Mycoplasma mycoides subsp. mycoides (Mmm), a pathogenic wall-less bacterium, is the etiological agent of contagious bovine pleuropneumonia (CBPP). This highly contagious respiratory disease may develop in severe pneumonia, with associated high mortality rates in cattle. Mmm can display different immune evasion mechanisms; in addition, a host uncontrolled inflammatory response stands for lung lesions and chronic carrier animals.

Macrophages are among the most important lines of defense against Mmm of the lower respiratory tract. Although their importance in defense and immune response modulation is known, results about their role and mechanisms of action are scarce and sometimes conflicting.

In the present study, Totté et al. (1) aimed to investigate the interaction of bovine macrophages (isolated from cattle peripheral blood mononuclear cells) with Mmm, under in vitro conditions. The authors highlight that the study was performed under physiological conditions (in the presence of complement prepared from the same cell donor).

In their study, using different approaches, the authors provide interesting and original results, proposing a pivotal role of complement in controlling the inflammatory response, which is crucial in the CBPP pathogenesis. 

The authors reported that macrophages did not kill Mmm in the presence of a non-bactericidal concentration of bovine serum. However, Mmm inactivation was observed when antiserum from CBPP convalescent animals was used. They also observed that Mmm induced the production of TNF by macrophages (when a high MOI was assessed). However, complement could even abolish Mmm-induced TNF response when used at bactericidal activity concentrations. This role of complement could be combined with the development of potentially protective antibodies against particular Mmm antigens involved in the interaction with identified macrophage receptors to propose control strategies against CBPP. 

Overall, the study by Totté et al. provides new fundamental insight for the research on preventive or therapeutic strategies for a poorly understood disease that still represents a serious concern for livestock production. 

REFERENCES

1. Totté, P., Bonnefois, T., Manso-Silván, L. Interactions between Mycoplasma mycoides subsp. mycoides and bovine macrophages under physiological conditions. bioRxiv 2022.12.06.519279, ver. 2 peer-reviewed and recommended by Peer Community In Microbiology. https://doi.org/10.1101/2022.12.06.519279

The Burkholderia sensu lato group is predominant in the rhizosphere of rice. It includes both plant growth promoting rhizobacteria (typically members of the Paraburkholderia genus) and phytopathogens (typically members of the Burkholderia genus). Better understanding the interaction between Burkholderia sensu lato and their host plant is therefore crucial to advance our knowledge of the ecology of rice, a plant that feeds more than half of the humans on the planet.

The perception of root exudates from their host is key for rhizobacteria. Is the response to root exudates more related to the phylogeny of the bacteria, i.e. genus-dependent, or is it strain-specific? This question is not trivial for the Burkholderia sensu lato group, which has experienced shifting outlines over the last twenty years. During the early stages of rice root colonization, Wallner et al. [1] investigated the transcriptomic regulation of three strains of each Burkholderia and Paraburkholderia genera, in addition to a genomic comparison, in order to better understand their early colonization strategies. 

While these six strains possess a large proportion of gene homologues, their experiment shows their response to root exudates to be strain-specific. In the study, rice root exudates affected several metabolic pathways of interest in most strains, noticeably including i) the Entner-Doudoroff pathway, which had never been reported to be activated in relation to root colonization and ii) the putrescine pathway, which may reflect signaling controlling root colonization. 

The work by Wallner et al. provides new insights on the strain-level response of the transcriptomic regulation of Burkholderia sensu lato in response to root exudates in the early stages of root colonization. Beyond this, the next steps will hopefully shed light on what happens in more complex environments, within a complex bacterial community and during later colonization stages.

Reference

Wallner A, Klonowska A, Guigard L, King E, Rimbault I, Ngonkeu E, Nguyen P, Béna G, Moulin L (2022) Comparative genomics and transcriptomic response to root exudates of six rice root-associated Burkholderia sensu lato species. BioRxiv, 2022.10.04.510755, version 2 peer-reviewed and recommended by PCI Microbiol. https://doi.org/10.1101/2022.10.04.510755