To examine the disease-causing potential of the patients’ altered microflora, the researchers fed lab-grown cultures of six patients’ microbes to C. elegans roundworms—established models for studying the virulence activities of mammalian pathogens. Although individual bacterial taxa caused little harm to the worms on their own, most Candida isolates killed the worms within 40 hours of ingestion. The combination ofCandida and bacteria, however, appeared to keep pathogenesis in check.
“They actually considered the fungal component of the microbiome, which has been routinely ignored in hundreds of microbiome studies,” said Michael Lorenz, a microbiologist at the University of Texas Health Science Center at Houston who was not in involved in the study. “So the finding that there are these interactions between the bacterial and fungal components is one that people should be very aware of.”
To mimic the conditions in an ICU patient’s gut, the researchers grew the microbes in the presence of an opioid. Opioids often enter the gut in critically ill patients as part of a stress response, and are known to interact with the quorum-sensing signals that regulate bacterial virulence. Indeed, in two Candida-bacteria combinations, opioid treatment shifted bacterial behavior from commensal to pathogenic, killing a substantial proportion of worms.