Environmental Burkholderia cenocepacia Strain Enhances Fitness by Serial Passages during Long-Term Chronic Airways Infection in Mice

Abstract

Burkholderia cenocepacia is an important opportunistic pathogen in cystic fibrosis (CF) patients, and has also been isolated from natural environments. In previous work, we explored the virulence and pathogenic potential of environmental B. cenocepacia strains and demonstrated that they do not differ from clinical strains in some pathogenic traits. Here, we investigated the ability of the environmental B. cenocepacia Mex1 strain, isolated from the maize rhizosphere, to persist and increase its virulence after serial passages in a mouse model of chronic infection. B. cenocepacia Mex1 strain, belonging to the recA lineage IIIA, was embedded in agar beads and challenged into the lung of C57Bl/6 mice. The mice were sacrificed after 28 days from infection and their lungs were tested for bacterial loads. Agar beads containing the pool of B. cenocepacia colonies from the four sequential passages were used to infect the mice. The environmental B. cenocepacia strain showed a low incidence of chronic infection after the first passage; after the second, third and fourth passages in mice, its ability to establish chronic infection increased significantly and progressively up to 100%. Colonial morphology analysis and genetic profiling of the Mex1-derived clones recovered after the fourth passage from infected mice revealed that they were indistinguishable from the challenged strain both at phenotypic and genetic level. By testing the virulence of single clones in the Galleria mellonella infection model, we found that two Mex1-derived clones significantly increased their pathogenicity compared to the parental Mex1 strain and behaved similarly to the clinical and epidemic B. cenocepacia LMG16656T. Our findings suggest that serial passages of the environmental B. cenocepacia Mex1 strain in mice resulted in an increased ability to determine chronic lung infection and the appearance of clonal variants with increased virulence in non-vertebrate hosts.

Airway disease phenotypes in animal models of cystic fibrosis

“…In addition to bacterial challenge being performed in CF mice, models of respiratory infection have also been established in normal rodents by using key CF-related bacterial species. Mouse models of chronic Burkholderia cenocepacia [62] and P. aeruginosa [63] infection have recently proven useful for investigating hostpathogen interactions of bacterial species that are pertinent to CF lung disease. Similarly, a rat model of chronic P. aeruginosa infection that exhibits pathologic features of human P. aeruginosa pulmonary infection has also been developed [64].…”

Biofilm and motility in response to environmental and host-related signals in Gram negative opportunistic pathogens

“…It is likely that long term adaptation to unfavourable environmental conditions are the ultimate drive to the emergence of more host‐adapted, and often more pathogenic, mutant variants (Bragonzi et al . ). In the perspective of long‐term adaptation to the host, it should be reminded that B. mallei , like many other bacterial pathogens, has taken a ‘reductionist’ approach, reducing its genome size which, although still rather large at its 5·8 Mbp, it is significantly smaller than B. thailandensis (6·7 Mbp) and B. pseudomallei (7·2 Mbp), and carries many inactivated or non‐functional genes such as the flagellar operons (Nierman et al .…”

“…The observation that growth conditions largely unfavourable for bacteria with a strong preference for aerobic respiration can trigger such an energy costing process strongly suggests that EPS production represents an important response to stress conditions and a possible defence mechanism from environmental predators and host immune response, in line with what already described for other bacteria. It is likely that long term adaptation to unfavourable environmental conditions are the ultimate drive to the emergence of more host-adapted, and often more pathogenic, mutant variants (Bragonzi et al 2017). In the perspective of long-term adaptation to the host, it should be reminded that B. mallei, like many other bacterial pathogens, has taken a 'reductionist' approach, reducing its genome size which, although still rather large at its 5Á8 Mbp, it is significantly smaller than B. thailandensis (6Á7 Mbp) and B. pseudomallei (7Á2 Mbp), and carries many inactivated or non-functional genes such as the flagellar operons (Nierman et al 2004).…”

Lung Diseases: Chronic Respiratory Infections

“…Bragonzi A et al [8] Enviromental Burkholderia cenocepacia strain enhances fitness by serial passages during long-term chronic airway infections in mice…”