Inactivation of the Glycoside Hydrolase NagZ Attenuates Antipseudomonal beta-Lactam Resistance in Pseudomonas aeruginosa
Bibliographic record
Abstract
Pseudomonas aeruginosa is a versatile Gram-negative opportunistic pathogen notorious for its ability to chronically colonize and deteriorate the pulmonary function of the cystic fibrosis lung. It exhibits high resistance to beta-lactam antibiotics, including cephalosporins and monobactams, via induction of their chromosomally encoded AmpC beta-lactamase. Regulation of ampC expression is coupled to the bacterial cell wall recycling pathway by the activity of NagZ, a glycosidase that produces 1,6-anhydroMurNAc-tri-(or penta-) peptides from internalized peptidoglycan metabolites. During beta-lactam therapy, this tripeptide rapidly concentrates in the bacterial cytosol to levels sufficient for it to bind and activate AmpR, the transcriptional activator of ampC. P. aeruginosa also encodes three ampD genes, each expressing an N-acetylmuramyl-L-amidase that cleaves the peptide stems from 1,6-anhydroMurNAc or GlcNAc 1,6-anhydroMurNAc. AmpD thus suppresses 1,6-anhydroMurNAc-peptide accumulation and moderates ampC induction. Selection of AmpD null mutants during therapy thus causes chronic hyperproduction of beta-lactamase, presumably from an increase in NagZ product, and have been identified in P. aeruginosa strains isolated from chronically infected CF patients. Mutants harboring an inactivated nagZ gene in a wild-type P. aeruginosa background were isolated and were found to have increased antibiotic susceptibility to antipseudomonal beta-lactams. Inactivating nagZ in a triple ampD mutant substantially decreased the expression of ampC and rendered these high-level resistant strains susceptible to antipseudomonal beta-lactams at wild-type strain levels. This brings the susceptibility of the P. aeruginosa strains down to the beta-lactam therapy range accepted by CLSI for use in cystic fibrosis patients suffering from chronic Pseudomonas aeruginosa infections. To assess whether P. aeruginosa expresses more than one N-acetyl-beta-glucosaminidase that could contribute to the production of the activating tripeptide, residual activity assays were conducted on nagZ deficient mutants. Mutants were devoid of activity so it was concluded that P. aeruginosa expresses only the one N-acetyl-beta-glucosaminidase in study, NagZ. Complementation studies using the wild type nagZ gene restored the wild type phenotypes, particularly evident in the triple ampD null mutants. These findings suggest that NagZ activity is required for ampC induction, and that an intricate balance exists between NagZ and AmpD activity to regulate the concentration of the inducer molecule 1,6-anhydroMurNAc-tripeptide.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".