Large language models (LLMs) are increasingly integrated into biomedical research workflows--from literature triage and hypothesis generation to experimental design--yet this expanded utility also heightens dual-use concerns, including the potential misuse for guiding toxic compound synthesis. In response, this study shows a Biosecurity Agent that comprises four coordinated modes across the model lifecycle: dataset sanitization, preference alignment, run-time guardrails, and automated red teaming. For dataset sanitization (Mode 1), evaluation is conducted on CORD-19, a COVID-19 Open Research Dataset of coronavirus-related scholarly articles. We define three sanitization tiers--L1 (compact, high-precision), L2 (human-curated biosafety terms), and L3 (comprehensive union)--with removal rates rising from 0.46% to 70.40%, illustrating the safety-utility trade-off. For preference alignment (Mode 2), DPO with LoRA adapters internalizes refusals and safe completions, reducing end-to-end attack success rate (ASR) from 59.7% to 3.0%. At inference (Mode 3), run-time guardrails across L1-L3 show the expected security-usability trade-off: L2 achieves the best balance (F1 = 0.720, precision = 0.900, recall = 0.600, FPR =0.067), while L3 offers stronger jailbreak resistance at the cost of higher false positives. Under continuous automated red-teaming (Mode 4), no successful jailbreaks are observed under the tested protocol. Taken together, our biosecurity agent offers an auditable, lifecycle-aligned framework that reduces attack success while preserving benign utility, providing safeguards for the use of LLMs in scientific research and setting a precedent for future agent-level security protections.