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The inoculum effect and band-pass bacterial response to periodic antibiotic treatment.

Tan C, Smith RP, Srimani JK, Riccione KA, Prasada S, Kuehn M, You L - Mol. Syst. Biol. (2012)

Bottom Line: The inoculum effect (IE) refers to the decreasing efficacy of an antibiotic with increasing bacterial density.A critical requirement for this bistability is sufficiently fast degradation of ribosomes, which can result from antibiotic-induced heat-shock response.Our proposed mechanism for the IE may be generally applicable to other bacterial species treated with antibiotics targeting the ribosomes.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Engineering, Duke University, Durham, NC, USA.

ABSTRACT
The inoculum effect (IE) refers to the decreasing efficacy of an antibiotic with increasing bacterial density. It represents a unique strategy of antibiotic tolerance and it can complicate design of effective antibiotic treatment of bacterial infections. To gain insight into this phenomenon, we have analyzed responses of a lab strain of Escherichia coli to antibiotics that target the ribosome. We show that the IE can be explained by bistable inhibition of bacterial growth. A critical requirement for this bistability is sufficiently fast degradation of ribosomes, which can result from antibiotic-induced heat-shock response. Furthermore, antibiotics that elicit the IE can lead to 'band-pass' response of bacterial growth to periodic antibiotic treatment: the treatment efficacy drastically diminishes at intermediate frequencies of treatment. Our proposed mechanism for the IE may be generally applicable to other bacterial species treated with antibiotics targeting the ribosomes.

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Kanamycin, but not chloramphenicol, led to IE in E. coli strain BL21. (A) Bacteria exhibited IE with kanamycin (6–10 μg/ml). Red and black lines represent high and low initial cell densities, respectively. Dark gray regions indicate that populations exhibited IE. Light gray regions indicate that both populations went extinct. (B) Bacteria did not exhibit IE with chloramphenicol. At both low and high initial densities, bacterial populations either survived or went extinct depending upon the applied concentration of chloramphenicol. See additional results in Supplementary Figure S2. (C) Dose response of bacterial growth rates with increasing concentrations of kanamycin. Red and black lines represent high and low initial cell density, respectively. Maximum growth rates were used as more sensitive metrics of bacterial growth as compared with absolute differences in optical densities. (D) Dose response of bacterial growth rates with increasing concentrations of chloramphenicol. Source data is available for this figure in the Supplementary Information.
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f2: Kanamycin, but not chloramphenicol, led to IE in E. coli strain BL21. (A) Bacteria exhibited IE with kanamycin (6–10 μg/ml). Red and black lines represent high and low initial cell densities, respectively. Dark gray regions indicate that populations exhibited IE. Light gray regions indicate that both populations went extinct. (B) Bacteria did not exhibit IE with chloramphenicol. At both low and high initial densities, bacterial populations either survived or went extinct depending upon the applied concentration of chloramphenicol. See additional results in Supplementary Figure S2. (C) Dose response of bacterial growth rates with increasing concentrations of kanamycin. Red and black lines represent high and low initial cell density, respectively. Maximum growth rates were used as more sensitive metrics of bacterial growth as compared with absolute differences in optical densities. (D) Dose response of bacterial growth rates with increasing concentrations of chloramphenicol. Source data is available for this figure in the Supplementary Information.

Mentions: Altogether, based on our model (Equation 1) and the experimental data, we expected kanamycin to exhibit IE (δlow, Figure 1D), but not chloramphenicol (δhigh, Figure 1D). Indeed, bacteria exhibited IE when treated with kanamycin (Figure 2A and C; Supplementary Figure S2A and I). At 6–10 μg/ml kanamycin, populations with low initial densities (black lines) did not grow after 24 h. However, those with high initial densities (red lines) grew. In contrast, bacteria did not exhibit IE when treated with chloramphenicol (Figure 2B and D; Supplementary Figure S2B, C, and J). At each concentration of chloramphenicol tested, bacterial populations either grew or did not grow regardless of their initial density. As such, kanamycin, which causes HSR and ribosome degradation, resulted in IE, whereas chloramphenicol, which does not cause HSR and ribosome degradation, did not.


