Long-term starvation-induced loss of antibiotic resistance in bacteria.

Escherichia coli, Pseudomonas fluorescens, and aPseudomonas sp. strain 133B containing the pSa plasmid were starved in well water for up to 523 days. There were two patterns of apparent antibiotic resistance loss observed. InPseudomonas sp. strain 133B, there was no apparent loss of antibiotic resistance even after starvation for 340 days. InE. coli, by day 49 there was a ten-fold difference between the number of cells that would grow on antibiotic- and nonantibiotic-containing plates. However, over 76% of the cells that apparently lost their antibiotic resistance were able to express antibiotic resistance after first being resuscitated on non-selective media. By day 523, only 12% of these cells were able to express their antibiotic resistance after being resuscitated. After starvation for 49 days, cells that could not grow on antibiotic medium even after resuscitation, showed a permanent loss of chloramphenicol (Cm) resistance but retained resistance to kanamycin (Km) and streptomycin (Sm). Restriction enzyme digests show that a 2.5 to 3.0 Kb region from map location 12.5 to 15.5 Kb was deleted. This coincides with the 2.5 Kb reduction in plasmid size observed in 3 isolates that had lost antibiotic resistance after starvation for 49 days.