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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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Related in: MedlinePlus

Antibiotic inhibition of the ribosome can lead to growth bistability. (A) Inhibition of ribosomes (C) by an antibiotic (A). CA represents complex of C and A, and * represents degradation products of C. The model accounts for the positive feedback of C, the transport of A across the bacterial membrane, the binding of A and C, and the degradation of C (left panel). The complex model (middle panel) corresponds to a basic network motif (right panel) that can generate bistability. See Supplementary Figure S1 for additional material. (B) With δ=10−4 (Equation 1), the system has one stable steady state. (C) With δ=10−6 (Equation 1), the system has two stable steady states. Green circles indicate stable steady states. Red circles indicate unstable steady states. The black lines represent the synthesis rate of C (first and second right hand side (RHS) terms of Equation 1). The dotted lines represent the decay and inhibition of C (third RHS term of Equation 1). φ=5 × 10−6 and γ=10−4 (Equation 1). (D) The region of IE shrinks and shifts to higher values of φ (antibiotic concentration) with increasing δ (degradation of C). At a high δ (slow degradation of C), a bacterial population would survive or go extinct regardless of its initial density (no IE). At a low δ (fast degradation of C), a bacterial population would exhibit IE (gray area). Red lines represent populations with low initial density (γ=10−5), green lines represent populations with high initial density (γ=10−4). c0=1, α=0.001, κ=0.5, δ=10−6 (full lines), δ=10−5 (dashed lines), and δ=10−4 (dotted lines) (Equation 1).
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f1: Antibiotic inhibition of the ribosome can lead to growth bistability. (A) Inhibition of ribosomes (C) by an antibiotic (A). CA represents complex of C and A, and * represents degradation products of C. The model accounts for the positive feedback of C, the transport of A across the bacterial membrane, the binding of A and C, and the degradation of C (left panel). The complex model (middle panel) corresponds to a basic network motif (right panel) that can generate bistability. See Supplementary Figure S1 for additional material. (B) With δ=10−4 (Equation 1), the system has one stable steady state. (C) With δ=10−6 (Equation 1), the system has two stable steady states. Green circles indicate stable steady states. Red circles indicate unstable steady states. The black lines represent the synthesis rate of C (first and second right hand side (RHS) terms of Equation 1). The dotted lines represent the decay and inhibition of C (third RHS term of Equation 1). φ=5 × 10−6 and γ=10−4 (Equation 1). (D) The region of IE shrinks and shifts to higher values of φ (antibiotic concentration) with increasing δ (degradation of C). At a high δ (slow degradation of C), a bacterial population would survive or go extinct regardless of its initial density (no IE). At a low δ (fast degradation of C), a bacterial population would exhibit IE (gray area). Red lines represent populations with low initial density (γ=10−5), green lines represent populations with high initial density (γ=10−4). c0=1, α=0.001, κ=0.5, δ=10−6 (full lines), δ=10−5 (dashed lines), and δ=10−4 (dotted lines) (Equation 1).

Mentions: We first considered the dynamics of an antibiotic that specifically targets and interacts with the ribosome (Figure 1A). In each bacterium, the accumulation of the ribosome (C) can be described as a positive feedback loop (Tadmor and Tlusty, 2008). The antibiotic (A) can prevent the activation of the positive feedback by binding to C, resulting in a basic regulatory motif able to generate bistability (Thron, 1997; Figure 1A). For certain antibiotics, inhibition of C induces the heat-shock response (HSR) due to mistranslated proteins (Goff and Goldberg, 1985; Vanbogelen and Neidhardt, 1990). HSR in Escherichia coli upregulates chaperone proteins (e.g., DnaK) and proteases (lon and ClpP; Goff et al, 1984; Parsell and Lindquist, 1993). These proteases can target ribosomal proteins for increased degradation when bacteria are stressed (Kuroda et al, 2001). Heat shock, as well as treatment with certain antibiotics, has also been shown to cause degradation of ribosomal RNA (rRNA; Dubin, 1964; Suzuki and Kilgore, 1967; Rosenthal and Iandolo, 1970; Tolker-Nielsen and Molin, 1996; Sykes et al, 2010) and ribosomal proteins (Sykes et al, 2010). Therefore, degradation of C (ribosomes) can be increased when these antibiotics stimulate HSR (see Supplementary information). The above interactions can be captured by a simple mathematical model (Equation 1; Supplementary Equation S13):


