Exposure to One Antibiotic Leads to Acquisition of Resistance to Another Antibiotic via Quorum Sensing Mechanisms

Abstract

The vancomycin-resistant Enterococci (VRE) have progressively become a severe medical problem. Although clinics have started to reduce vancomycin prescription, vancomycin resistance has not been contained. We found that the transfer of vancomycin resistance in Enterococcus faecalis increased more than 30-fold upon treatment by streptomycin. Notably, treatment with an antibiotic caused the bacteria to become resistant to another. The response was even stronger in the well-studied plasmid pCF10 and the number of transconjugants increased about 100,000-fold. We tested four different antibiotics, and all of them induced conjugal response. Through a mathematical model based on gene regulation, we found a plausible explanation. Via quorum sensing, the change of the cell density triggers the conjugation. Moreover, we searched for generality and found a similar strategy in Bacillus subtilis. The outcome of the present study suggests that even common antibiotics must not be overused.

Keywords: conjugal genes; dissemination of drug resistantance; nosocomial infection; post-antibiotic era; prgQ; prgX; selection pressure; side effect.

Figure 1

The main regulation of conjugal genes in plasmid pCF10. Peptides cCF10 and iCF10, through PrgX protein, determine the expression level of prgQ. The induction of prgQ results in the production of Q_L RNA, which triggers the conjugation.

Figure 2

Administration of antibiotics affects the conjugation rate. (A) An administration of streptomycin of 250 μg/ml or 500 μg/ml increased the conjugation of plasmid pMG2200 encoding vancomycin resistance. (B) An administration of spectinomycin of 25 μg/ml or 100 μg/ml leads to an increase in the conjugation of plasmid pCF10 (***indicates p < 0.001, in comparison to the case without an antibiotic).

Figure 3

The response of conjugal genes to different types of antibiotics. (A) In Enterococcus faecalis with plasmid pBK2, administration of 5 μg/ml spectinomycin, 5 μg/ml tetracycline, or 0.03 μg/ml erythromycin caused the conjugal response. (B) In Lactococcus lactis with plasmid pBK2, administration of spectinomycin results in the conjugal response. (C) An administration of streptomycin on the vulnerable donor cells OG1RF, resistant recipients OG1X, and vulnerable recipients OG1Sp increased the conjugation frequency. The transconjugant (1) and (2) are OG1X(pCF10) and OG1Sp(pCF10), respectively (***indicates p < 0.001, in comparison to the case without an antibiotic).

Figure 4

The effect of cell density on conjugal response. (A) The simulation results show that low donor density leads to a high expression level. Note that we use the inverted scale of the x-axis. For the y-axis, the normalized Q_L is the expression levels divided by the lowest one obtained at a cell density of 10¹¹ CFU/ml. (B) In Enterococcus faecalis with plasmid pBK2, low cell density results in the high expression of the conjugal gene. For the x-axis, the normalized cell density is the cell density divided by the overnight cell density (***indicates p < 0.001, in comparison to the case with a normalized cell density of one).

Figure 5

Low donor density leads to a high conjugation rate. In x-axis, high density is a 1:10 dilution to the overnight donor culture and low density are 1:10,000,000 and 1:100 to an overnight donor culture of FA2-2 (pMG2200) and OG1RF (pCF10), respectively (***indicates p < 0.001, in comparison to the case with high cell density).