Extracellular vesicle (EV) research in Gram-positive bacteria, mycobacteria and fungi was neglected until recently, owing to the presumption that vesicles could not traverse the thick cell walls found in these organisms. EVs are now understood to be produced by all types of microorganism, including those with thick cell walls, and are biologically active. EVs from bacteria, mycobacteria and fungi contain virulence factors, such as toxins, that are involved in pathogenesis and elicit strong host immune responses. For example, Cryptococcus neoformans EVs carry the capsular polysaccharide glucuronoxylomannan, which is an important virulence factor. Interaction of EVs with the host is specific to the microorganism from which the EVs were produced and is based on the lipid content and cargo of the EVs. Research into EVs produced by microorganisms with thick cell walls is a very young field. By learning how these microorganisms use EVs, we hope that researchers will gain insight into pathogenesis, therapeutics and vaccines.

Severe acute respiratory syndrome coronavirus 2 (SARS–CoV-2), the cause of coronavirus disease 2019 (COVID-19), has spurred a global health crisis. To date, there are no proven options for prophylaxis for those who have been exposed to SARS–CoV-2, nor therapy for those who develop COVID-19. Immune (i.e., “convalescent”) plasma refers to plasma that is collected from individuals following resolution of infection and development of antibodies. Passive antibody administration through transfusion of convalescent plasma may offer the only short-term strategy for conferring immediate immunity to susceptible individuals. There are numerous examples in which convalescent plasma has been used successfully as postexposure prophylaxis and/or treatment of infectious diseases, including other outbreaks of coronaviruses (e.g., SARS-1, Middle East respiratory syndrome [MERS]). Convalescent plasma has also been used in the COVID-19 pandemic; limited data from China suggest clinical benefit, including radiological resolution, reduction in viral loads, and improved survival. Globally, blood centers have robust infrastructure for undertaking collections and constructing inventories of convalescent plasma to meet the growing demand. Nonetheless, there are nuanced challenges, both regulatory and logistical, spanning donor eligibility, donor recruitment, collections, and transfusion itself. Data from rigorously controlled clinical trials of convalescent plasma are also few, underscoring the need to evaluate its use objectively for a range of indications (e.g., prevention vs. treatment) and patient populations (e.g., age, comorbid disease). We provide an overview of convalescent plasma, including evidence of benefit, regulatory considerations, logistical work flow, and proposed clinical trials, as scale-up is brought underway to mobilize this critical resource.

Existing definitions of microbial pathogenicity and virulence are inadequate to explain many infectious diseases and do not incorporate the contribution of the host to these processes. A new theoretical approach to understanding microbial pathogenesis — the damage-response framework – is proposed. The damage-response framework differs from other views of microbial pathogenesis as it is neither microorganism-centred nor host-centred. Instead, the damage-response framework is based on the fact that microbial pathogenesis is the outcome of an interaction between a host and a microorganism, and uses host damage as a common principle that incorporates the role of both the host and the microorganism. The host-immune response can augment or delimit the nature and amount of host damage resulting from a host–microorganism interaction. Therefore, in the damage-response framework, pathogens are classified by the amount, or degree, of host damage that results from host–microorganism interactions as a function of the host-immune response. Six different classes are proposed, and are depicted in parabolic damage-response curves that represent the amount of host damage as a function of the intensity and degree of the host response. The amount or degree of host damage that results from the host–microorganism interaction as a function of time can be used to define and characterize the outcome of infection as the states of commensalism, colonization, latency or disease. The damage-response framework is based on clinical and experimental observations of the outcome of host–microorganism interactions. Its associated classifications and predictions can be subjected to further experimental studies to validate or refute its ability to account for the contributions of both host and microorganism to microbial pathogenesis. The damage-response framework of microbial pathogenesis could assist in the design of vaccines and immunotherapies and in the characterization of new infectious diseases. Its simplified classification system is a useful educational tool. Additionally, use of the damage-response framework could foster collaboration between investigators in different, and at present separate, areas of microbial pathogenesis research.

Melanins are enigmatic pigments that are produced by a wide variety of microorganisms including several species of pathogenic bacteria, fungi and helminthes. The study of melanin is difficult because...