CF-associated Bacteria and Fungi Affect Each Other’s Growth, According to In Vitro Study

Physical and signaling interactions between two pathogen groups that often affect patients with cystic fibrosis (CF) — Pseudomonas aeruginosa bacteria and Scedosporium fungi — result in a growth effect (inhibition or enhancement) of these microorganisms, according to an in vitro study.

CF therapies such as antibiotics or corticosteroids also affected the growth of the fungi, the researchers found, highlighting the importance of a carefully selected therapy for CF patients.

The study, “In vitro Interactions of Pseudomonas aeruginosa With Scedosporium Species Frequently Associated With Cystic Fibrosis,” was published in the journal Frontiers in Microbiology.

CF patients often receive antibiotics due to disease-associated susceptibility to bacterial infections by pathogens such as Pseudomonas aeruginosa and Staphylococcus aureus. However, antibiotic therapy may perturb the polymicrobial (many types of microorganisms) environment, and consequently allow other pathogens such as fungi to establish.

After Aspergillus fumigatus, the Scedosporium apiospermum complex is the second most common filamentous (with filaments) fungi colonizing the airways and sputum of CF patients with fungal pulmonary infections. The complex is part of a 10-member genus called the Scedosporium.

Although prevalent in CF, little is known about the risk factors and causes associated with Scedosporium.

To better understand the cause of fungal infections in CF, the researchers evaluated the direct and indirect effects of P. aeruginosa as well as commonly used CF-related therapies — such as antibiotics and corticosteroids (used to reduce lung inflammation) — on fungal growth.

The researchers tested two P. aeruginosa bacterial strains (named ATCC 27853 and ATCC 19429), and 10 different strains from the Scedosporium genus to test possible inhibitory or enhancing effects. The investigators chose these two bacterial strains based on their different phenotypes and sources of isolation (blood and urine samples, respectively).

The team first tested the effects exerted by physical contact between these bacteria and fungi. They did so by performing a so-called disk inhibition test, which measures the growth of the strains upon physical contact.

Results showed that physical contact inhibited both bacterial strains (about 60% inhibition) and fungal strains (100% inhibition). Still, the inhibitory effects showed substantial variations depending on the exposed growth medium and tested strains. For example, the inhibition weakened and became more strain-dependent when the researchers used synthetic CF sputum medium, a medium originally developed to mimic the conditions in CF patients.

The researchers next assessed a possible indirect effect by diffusible media-soluble molecules from the two bacterial strains. They found that the bacteria’s soluble molecules have a growth-inhibitory impact on fungal isolates, and that particles from the blood-derived ATCC 27853 had the most potent inhibitory activity.

When further testing two known bacterial signal molecules, called diffusible signal factor and pyocyanin, the researchers found that both molecules inhibited fungal germination (development from spores) and growth rate.

In contrast, the team found that bacterial volatile organic compounds (VOCs) stimulated the growth of the fungi.

“According to our results, P. aeruginosa either inhibited or enhanced the growth of scedosporia depending on the culture conditions and the mode of interactions. When the two pathogens were cultured physically separately from each other… the presence of the bacteria was able to stimulate the growth of several fungal isolates. While in direct physical contact, bacterial strains inhibited the fungal growth,” the researchers wrote.

Regarding the use of corticosteroids and antibiotics, results showed that Scedosporium growth rates were in general unaffected by hydrocortisone treatment, except for two strains (S. angustum and S. boydii) that showed a small yet significant reduction. Co-incubation with the corticosteroids prednisone and methylprednisolone resulted in reduced growth in the majority of the tested strains.

In that respect, the team highlighted previous studies showing that “inhaled corticosteroids surprisingly decreased the likelihood of a fungal infection, which is roughly in line with our in vitro data,” they wrote.

Researchers then tested three antibiotics — tobramycin, ceftazidime, and flucloxacillin — used to treat bacterial infections in CF. While the presence of tobramycin showed a dose-dependent effect on the growth rate of the strains, ceftazidime and flucloxacillin had highly variable results, leading to both higher and lower growth rates.

“In conclusion, we confirmed that P. aeruginosa inhibited the fungal growth in direct contact with Scedosporium species and A. fumigatus. Besides, we observed that without any physical contact, P. aeruginosa was able to enhance the growth of scedosporia via VOCs,” the researchers wrote.

The team also emphasized that the results suggest “that both antibacterial agents and corticosteroids might affect the growth properties of filamentous fungi underlying the importance of a carefully selected therapy for patients with CF.”