUSIONS

By spatial distancing, stressing PAO1 with aztreonam caused bacteria to become multi-drug hyper-susceptible rather than the reported multi-drug resistant. The biofilm formation by the hyper-susceptible strain was inhibited, rather than promoted, by sub-lethal concentrations of multiple antibiotics.