Antimicrobial resistance (AMR) presents a growing global threat, driven by widespread antibiotic misuse across human and veterinary medicine. Companion animals, particularly dogs and cats, harbor complex natural microbiota—including skin, mucosal, and gastrointestinal communities—that are essential to their health yet also serve as reservoirs of antibiotic resistance genes (ARGs). These ARGs can spread through horizontal gene transfer (HGT), especially during bacterial imbalances such as endogenous infections or surgical interventions, increasing the risk of difficult-to-treat infections. Documented zoonotic and anthroponotic transmissions of resistant strains such as MRSA, MRSP, and ESBL-producing E. coli highlight the bidirectional nature of ARG flow between animals and humans. This underscores the critical importance of the One Health approach, which promotes interdisciplinary collaboration to monitor, understand, and combat AMR across the human–animal-environment interface. Key mechanisms of ARG dissemination, the role of companion animal microbiota, and real-world examples of resistance transfer between species illustrate the complexity and urgency of addressing AMR. Targeted surveillance, rational antibiotic use, and public awareness are essential to preserving antimicrobial efficacy and safeguarding both human and animal populations.

The detection of circulating tumor DNA (ctDNA) and circulating tumor cells (CTCs) is a minimally invasive approach for diagnosing and monitoring cancer. These liquid biopsy-based strategies enable the identification of primary or metastatic tumors and provide valuable information on tumor biology and treatment response. A variety of techniques are employed to analyze components obtained through liquid biopsy. While CTCs are typically detected using cell-based methods, ctDNA is commonly analyzed using highly sensitive molecular approaches, including quantitative PCR (qPCR), digital PCR (dPCR), and next-generation sequencing (NGS). Extracellular vesicles (EVs), which carry tumor-derived nucleic acids, proteins, and other biomolecules, are increasingly investigated as potential biomarkers, while tumor-educated platelets (TEPs) can reflect tumor-associated molecular changes and may provide additional information for cancer detection and monitoring. Furthermore, emerging multi-cancer early detection (MCED) approaches aim to identify molecular signatures associated with multiple cancer types from a single blood sample, highlighting the broader diagnostic potential of liquid biopsy. Due to the high specificity of tumors and somatic mutations, ctDNA serves as a real-time biomarker for tracking cancer progression. The advantages of ctDNA diagnostics include its low invasiveness and its capacity to detect cancer at early stages. In addition to confirming the presence of a tumor, liquid biopsy approaches facilitate the assessment of malignancy and the evaluation of treatment response. In the field of human medicine, ctDNA plays a pivotal role in diagnostic procedures. It is utilized not only for screening tests that detect the presence of cancer but also for the development of targeted treatment protocols for specific patients. These protocols are informed by the detection of genomic alterations and are designed to monitor the response to therapy over the course of treatment. Similarly, CTC, EV, TEP, and MCED approaches are being investigated as complementary tools for cancer detection, molecular characterization, prognosis, and longitudinal disease monitoring. In the domain of veterinary medicine, ctDNA has been instrumental in the diagnosis of various neoplasms, including osteosarcomas, hemangiosarcomas, lymphomas, canine mammary tumors, and melanomas. Other liquid biopsy components, including CTCs and EVs, also show promise for the detection and characterization of tumors in companion animals, although their clinical application remains less developed than in human medicine. Another significant application is in the detection of minimal residual disease (MRD), where a small number of cancer cells remain undetectable by conventional tests, such as blood counts. This underscores the significance of advanced ctDNA analysis. However, challenges persist, including the low concentration of ctDNA in blood, the low ratio of mutated to normal DNA fragments, potential contamination, biological variability, and the need for standardized protocols. This review synthesizes the current knowledge on liquid biopsy, including ctDNA, CTCs, EVs, TEPs, and emerging MCED approaches, and the potential for transferring these technologies from human medicine to animal medicine. Continued research is necessary to enhance the sensitivity and specificity of detection, which could facilitate early cancer diagnosis in animals and improve survival through timely treatment.

The body’s microbiota plays a fundamental role in maintaining homeostasis and influences immune function, metabolism, and tissue integrity. A growing body of research suggests that fluctuations in the composition and abundance of individual microbiota populations may influence cancer development and the effectiveness of therapy. The condition of microbiota dysbiosis has been demonstrated to induce chronic inflammation, immune system dysregulation, and, most significantly, modulation of molecular pathways that promote tumorigenesis. The efficacy and toxicity of cancer treatment can be influenced by the composition of the microbiota. Bacteria can modify the effectiveness and toxicity of chemotherapy and immunotherapy by affecting drug metabolism and the body’s immune response. In contrast, the development of anticancer therapies that utilize bacteria is gaining increasing interest. This alternative to conventional treatment utilizes the natural ability of certain bacterial species to selectively colonize hypoxic and necrotic environments. The exploration of natural and genetically modified bacteria as vectors for the delivery of cytotoxins, immunomodulators, or therapeutic genes in the combat of cancer is a current area of research. In addition, their capacity to stimulate an antitumor immune response is also exploited. Preclinical investigations in animals have demonstrated the efficacy of this therapeutic approach, underscoring the promise of bacterial therapies as either an adjunct to conventional treatment or as a standalone strategy for combating cancer. This article synthesizes the current knowledge regarding the role of microbiota in carcinogenesis in animals and discusses recent developments in the field of bacterial therapies. The text also addresses the challenges, safety considerations, and future perspectives associated with translating microbiota-targeted and bacterial therapies into veterinary and comparative oncology.