Bacterial endosymbionts influence fungal transcriptional profiles with implications for host response in the human fungal pathogensRhizopus microsporusandRhizopus delemar

Abstract

Mucorales spores, the causative agents of mucormycosis, interact with the innate immune system to cause acute, chronic, or resolving infection. Understanding the factors that influence disease initiation and progression is key to understanding mucormycosis and developing new treatments. Complicating this, mucormycosis can be caused by a number of species that span the Mucorales genus and may be host to bacterial endosymbionts. This study sets out to examine the differences between two species in the Mucorales order by characterising their differential interactions with the innate immune system, and their interactions with environmental bacterial endosymbionts. Through a holistic approach, this study examines the transcriptional responses ofRhizopus delemarandRhizopus microsporus,two of the most commonly diagnosed species, to innate immune cells. This study also examines the immune cell response and assesses the variation in these responses, given the presence or absence of bacterial endosymbionts within the fungi. We see that the fungal response is driven by interaction with innate immune cells. The effect of the bacterial endosymbiont on the fungus is species specific, with a minimal in the absence of stress, but strongly influencing fungal transcripts during interaction with innate immune cells. In contrast, we observe that the macrophage response varies depending on the infecting fungal species, but also depending on endosymbiont status. The most successful macrophages elicit a pro-inflammatory response, and we see that through germination inhibition macrophage survival is enhanced. This work reveals species-specific host responses to related Mucorales spores and shows that bacterial endosymbionts impact the innate immune cell response.

Environmental interactions with amoebae as drivers of bacterial-fungal endosymbiosis and pathogenicity

“…Loss of the endosymbiont had profound impacts on R. microsporus response to phagocytosis, leading to altered regulation of 359 genes, including inappropriate repression of genes involved in siderophore activity and iron scavenging. 72 We observed a similar global change in the intensity of the fungal response to phagocytosis in a cured isolate of R. delemar lacking an unculturable endosymbiont with high 16S sequence homology to other unculturable bacteria (Accession number MK573034). 72 Together, our findings support a mutualistic lifestyle for R. pickettii and R. microsporus .…”

“…72 We observed a similar global change in the intensity of the fungal response to phagocytosis in a cured isolate of R. delemar lacking an unculturable endosymbiont with high 16S sequence homology to other unculturable bacteria (Accession number MK573034). 72 Together, our findings support a mutualistic lifestyle for R. pickettii and R. microsporus .…”

“…Specifically, loss of the endosymbiont was associated with reduced expression of a gene implicated in autophagy. 72 Fungal autophagy has been shown to influence lipid biogenesis, morphogenesis, development, and virulence. 110 Consistent with a change in lipid biogenesis, we observe changes in the fungal plasma membrane in cured spores, as well as increased sensitivity to Amphotercin B, indicative of altered ergosterol content.…”

“…Further in vivo and in vitro investigation revealed the bacterial endosymbiont to be the producer of an antagonistic compound with anti-phagocytic activity, which we term Phagocin R. In a companion paper, Sephton-Clark et al investigate the impact of endosymbiont status on fungal and macrophage transcriptional profiles, revealing altered macrophage polarization in response to wild type vs cured R. microsporus spores. 72 Here we investigate the consequences of endosymbiont status on infection outcome and demonstrate that this bacterial endosymbiont contributes to both fungal stress resistance and immune evasion during the earliest stages of infection, enabling fungal pathogenesis.…”

Understanding Mucormycoses in the Age of “omics”

“…Specifically, Ralstonia pickettii promotes the ability of R. microsporus to survive killing by macrophages ( Itabanga et al, 2019 ). In a transcriptome-focused companion paper, Sephton-Clark et al (2019) examined the transcriptional response of J774.1 macrophages following phagocytosis of R. delemar and R. microsporus either with or without their bacterial endosymbionts living inside them. This study revealed that endosymbiont-cured R. microsporus elicited a much stronger pro-inflammatory response than did R. microsporus which contained the bacterial endosymbiont, consistent with increased ability of macrophages to kill R. pickettii -cured R. microsporus .…”