Multiple differences in pathogen-host cell interactions following a bacterial host shift

Extracellular Vesicles From Mycoplasma gallisepticum: Modulators of Macrophage Activation and Virulence

Extracellular vesicles (EVs) mediate intercellular communication by transporting proteins. To investigate the pathogenesis of Mycoplasma gallisepticum, a major threat to the poultry industry, we isolated and characterized M. gallisepticum-produced EVs. Our study highlights the significant impact of M. gallisepticum-derived EVs on immune function and macrophage apoptosis, setting them apart from other M. gallisepticum metabolites. These EVs dose-dependently enhance M. gallisepticum adhesion and proliferation, simultaneously modulating Toll-like receptor 2 and interferon γ pathways and thereby inhibiting macrophage activation. A comprehensive protein analysis revealed 117 proteins in M. gallisepticum-derived EVs, including established virulence factors, such as GapA, CrmA, VlhA, and CrmB. Crucially, these EV-associated proteins significantly contribute to M. gallisepticum infection. Our findings advance our comprehension of M. gallisepticum pathogenesis, offering insights for preventive strategies and emphasizing the pivotal role of M. gallisepticum-derived EVs and their associated proteins. This research sheds light on the composition and crucial role of M. gallisepticum-derived EVs in M. gallisepticum pathogenesis, aiding our fight against M. gallisepticum infections.

CHANGES IN TISSUE TROPISM OF MYCOPLASMA GALLISEPTICUM FOLLOWING HOST JUMP

Mycoplasma gallisepticum, a pathogen of worldwide economic importance in poultry, is recovered in chickens, especially from the respiratory tract. Some strains, however, are specialized to other tissues and because it jumps from poultry to wild birds, the new strains also cause severe conjunctivitis in new hosts. Nevertheless, most studies of M. gallisepticum in wild birds use choanal swabs or combine choanal and conjunctival swabs to quantify bacterial load. Because the clinical signs associated with M. gallisepticum infection differ markedly between poultry and House Finches (Haemorhous mexicanus), we compared the bacterial load in choanal and conjunctival samples following experimental inoculation of House Finches with M. gallisepticum isolates originating from poultry or from House Finches. This allowed us to test two hypotheses: M. gallisepticum changed tissue tropism, or M. gallisepticum simply expanded its within-host niche. By comparing bacterial loads from choanal and conjunctival swabs in birds inoculated with one of a suite of M. gallisepticum isolates, we found support for hypothesis 2. The choanal loads in House Finches did not differ between isolates, while the conjunctival loads of birds inoculated with poultry isolates were lower than in birds inoculated with House Finch isolates. When measuring the bacterial load of M. gallisepticum in birds, it is important to sample and analyze separately choanal and conjunctival swabs, as quantifying bacterial loads in pooled samples may not provide reliable information on differences in virulence.

The dynamics of evolutionary rescue from a novel pathogen threat in a host metapopulation

When a novel disease strikes a naïve host population, there is evidence that the most immediate response can involve host evolution while the pathogen remains relatively unchanged. When hosts also live in metapopulations, there may be critical differences in the dynamics that emerge from the synergy among evolutionary, ecological, and epidemiological factors. Here we used a Susceptible-Infected-Recovery model to explore how spatial and temporal ecological factors may drive the epidemiological and rapid-evolutionary dynamics of host metapopulations. For simplicity, we assumed two host genotypes: wild type, which has a positive intrinsic growth rate in the absence of disease, and robust type, which is less likely to catch the infection given exposure but has a lower intrinsic growth rate in the absence of infection. We found that the robust-type host would be strongly selected for in the presence of disease when transmission differences between the two types is large. The growth rate of the wild type had dual but opposite effects on host composition: a smaller increase in wild-type growth increased wild-type competition and lead to periodical disease outbreaks over the first generations after pathogen introduction, while larger growth increased disease by providing more susceptibles, which increased robust host density but decreased periodical outbreaks. Increased migration had a similar impact as the increased differential susceptibility, both of which led to an increase in robust hosts and a decrease in periodical outbreaks. Our study provided a comprehensive understanding of the combined effects among migration, disease epidemiology, and host demography on host evolution with an unchanging pathogen. The findings have important implications for wildlife conservation and zoonotic disease control.

Levels of pathogen virulence and host resistance both shape the antibody response to an emerging bacterial disease

Quantifying variation in the ability to fight infection among free-living hosts is challenging and often constrained to one or a few measures of immune activity. While such measures are typically taken to reflect host resistance, they can also be shaped by pathogen effects, for example, if more virulent strains trigger more robust immune responses. Here, we test the extent to which pathogen-specific antibody levels, a commonly used measure of immunocompetence, reflect variation in host resistance versus pathogen virulence, and whether these antibodies effectively clear infection. House finches (Haemorhous mexicanus) from resistant and susceptible populations were inoculated with > 50 isolates of their novel Mycoplasma gallisepticum pathogen collected over a 20-year period during which virulence increased. Serum antibody levels were higher in finches from resistant populations and increased with year of pathogen sampling. Higher antibody levels, however, did not subsequently give rise to greater reductions in pathogen load. Our results show that antibody responses can be shaped by levels of host resistance and pathogen virulence, and do not necessarily signal immune clearance ability. While the generality of this novel finding remains unclear, particularly outside of mycoplasmas, it cautions against using antibody levels as implicit proxies for immunocompetence and/or host resistance.