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Separation of abnormal cell wall composition from penicillin resistance through genetic transformation of
A Severin1, A M Figueiredo, A Tomasz
1Laboratory of Microbiology, The Rockefeller University, New York, New York, USA.
Abstract:
Compared with most penicillin-susceptible isolates of Streptococcus pneumoniae, penicillin-resistant clinical isolate Hun 663 contains mosaic penicillin-binding protein (PBP) genes encoding PBPs with reduced penicillin affinities, anomalous molecular sizes, and also cell walls of unusual chemical composition. Chromosomal DNA prepared from Hun 663 was used to transform susceptible recipient cells to donor level penicillin resistance, and a resistant transformant was used next as the source of DNA in the construction of a second round of penicillin-resistant transformants. The greatly reduced penicillin affinity of the high-molecular-weight PBPs was retained in all transformants through both genetic crosses. On the other hand, PBP pattern and abnormal cell wall composition, both of which are stable, clone-specific properties of strain Hun 663, were changed: individual transformants showed a variety of new, abnormal PBP patterns. Furthermore, while the composition of cell walls resembled that of the DNA donor in the first-round transformants, it became virtually identical to that of susceptible pneumococci in the second-round transformants. The findings indicate that genetic elements encoding the low affinity of PBPs and the penicillin resistance of the bacteria are separable from determinants that are responsible for the abnormal cell wall composition that often accompanies penicillin resistance in clinical strains of pneumococci.
Insights
Penicillin resistance in Streptococcus pneumoniae can be separated from abnormal cell wall composition. Genetic elements for low penicillin-binding protein (PBP) affinity and resistance do not determine cell wall changes.
Area of Science:
- Microbiology
- Molecular Biology
- Genetics
Background:
- Penicillin-resistant Streptococcus pneumoniae often exhibits altered penicillin-binding proteins (PBPs) and cell wall composition.
- Strain Hun 663, a penicillin-resistant isolate, displays mosaic PBP genes, reduced PBP penicillin affinity, and unusual cell wall chemistry.
Purpose of the Study:
- To investigate the genetic basis of penicillin resistance and cell wall composition in Streptococcus pneumoniae.
- To determine if the genetic determinants for PBP low affinity and penicillin resistance are linked to those responsible for abnormal cell wall composition.
Main Methods:
- Genetic transformation experiments using chromosomal DNA from penicillin-resistant strain Hun 663.
- Analysis of penicillin-binding protein (PBP) patterns and penicillin affinities in transformants.
- Chemical composition analysis of cell walls in resistant isolates and transformants.
Main Results:
- Reduced penicillin affinity of high-molecular-weight PBPs was consistently transferred to transformants.
- PBP patterns and cell wall composition varied among transformants, indicating they are separable from resistance determinants.
- First-round transformants showed donor-like cell wall composition, while second-round transformants had cell walls similar to susceptible strains.
Conclusions:
- Genetic elements conferring low PBP affinity and penicillin resistance are separable from determinants of abnormal cell wall composition.
- Penicillin resistance in Streptococcus pneumoniae can occur independently of significant alterations in cell wall chemistry.