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)

Antibiotic inhibition of the ribosome can lead to growth bistability. (A) Inhibition of ribosomes (C) by an antibiotic (A). CA represents complex of C and A, and * represents degradation products of C. The model accounts for the positive feedback of C, the transport of A across the bacterial membrane, the binding of A and C, and the degradation of C (left panel). The complex model (middle panel) corresponds to a basic network motif (right panel) that can generate bistability. See Supplementary Figure S1 for additional material. (B) With δ=10−4 (Equation 1), the system has one stable steady state. (C) With δ=10−6 (Equation 1), the system has two stable steady states. Green circles indicate stable steady states. Red circles indicate unstable steady states. The black lines represent the synthesis rate of C (first and second right hand side (RHS) terms of Equation 1). The dotted lines represent the decay and inhibition of C (third RHS term of Equation 1). φ=5 × 10−6 and γ=10−4 (Equation 1). (D) The region of IE shrinks and shifts to higher values of φ (antibiotic concentration) with increasing δ (degradation of C). At a high δ (slow degradation of C), a bacterial population would survive or go extinct regardless of its initial density (no IE). At a low δ (fast degradation of C), a bacterial population would exhibit IE (gray area). Red lines represent populations with low initial density (γ=10−5), green lines represent populations with high initial density (γ=10−4). c0=1, α=0.001, κ=0.5, δ=10−6 (full lines), δ=10−5 (dashed lines), and δ=10−4 (dotted lines) (Equation 1).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Antibiotic inhibition of the ribosome can lead to growth bistability. (A) Inhibition of ribosomes (C) by an antibiotic (A). CA represents complex of C and A, and * represents degradation products of C. The model accounts for the positive feedback of C, the transport of A across the bacterial membrane, the binding of A and C, and the degradation of C (left panel). The complex model (middle panel) corresponds to a basic network motif (right panel) that can generate bistability. See Supplementary Figure S1 for additional material. (B) With δ=10−4 (Equation 1), the system has one stable steady state. (C) With δ=10−6 (Equation 1), the system has two stable steady states. Green circles indicate stable steady states. Red circles indicate unstable steady states. The black lines represent the synthesis rate of C (first and second right hand side (RHS) terms of Equation 1). The dotted lines represent the decay and inhibition of C (third RHS term of Equation 1). φ=5 × 10−6 and γ=10−4 (Equation 1). (D) The region of IE shrinks and shifts to higher values of φ (antibiotic concentration) with increasing δ (degradation of C). At a high δ (slow degradation of C), a bacterial population would survive or go extinct regardless of its initial density (no IE). At a low δ (fast degradation of C), a bacterial population would exhibit IE (gray area). Red lines represent populations with low initial density (γ=10−5), green lines represent populations with high initial density (γ=10−4). c0=1, α=0.001, κ=0.5, δ=10−6 (full lines), δ=10−5 (dashed lines), and δ=10−4 (dotted lines) (Equation 1).
Mentions: We first considered the dynamics of an antibiotic that specifically targets and interacts with the ribosome (Figure 1A). In each bacterium, the accumulation of the ribosome (C) can be described as a positive feedback loop (Tadmor and Tlusty, 2008). The antibiotic (A) can prevent the activation of the positive feedback by binding to C, resulting in a basic regulatory motif able to generate bistability (Thron, 1997; Figure 1A). For certain antibiotics, inhibition of C induces the heat-shock response (HSR) due to mistranslated proteins (Goff and Goldberg, 1985; Vanbogelen and Neidhardt, 1990). HSR in Escherichia coli upregulates chaperone proteins (e.g., DnaK) and proteases (lon and ClpP; Goff et al, 1984; Parsell and Lindquist, 1993). These proteases can target ribosomal proteins for increased degradation when bacteria are stressed (Kuroda et al, 2001). Heat shock, as well as treatment with certain antibiotics, has also been shown to cause degradation of ribosomal RNA (rRNA; Dubin, 1964; Suzuki and Kilgore, 1967; Rosenthal and Iandolo, 1970; Tolker-Nielsen and Molin, 1996; Sykes et al, 2010) and ribosomal proteins (Sykes et al, 2010). Therefore, degradation of C (ribosomes) can be increased when these antibiotics stimulate HSR (see Supplementary information). The above interactions can be captured by a simple mathematical model (Equation 1; Supplementary Equation S13):

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