On sabbatical four years ago, Dianne K. Newman, PhD, came up with an idea to apply geochemistry tools to infectious diseases. At that point Newman, a professor of biology and geobiology at the California Institute of Technology, already had been working in Pseudomonas biology for a decade, and her laboratory work suggested that some findings may be relevant in infectious diseases of the lungs, like cystic fibrosis (CF).
“There are very few measurements of environmental chemistry at the microscale within this habitat,” Newman says. “Much of infectious disease research is done in the lab, with few measurements performed in the actual environment within the human host where pathogens reside.” As a result, “much of how we set up experiments is just guessing at what appropriate conditions might be. We need to get past that and characterize the environment to design more relevant studies in the lab.”
Her friend and colleague Wiebke Ziebis, PhD, an associate professor of biological sciences at the University of Southern California and a marine biogeochemist who mainly studies sediments on the ocean floor, signed on, bringing her skills with microsensors to the table. With funding from the National Institutes of Health and the Howard Hughes Medical Institute, they set out to examine the microbiologic environment of sputum in CF patients.
Their study, published in mBio this week, evaluated sputum samples from 22 pediatric CF patients seen at Children’s Hospital Los Angeles. It found that microbes contributing to CF can survive in mucus that is chemically heterogeneous, including significant portions that are largely devoid of oxygen. Moreover, the sputum microenvironment can differ between patients, and even within the same patient at different points of time. A number of samples contained the gas hydrogen sulfide, a form of sulfur that reacts with and removes oxygen from the environment. Patients with detectable hydrogen sulfide in their sputum tended to have less severe disease.
The results shed a light on the conditions under which CF microbes can survive, Newman says: “The diversity and adaptation of disease-causing microorganisms within the CF lung environment, in part, is what renders CF infections so difficult to eradicate. Few studies have attempted to characterize the chemistry of mucus collecting in CF airways, yet such measurements are essential if we are to understand how microorganisms survive in the lung and impact the microenvironment.”
The authors used microsensors to measure high-resolution profiles of the oxygen and sulfide levels of 48 fresh sputum samples from the patients. They also looked at the samples’ chemistry and their oxidation-reduction potential, a measurement of an environment’s tendency to give or receive electrons. The samples were found to have just a very thin layer of oxygen at the surface, with the rest depleted of oxygen.
Thirty-two samples also were cultured for dominant CF disease-causing microorganisms by the hospital’s clinical microbiology laboratory. Thirteen samples harbored Pseudomonas aeruginosa, 12 had Staphylococcus aureus, five were positive for both and two had neither. (image: P. aeruginosa, a dominant CF disease-causing microbe)
“We found oxygen only at the very narrow interface between the air and samples,” says Ziebis. “It’s not only a stratified environment, with different microbial communities at different depths of the sputum, but also temporarily dynamic – there were differences not only between patients but also at different time points for the same patients.”
Further study is needed to determine whether particular metabolic fingerprints correlate with disease progression and, if so, which treatments would be most effective under these conditions, the authors said. “A greater diversity of metabolic survival strategies need to be considered and understood, including ones that operate solely under no-oxygen conditions, because that represents an important reservoir within this habitat,” Newman says.
-- Karen Blum