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Compensatory evolution of pbp mutations restores the fitness cost imposed by β-lactam resistance in Streptococcus pneumoniae.

Albarracín Orio AG, Piñas GE, Cortes PR, Cian MB, Echenique J - PLoS Pathog. (2011)

Bottom Line: Thus, these compensatory combinations of pbp mutant alleles resulted in an increase in the level and spectrum of β-lactam resistance.The clinical origin of the pbp mutations suggests that this intergenic compensatory process is involved in the persistence of β-lactam resistance among circulating strains.We propose that this compensatory mechanism is relevant for β-lactam resistance evolution in Streptococcus pneumoniae.

View Article: PubMed Central - PubMed

Affiliation: Departamento de Bioquímica Clínica - CIBICI (CONICET), Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Córdoba, Argentina.

ABSTRACT
The prevalence of antibiotic resistance genes in pathogenic bacteria is a major challenge to treating many infectious diseases. The spread of these genes is driven by the strong selection imposed by the use of antibacterial drugs. However, in the absence of drug selection, antibiotic resistance genes impose a fitness cost, which can be ameliorated by compensatory mutations. In Streptococcus pneumoniae, β-lactam resistance is caused by mutations in three penicillin-binding proteins, PBP1a, PBP2x, and PBP2b, all of which are implicated in cell wall synthesis and the cell division cycle. We found that the fitness cost and cell division defects conferred by pbp2b mutations (as determined by fitness competitive assays in vitro and in vivo and fluorescence microscopy) were fully compensated by the acquisition of pbp2x and pbp1a mutations, apparently by means of an increased stability and a consequent mislocalization of these protein mutants. Thus, these compensatory combinations of pbp mutant alleles resulted in an increase in the level and spectrum of β-lactam resistance. This report describes a direct correlation between antibiotic resistance increase and fitness cost compensation, both caused by the same gene mutations acquired by horizontal transfer. The clinical origin of the pbp mutations suggests that this intergenic compensatory process is involved in the persistence of β-lactam resistance among circulating strains. We propose that this compensatory mechanism is relevant for β-lactam resistance evolution in Streptococcus pneumoniae.

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Electron microscopy analysis of cell morphology of pbp mutants.Microphotographs of the wild-type strain (Cp1015) revealed the typical morphology of S. pneumoniae, with a correct septum placement, division and symmetry of daughter cells. In the pbp2b28 mutant, two subpopulations were identified, namely rod-like and coccoid cells. The rod-like shaped cells showed multiple septa with incorrect formation and placement. The coccoid-shape cells exhibited multiple septa and intracellular accumulation of the cell wall, with the atypical septum localization resulting in irregular cell divisions and asymmetrical daughter cells (see arrows). The pbp2b28 pbp2x28 mutant partially restored the coccoid morphology, but conserved some cell alterations similar to those of the single pbp2b28 mutant. The triple pbp mutant showed a similar morphology to wt cells. Additional microphotographs are shown in Figs. S12, S13, S14 and S15 in Supporting Information S1 (bar scale, 1.6 µm).
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ppat-1002000-g003: Electron microscopy analysis of cell morphology of pbp mutants.Microphotographs of the wild-type strain (Cp1015) revealed the typical morphology of S. pneumoniae, with a correct septum placement, division and symmetry of daughter cells. In the pbp2b28 mutant, two subpopulations were identified, namely rod-like and coccoid cells. The rod-like shaped cells showed multiple septa with incorrect formation and placement. The coccoid-shape cells exhibited multiple septa and intracellular accumulation of the cell wall, with the atypical septum localization resulting in irregular cell divisions and asymmetrical daughter cells (see arrows). The pbp2b28 pbp2x28 mutant partially restored the coccoid morphology, but conserved some cell alterations similar to those of the single pbp2b28 mutant. The triple pbp mutant showed a similar morphology to wt cells. Additional microphotographs are shown in Figs. S12, S13, S14 and S15 in Supporting Information S1 (bar scale, 1.6 µm).

Mentions: Curiously, 46% of pbp2b28 cells had a rod-like shape (Fig. 2A), with the rest showing an apparent wild-type morphology. Electron microscopy revealed that the coccoid-shaped cells exhibited a variety of cell wall defects, including an abnormal septum position, atypical intracellular structures and frequent asymmetrical divisions (Figs. 3 and S13 in Supporting Information S1) compared with the wild-type strain (Figs. 3 and S12 in Supporting Information S1), with the rod-shaped cells exhibiting multiple septa (Figs. 3 and S13 in Supporting Information S1). Moreover, the double pbp2b28 pbp2x28 mutant displayed a coccoid morphology but with atypical septum localizations and peptidoglycan accumulation (Figs. 3 and S14 in Supporting Information S1). However, the triple pbp mutants had wild-type cell morphology and showed no ultrastructural alterations (Figs. 3 and S15 in Supporting Information S1).


