Selective pressure of various levels of erythromycin on the development of antibiotic resistance.

This study evaluated microbial fitness under selective pressure of various erythromycin concentrations and the development of resistance genes in Escherichia coli (E. coli) and Enterococcus faecalis (E. faecalis). Eight different concentrations of erythromycin were applied to the environment of erythromycin-resistant strains. The development of erythromycin resistance genes and gene expression were evaluated with plate counting method (PCM), fluorescence in situ hybridization (FISH), and quantitative polymerase chain reaction (qPCR). The results indicated that bacterial growth and adaptation were influenced by bacterial fitness in response to different levels of erythromycin concentrations. Furthermore, the concentration at one minimum inhibitory concentration (1x MIC) was the most effective concentration to select for antibiotic resistance for E.coli, while 4x MIC was the most effective concentration to select for antibiotic resistance for E. faecalis. Total cell densities, measured by qPCR, FISH, and PCM, decreased with increasing erythromycin concentrations. Conversely, resistant bacteria and erythromycin ribosome methylase (erm) gene abundance increased with sub-MIC erythromycin concentrations. Methylated 23S rRNA decreased with increasing erythromycin concentrations. In summary, erythromycin-resistant E. coli and E. faecalis strains adapted to the selective pressure of varying erythromycin concentrations by acquiring and proliferating antibiotic-resistant genes. These results indicate that the development of antibiotic resistance is closely linked to antibiotic concentrations and highlight the significance of selective windows in the emergence and persistence of antibiotic resistance under varying antibiotic concentrations.