Acidic pH modulates Burkholderia cenocepacia antimicrobial susceptibility in the cystic fibrosis nutritional environment

Burkholderia cenocepacia is an opportunistic pathogen correlated with increased disease severity and mortality in cystic fibrosis (CF) patients. One major difficulty when treating these infections in CF is the poor relationship between in vitro susceptibility and clinical outcome. Previous analysis of CF sputum samples showed elevated levels of zinc and iron ions and an acidic pH (2.9-6.5) compared to healthy individuals. Additionally, B. cenocepacia grows at an acidic pH (similar to 3.5) in vitro and persists for 24 h in the intracellular acid compartments of amoebas and macrophages. We seek to understand the impact of acidic pH and increased zinc and iron concentrations on antibiotic susceptibility in the CF nutritional environment. The Synthetic Cystic Fibrosis sputum Media (SCFM) was modified to represent the acidic pH and increased zinc and iron concentrations found in CF sputum (SCFM-FeZn). We found that the nutritional environment modulated B. cenocepacia susceptibility to antimicrobials, and more strikingly, acidic pH decreased susceptibility to most antimicrobials used clinically to treat these infections. Finally, we assessed susceptibility against a custom compound library of antimicrobials using SCFM-FeZn agar plates. Out of 591 antibiotics, only 18 were active against B. cenocepacia at both neutral and acidic pH. Four of these compounds (novobiocin, coumermycin, mitomycin C, and streptonigrin) were selected for susceptibility assays in liquid culture. Here, we show that acidic pH modulates antimicrobial susceptibility in the CF nutritional environment and that susceptibility testing in media that mimic the host nutritional environment could pave the way to finding new therapies against infections in CF. IMPORTANCE Burkholderia cenocepacia causes severe infections in cystic fibrosis (CF) patients. CF patients are prone to reoccurring infections due to the accumulation of mucus in their lungs, where bacteria can adhere and grow. Some of the antibiotics that inhibit B. cenocepacia in the laboratory are not effective for CF patients. A major contributor to poor clinical outcomes is that antibiotic testing in laboratories occurs under conditions that are different from those of sputum. CF sputum may be acidic and have increased concentrations of iron and zinc. Here, we used a medium that mimics CF sputum and found that acidic pH decreased the activity of many of the antibiotics used against B. cenocepacia. In addition, we assessed susceptibility to more than 500 antibiotics and found four active compounds against B. cenocepacia. Our findings give a better understanding of the lack of a relationship between susceptibility testing and the clinical outcome when treating B. cenocepacia infections.