Compensatory evolution of pbp mutations restores the fitness cost imposed by β-lactam resistance in Streptococcus pneumoniae.

Albarracín Orio AG, Piñas GE, Cortes PR, Cian MB, Echenique J - PLoS Pathog. (2011)

Electron microscopy analysis of cell morphology of pbp mutants.Microphotographs of the wild-type strain (Cp1015) revealed the typical morphology of S. pneumoniae, with a correct septum placement, division and symmetry of daughter cells. In the pbp2b28 mutant, two subpopulations were identified, namely rod-like and coccoid cells. The rod-like shaped cells showed multiple septa with incorrect formation and placement. The coccoid-shape cells exhibited multiple septa and intracellular accumulation of the cell wall, with the atypical septum localization resulting in irregular cell divisions and asymmetrical daughter cells (see arrows). The pbp2b28 pbp2x28 mutant partially restored the coccoid morphology, but conserved some cell alterations similar to those of the single pbp2b28 mutant. The triple pbp mutant showed a similar morphology to wt cells. Additional microphotographs are shown in Figs. S12, S13, S14 and S15 in Supporting Information S1 (bar scale, 1.6 µm).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3040684&req=5

ppat-1002000-g003: Electron microscopy analysis of cell morphology of pbp mutants.Microphotographs of the wild-type strain (Cp1015) revealed the typical morphology of S. pneumoniae, with a correct septum placement, division and symmetry of daughter cells. In the pbp2b28 mutant, two subpopulations were identified, namely rod-like and coccoid cells. The rod-like shaped cells showed multiple septa with incorrect formation and placement. The coccoid-shape cells exhibited multiple septa and intracellular accumulation of the cell wall, with the atypical septum localization resulting in irregular cell divisions and asymmetrical daughter cells (see arrows). The pbp2b28 pbp2x28 mutant partially restored the coccoid morphology, but conserved some cell alterations similar to those of the single pbp2b28 mutant. The triple pbp mutant showed a similar morphology to wt cells. Additional microphotographs are shown in Figs. S12, S13, S14 and S15 in Supporting Information S1 (bar scale, 1.6 µm).
Mentions: Curiously, 46% of pbp2b28 cells had a rod-like shape (Fig. 2A), with the rest showing an apparent wild-type morphology. Electron microscopy revealed that the coccoid-shaped cells exhibited a variety of cell wall defects, including an abnormal septum position, atypical intracellular structures and frequent asymmetrical divisions (Figs. 3 and S13 in Supporting Information S1) compared with the wild-type strain (Figs. 3 and S12 in Supporting Information S1), with the rod-shaped cells exhibiting multiple septa (Figs. 3 and S13 in Supporting Information S1). Moreover, the double pbp2b28 pbp2x28 mutant displayed a coccoid morphology but with atypical septum localizations and peptidoglycan accumulation (Figs. 3 and S14 in Supporting Information S1). However, the triple pbp mutants had wild-type cell morphology and showed no ultrastructural alterations (Figs. 3 and S15 in Supporting Information S1).

Bottom Line: Thus, these compensatory combinations of pbp mutant alleles resulted in an increase in the level and spectrum of β-lactam resistance.The clinical origin of the pbp mutations suggests that this intergenic compensatory process is involved in the persistence of β-lactam resistance among circulating strains.We propose that this compensatory mechanism is relevant for β-lactam resistance evolution in Streptococcus pneumoniae.

View Article: PubMed Central - PubMed

Affiliation: Departamento de Bioquímica Clínica - CIBICI (CONICET), Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Córdoba, Argentina.

ABSTRACT
The prevalence of antibiotic resistance genes in pathogenic bacteria is a major challenge to treating many infectious diseases. The spread of these genes is driven by the strong selection imposed by the use of antibacterial drugs. However, in the absence of drug selection, antibiotic resistance genes impose a fitness cost, which can be ameliorated by compensatory mutations. In Streptococcus pneumoniae, β-lactam resistance is caused by mutations in three penicillin-binding proteins, PBP1a, PBP2x, and PBP2b, all of which are implicated in cell wall synthesis and the cell division cycle. We found that the fitness cost and cell division defects conferred by pbp2b mutations (as determined by fitness competitive assays in vitro and in vivo and fluorescence microscopy) were fully compensated by the acquisition of pbp2x and pbp1a mutations, apparently by means of an increased stability and a consequent mislocalization of these protein mutants. Thus, these compensatory combinations of pbp mutant alleles resulted in an increase in the level and spectrum of β-lactam resistance. This report describes a direct correlation between antibiotic resistance increase and fitness cost compensation, both caused by the same gene mutations acquired by horizontal transfer. The clinical origin of the pbp mutations suggests that this intergenic compensatory process is involved in the persistence of β-lactam resistance among circulating strains. We propose that this compensatory mechanism is relevant for β-lactam resistance evolution in Streptococcus pneumoniae.

Show MeSH
Related in: MedlinePlus