Burkholderia cenocepacia induces neutrophil necrosis in chronic granulomatous disease.

Burkholderia cepacia complex is a life-threatening group of pathogens for patients with chronic granulomatous disease (CGD), whose phagocytes are unable to produce reactive oxygen species (ROS). Unlike other CGD pathogens, B. cepacia complex is particularly virulent, characteristically causing septicemia, and is the bacterial species responsible for most fatalities in these patients. We found that a nonmucoid Burkholderia cenocepacia (a predominant species in the B. cepacia complex) isolate was readily ingested by normal human neutrophils under nonopsonic conditions and promoted apoptosis in these cells. The proapoptotic effect was not due to secreted bacterial products, but was dependent on bacterial viability. Phagocytosis was associated with a robust production of ROS, and the apoptotic neutrophils could be effectively cleared by monocyte-derived macrophages. The proapoptotic effect of B. cenocepacia was independent of ROS production because neutrophils from CGD patients were rendered apoptotic to a similar degree as control cells after challenge. More importantly, neutrophils from CGD patients, but not from normal individuals, were rendered necrotic after phagocytosis of B. cenocepacia. The extreme virulence of B. cepacia complex bacteria in CGD, but not in immunocompetent hosts, could be due to its necrotic potential in the absence of ROS.

Biofilm formation and acyl homoserine lactone production in the Burkholderia cepacia complex.

Acyl homoserine lactone (acyl-HSL)-mediated gene regulation has been shown to influence biofilm formation in one Burkholderia cepacia cystic fibrosis isolate, but it is not known whether this relationship is a consistent feature of the several genomic species that make up the B. cepacia complex (BCC). We screened strains belonging to genomovars I to V of the BCC for biofilm formation on an abiotic surface and for acyl-HSL synthesis. We determined that organisms from each of these genomovars were capable of biofilm formation. Similarly, acyl-HSL was synthesized by organisms from each of genomovars I to V, with most isolates producing octanoyl-HSL in greatest abundance. When biofilms were grown in Luria broth, acyl-HSL synthesis and biofilm formation appeared to be associated, but these phenotypes were independent when the biofilms were grown in basal salts containing citrate. Genomovar V strains synthesized the greatest quantities of acyl-HSL, and genomovar II and III-A strains elaborated the most abundant biofilms. Quorum sensing may play a role in BCC pathogenesis, but it may not regulate biofilm formation under all growth conditions.

Production of exopolysaccharide by Burkholderia cenocepacia results in altered cell-surface interactions and altered bacterial clearance in mice.

Despite the characterization of some Burkholderia cepacia complex exopolysaccharides (EPSs), little is known about the role of EPSs in the pathogenicity of B. cepacia complex organisms. We describe 2 Burkholderia cenocepacia (genomovar III) isolates obtained from a patient with cystic fibrosis (CF): the nonmucoid isolate C8963 and the mucoid isolate C9343. Both isolates had identical random amplified polymorphic DNA patterns. C9343 produced a capsule composed of the EPSs PS-I and PS-II, as well as alpha -1,6-glucan. These isolates exhibited several phenotypic differences: C8963 synthesized octanoyl-homoserine lactone and produced biofilms, but C9343 did not; in a mouse model of pulmonary infection, C8963 was cleared more rapidly than was C9343; and C9343 interacted poorly with macrophages and neutrophils, compared with C8963, suggesting that the C9343 capsule interfered with cell-surface interactions. Overproduction of EPS by C9343 resulted in a mucoid appearance and interfered with cell-surface interactions and clearance in an animal model. This mucoid colonial appearance could enhance the persistence and virulence of this important CF-related pathogen.