CRISPR adaptation in <i>Streptococcus thermophilus</i> benefits from phage environmental DNA
Bibliographic record
Abstract
ABSTRACT The CRISPR-Cas system is a bacterial adaptive immune system that protects against infection by phages: viruses that infect bacteria. To develop immunity, bacteria integrate spacers—fragments of the invading nucleic acids—into their CRISPR array to serve as the basis for sequence-targeted DNA cleavage. However, upon infection, a phage quickly takes over the metabolism of the bacterium, leaving little time for the bacterium to acquire new spacers, transcribe them, and use them to cut the invading DNA. To develop CRISPR immunity, bacteria must be safely exposed to phage DNA. Phage infection releases environmental DNA (eDNA) which could be involved in the development of CRISPR immunity. Using Streptococcus thermophilus and phages 2972 and 858 as a model for CRISPR immunity, we show that eDNA is involved in CRISPR immunity, as generation of phage-immune bacterial colonies decreases with eDNA digestion. Furthermore, it is phage eDNA specifically that impacts CRISPR immunity since only its addition increases the generation of phage-immune colonies. We also show that the effect of eDNA is phage-specific, sequence-specific, and can even be traced to a region of the genome covering the early-expressed genes, which differ between phages 2972 and 858. However, we also show that eDNA is not used as a source of genetic information for spacer acquisition. These results link eDNA to the CRISPR-Cas system, providing a better understanding of the context of the emergence of CRISPR immunity and could inform our understanding of the mechanisms through which bacteria detect phage infection. IMPORTANCE How can a bacterial adaptive immune system (the CRISPR-Cas system) exist at all, when exposure to a virulent phage is so consistently lethal? We proposed that bacteria might actively sample their genetic environment for phage DNA through natural competence. In testing this hypothesis, we revealed that free phage DNA is important to CRISPR immunity—but not as the source of the immunological memory.
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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) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| 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".