Stenotrophomonas maltophilia phenotypic and genotypic diversity during a 10-year infection in the lungs of a cystic fibrosis patient

Introduction Stenotrophomonas maltophilia is one of the most common emerging multi-drug resistant pathogens found in the lungs of cystic fibrosis (CF) patient, although it is unclear whether it simply colonizes CF lungs or causes true infection leading to pulmonary inflammation and clinical deterioration. In several studies, we found evidences highly suggestive of the pathogenic role of S. maltophilia in CF patients. Particularly, this microorganism can grow as biofilm on CF-derived epithelial monolayer, probably because of a selective adaptation to CF airways. In contrast to P. aeruginosa CF lung colonization, where genetic adaptations leading to phenotypic variation are well known, the diversity generated by S. maltophilia persistence in the lungs is mostly unknown. Thus, the present study was carried out to understand the adaptive strategies developed by S. maltophilia for chronic colonization of the CF lung and to evaluate whether there is a typical phenotypic profile related to chronic infection. Materials and Methods Twelve strains isolated from the sputum of a CF patient over 10-year (2005-2014) were comparatively evaluated for growth rate, biofilm formation, motility, mutation frequencies, antibiotic resistance, virulence exhibited in Galleria mellonella and pathogenicity on human respiratory A549 epithelial cells. The epidemiological relatedness of the strains was also studied by pulsed-field gel electrophoresis (PFGE). Results PFGE showed over time the presence of 2 distinct groups, each consisting of 2 different pulsotypes. The pattern of evolution followed by S. maltophilia was dependent on pulsotype considered, with strains belonging to pulsotype 1.1 resulting to be the most adapted, being significantly changed in all traits considered. Pulsotype 2.1 strains showed variations in all traits but mutation frequency and twitching motility, sharing with pulsotype 1.1 the same trend for growth rate, biofilm formation, pathogenicity, and antibiotic resistance. All pulsotypes were affected in A549 pathogenicity and antibiotic susceptibility. Generally, S. maltophilia adaptation to CF lung led to increased growth rate and antibiotic resistance, whereas both in vivo and in vitro pathogenicity as well as biofilm formation were decreased. Conclusions Our results showed, for the first time, that S. maltophilia can successfully adapt to a highly stressful environment such as CF lung by paying a “biological cost”, as suggested by the presence of relevant genotypic and phenotypic heterogeneity within bacterial population. This indicates that S. maltophilia populations are significantly more complex and dynamic than can be described by the analysis of any single isolate and can fluctuate rapidly to changing selective pressures.