Mechanisms and applications of camelid variable heavy-chain nanobodies against bacterial and parasitic protozoal pathogens.

Camelid variable heavy-chain (VHH) nanobodies have emerged as a promising class of single-domain antibodies with significant potential for combating bacterial and parasitic infections. Their small molecular size, high solubility, exceptional physicochemical stability, and ability to recognize cryptic or conformational epitopes inaccessible to conventional antibodies enable unique therapeutic applications. This review summarizes recent advances in the biology, mechanisms of action, engineering, and translational potential of camelid VHH nanobodies against bacterial and parasitic pathogens. In bacterial infections, nanobodies primarily function as anti-virulence agents by neutralizing toxins, inhibiting bacterial adhesion and invasion, disrupting secretion systems, preventing biofilm formation, and targeting antimicrobial resistance determinants such as β-lactamases. In protozoal diseases, they exhibit diverse mechanisms including direct killing, inhibition of host-cell invasion, disruption of surface protein trafficking, interference with metabolic pathways, and blockade of transmission stages. We further discuss current approaches for nanobody generation, recombinant production, molecular engineering, and optimization, together with challenges related to pharmacokinetics, immunogenicity, manufacturing, and clinical translation. Finally, emerging applications involving artificial intelligence-assisted nanobody design, nanotechnology-based delivery systems, One Health strategies, and outbreak preparedness are highlighted. Collectively, camelid VHH nanobodies represent a versatile and highly engineerable platform with considerable potential to address antimicrobial resistance, and accelerate the development of next-generation antimicrobial therapeutics.