The inoculum effect and band-pass bacterial response to periodic antibiotic treatment.

Tan C, Smith RP, Srimani JK, Riccione KA, Prasada S, Kuehn M, You L - Mol. Syst. Biol. (2012)

Kanamycin, but not chloramphenicol, led to IE in E. coli strain BL21. (A) Bacteria exhibited IE with kanamycin (6–10 μg/ml). Red and black lines represent high and low initial cell densities, respectively. Dark gray regions indicate that populations exhibited IE. Light gray regions indicate that both populations went extinct. (B) Bacteria did not exhibit IE with chloramphenicol. At both low and high initial densities, bacterial populations either survived or went extinct depending upon the applied concentration of chloramphenicol. See additional results in Supplementary Figure S2. (C) Dose response of bacterial growth rates with increasing concentrations of kanamycin. Red and black lines represent high and low initial cell density, respectively. Maximum growth rates were used as more sensitive metrics of bacterial growth as compared with absolute differences in optical densities. (D) Dose response of bacterial growth rates with increasing concentrations of chloramphenicol. Source data is available for this figure in the Supplementary Information.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3472685&req=5

f2: Kanamycin, but not chloramphenicol, led to IE in E. coli strain BL21. (A) Bacteria exhibited IE with kanamycin (6–10 μg/ml). Red and black lines represent high and low initial cell densities, respectively. Dark gray regions indicate that populations exhibited IE. Light gray regions indicate that both populations went extinct. (B) Bacteria did not exhibit IE with chloramphenicol. At both low and high initial densities, bacterial populations either survived or went extinct depending upon the applied concentration of chloramphenicol. See additional results in Supplementary Figure S2. (C) Dose response of bacterial growth rates with increasing concentrations of kanamycin. Red and black lines represent high and low initial cell density, respectively. Maximum growth rates were used as more sensitive metrics of bacterial growth as compared with absolute differences in optical densities. (D) Dose response of bacterial growth rates with increasing concentrations of chloramphenicol. Source data is available for this figure in the Supplementary Information.
Mentions: Altogether, based on our model (Equation 1) and the experimental data, we expected kanamycin to exhibit IE (δlow, Figure 1D), but not chloramphenicol (δhigh, Figure 1D). Indeed, bacteria exhibited IE when treated with kanamycin (Figure 2A and C; Supplementary Figure S2A and I). At 6–10 μg/ml kanamycin, populations with low initial densities (black lines) did not grow after 24 h. However, those with high initial densities (red lines) grew. In contrast, bacteria did not exhibit IE when treated with chloramphenicol (Figure 2B and D; Supplementary Figure S2B, C, and J). At each concentration of chloramphenicol tested, bacterial populations either grew or did not grow regardless of their initial density. As such, kanamycin, which causes HSR and ribosome degradation, resulted in IE, whereas chloramphenicol, which does not cause HSR and ribosome degradation, did not.

Bottom Line: The inoculum effect (IE) refers to the decreasing efficacy of an antibiotic with increasing bacterial density.A critical requirement for this bistability is sufficiently fast degradation of ribosomes, which can result from antibiotic-induced heat-shock response.Our proposed mechanism for the IE may be generally applicable to other bacterial species treated with antibiotics targeting the ribosomes.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Engineering, Duke University, Durham, NC, USA.

ABSTRACT
The inoculum effect (IE) refers to the decreasing efficacy of an antibiotic with increasing bacterial density. It represents a unique strategy of antibiotic tolerance and it can complicate design of effective antibiotic treatment of bacterial infections. To gain insight into this phenomenon, we have analyzed responses of a lab strain of Escherichia coli to antibiotics that target the ribosome. We show that the IE can be explained by bistable inhibition of bacterial growth. A critical requirement for this bistability is sufficiently fast degradation of ribosomes, which can result from antibiotic-induced heat-shock response. Furthermore, antibiotics that elicit the IE can lead to 'band-pass' response of bacterial growth to periodic antibiotic treatment: the treatment efficacy drastically diminishes at intermediate frequencies of treatment. Our proposed mechanism for the IE may be generally applicable to other bacterial species treated with antibiotics targeting the ribosomes.

Show MeSH
Related in: MedlinePlus