Comparison of culture based sputum microbiology in cystic fibrosis and bronchiectasis: a case-control study

Objective Studies directly comparing sputum microbiology from subjects with cystic fibrosis (CF) and bronchiectasis without associated CF are scarce. Our aim was to compare the results from routine sputum microbiology using the same culture methods.

Methods Since Oct 2013 the same sputum culture methods for CF and bronchiectasis, including extensive culture for bacteria, mycobacteria and fungi, are used at our institution, allowing for direct comparison between both groups. Within a 3-yr period starting Oct 2013 patients, who were cared for in our respective outpatient clinics, were matched by age, gender and FEV₁. Bacterial and fungal species identification included microscopy, biochemical profiling, MALDI-TOF analysis or sequencing, if needed. In order to correct for differences in the number of sputum samples the relative proportion among all microorganisms within each group (CF or bronchiectasis) was calculated.

Results 28 CF and bronchiectasis patients were matched in a 1 : 1 ratio. In both groups, mean (SD) age was 32 (11) years (range 15 – 54), 16/28 were female; on average, patients had moderate airflow limitation (FEV₁ 64 – 65 (27 – 28) %predicted). CF patients differed significantly with regard to a lower BMI (21.1 vs. 24.0 kg/m²; p = 0.023) and a higher total number of sputum samples within the observational period (8 vs. 3; p = 0.022). Overall, comparable microbiological spectra including a variety of mold and Gram-negative bacilli were observed in both groups ([Fig. 1]). However, major differences in composition of groups were detected: in the CF group Staphylococcus aureus (SA) and Pseudomonas aeruginosa (PA) were most frequently detected constituting 23 and 21% of all microorganisms, whereas in the bronchiectasis group the proportion of both SA and PA was significantly lower, with 10 and 9% of total (p < 0.001, each).

Conclusion Currently, microbiological sputum surveillance programs for CF and bronchiectasis differ. However, applying the same extensive sputum culture methods as in CF patients may yield a similar microbiological spectrum in bronchiectasis, although clinical significance still needs to be established for many specific pathogens. The difference with regard to the proportion of SA and PA may point towards an increased susceptibility for these pathogens among patients with CF.