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Evidence for a purifying selection acting on the β-lactamase locus in epidemic clones of methicillin-resistant Staphylococcus aureus.

Milheiriço C, Portelinha A, Krippahl L, de Lencastre H, Oliveira DC - BMC Microbiol. (2011)

Bottom Line: Thirteen allotypes for blaZ, nine for blaI and 12 for blaR1 were found.In a total of 121 unique single-nucleotide polymorphisms (SNP) detected, no frameshift mutations were identified and only one nonsense mutation within blaZ was found in a MRSA strain.In addition, the data shows that the sensor-inducer blaR1 is the primary target for the accumulation of mutations in the bla locus, presumably to modulate the response to the presence of β-lactam antibiotic.

View Article: PubMed Central - HTML - PubMed

Affiliation: Laboratory of Molecular Genetics, Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa (ITQB/UNL), Oeiras, Portugal.

ABSTRACT

Background: The β-lactamase (bla) locus, which confers resistance to penicillins only, may control the transcription of mecA, the central element of methicillin resistance, which is embedded in a polymorphic heterelogous chromosomal cassette (the SCCmec element). In order to assess the eventual correlation between bla allotypes and genetic lineages, SCCmec types and/or β-lactam resistance phenotypes, the allelic variation on the bla locus was evaluated in a representative collection of 54 international epidemic methicillin-resistant Staphylococcus aureus (MRSA) clinical strains and, for comparative purposes, also in 24 diverse methicillin-susceptible S. aureus (MSSA) strains.

Results: Internal fragments of blaZ (the β-lactamase structural gene) were sequenced for all strains. A subset of strains, representative of blaZ allotypes, was further characterized by sequencing of internal fragments of the blaZ transcriptional regulators, blaI and blaR1. Thirteen allotypes for blaZ, nine for blaI and 12 for blaR1 were found. In a total of 121 unique single-nucleotide polymorphisms (SNP) detected, no frameshift mutations were identified and only one nonsense mutation within blaZ was found in a MRSA strain. On average, blaZ alleles were more polymorphic among MSSA than in MRSA (14.7 vs 11.4 SNP/allele). Overall, blaR1 was the most polymorphic gene with an average of 24.8 SNP/allele. No correlation could be established between bla allotypes and genetic lineages, SCCmec types and/or β-lactam resistance phenotypes. In order to estimate the selection pressure acting on the bla locus, the average dN/dS values were computed. In the three genes and in both collections dN/dS ratios were significantly below 1.

Conclusions: The data strongly suggests the existence of a purifying selection to maintain the bla locus fully functional even on MRSA strains. Although, this is in agreement with the notion that in most clinical MRSA strains mecA gene is under the control of the bla regulatory genes, these findings also suggest that the apparently redundant function of blaZ gene for the MRSA resistant phenotype is still important for these strains. In addition, the data shows that the sensor-inducer blaR1 is the primary target for the accumulation of mutations in the bla locus, presumably to modulate the response to the presence of β-lactam antibiotic.

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Cluster tree of blaI gene allotypes found in the MRSA and MSSA collections. See Figure 2 legend for details.
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Figure 3: Cluster tree of blaI gene allotypes found in the MRSA and MSSA collections. See Figure 2 legend for details.

Mentions: In agreement with what was observed for the blaZ gene, the cluster trees of blaI and blaR1 alleles found in our collections also showed no clustering according to MSSA/MRSA phenotype or genetic lineages (Figures 3 and 4). For those strains in which the alleles of the three genes were determined, we constructed a cluster tree with the concatenated sequences - see Figure 5. In spite of the relatively low number of allelic profiles, there was still no clear clustering of bla allotypes according to MSSA/MRSA phenotype or lineage, as the same allelic profile was present in different genetic lineages (e.g. profile 8/4/9 present in clonal complexes 5, 8 and 45) and, the same genetic lineage was characterized by profiles from different brunches (e.g. clonal cluster 8 characterized by profiles 8/4/9, 1/1/1, 3/3/6, etc.).


Evidence for a purifying selection acting on the β-lactamase locus in epidemic clones of methicillin-resistant Staphylococcus aureus.

Milheiriço C, Portelinha A, Krippahl L, de Lencastre H, Oliveira DC - BMC Microbiol. (2011)

Cluster tree of blaI gene allotypes found in the MRSA and MSSA collections. See Figure 2 legend for details.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3102608&req=5

Figure 3: Cluster tree of blaI gene allotypes found in the MRSA and MSSA collections. See Figure 2 legend for details.
Mentions: In agreement with what was observed for the blaZ gene, the cluster trees of blaI and blaR1 alleles found in our collections also showed no clustering according to MSSA/MRSA phenotype or genetic lineages (Figures 3 and 4). For those strains in which the alleles of the three genes were determined, we constructed a cluster tree with the concatenated sequences - see Figure 5. In spite of the relatively low number of allelic profiles, there was still no clear clustering of bla allotypes according to MSSA/MRSA phenotype or lineage, as the same allelic profile was present in different genetic lineages (e.g. profile 8/4/9 present in clonal complexes 5, 8 and 45) and, the same genetic lineage was characterized by profiles from different brunches (e.g. clonal cluster 8 characterized by profiles 8/4/9, 1/1/1, 3/3/6, etc.).

Bottom Line: Thirteen allotypes for blaZ, nine for blaI and 12 for blaR1 were found.In a total of 121 unique single-nucleotide polymorphisms (SNP) detected, no frameshift mutations were identified and only one nonsense mutation within blaZ was found in a MRSA strain.In addition, the data shows that the sensor-inducer blaR1 is the primary target for the accumulation of mutations in the bla locus, presumably to modulate the response to the presence of β-lactam antibiotic.

View Article: PubMed Central - HTML - PubMed

Affiliation: Laboratory of Molecular Genetics, Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa (ITQB/UNL), Oeiras, Portugal.

ABSTRACT

Background: The β-lactamase (bla) locus, which confers resistance to penicillins only, may control the transcription of mecA, the central element of methicillin resistance, which is embedded in a polymorphic heterelogous chromosomal cassette (the SCCmec element). In order to assess the eventual correlation between bla allotypes and genetic lineages, SCCmec types and/or β-lactam resistance phenotypes, the allelic variation on the bla locus was evaluated in a representative collection of 54 international epidemic methicillin-resistant Staphylococcus aureus (MRSA) clinical strains and, for comparative purposes, also in 24 diverse methicillin-susceptible S. aureus (MSSA) strains.

Results: Internal fragments of blaZ (the β-lactamase structural gene) were sequenced for all strains. A subset of strains, representative of blaZ allotypes, was further characterized by sequencing of internal fragments of the blaZ transcriptional regulators, blaI and blaR1. Thirteen allotypes for blaZ, nine for blaI and 12 for blaR1 were found. In a total of 121 unique single-nucleotide polymorphisms (SNP) detected, no frameshift mutations were identified and only one nonsense mutation within blaZ was found in a MRSA strain. On average, blaZ alleles were more polymorphic among MSSA than in MRSA (14.7 vs 11.4 SNP/allele). Overall, blaR1 was the most polymorphic gene with an average of 24.8 SNP/allele. No correlation could be established between bla allotypes and genetic lineages, SCCmec types and/or β-lactam resistance phenotypes. In order to estimate the selection pressure acting on the bla locus, the average dN/dS values were computed. In the three genes and in both collections dN/dS ratios were significantly below 1.

Conclusions: The data strongly suggests the existence of a purifying selection to maintain the bla locus fully functional even on MRSA strains. Although, this is in agreement with the notion that in most clinical MRSA strains mecA gene is under the control of the bla regulatory genes, these findings also suggest that the apparently redundant function of blaZ gene for the MRSA resistant phenotype is still important for these strains. In addition, the data shows that the sensor-inducer blaR1 is the primary target for the accumulation of mutations in the bla locus, presumably to modulate the response to the presence of β-lactam antibiotic.

Show MeSH
Related in: MedlinePlus