Bacterial capsules: no barrier against Bdellovibrio

This study investigated whether bacterial capsules protect Escherichia coli from predation by Bdellovibrio bacteriovorus, a gram-negative bacterium that invades and grows within other bacteria. Researchers compared predation rates on capsulated E. coli K29 and non-capsulated strains (K29- and ML35) using B. bacteriovorus strain 109J. Electron microscopy revealed that bdellovibrios successfully penetrated the thick polysaccharide capsule without enzymatic degradation of the capsular matrix. Both capsulated and non-capsulated prey cells were equally susceptible to predation, with similar turbidity decline patterns over 9 hours. Notably, the capsule remained intact during bdelloplast formation—the stage when bdellovibrios grow within prey cells—and multiple bdellovibrios could penetrate a single capsule. The authors concluded that unlike their protective role against bacteriophages and immune mechanisms, E. coli capsules do not serve as barriers against B. bacteriovorus predation. The active motility of bdellovibrios, propelled by flagella at high speeds, likely enables them to penetrate the gel-like capsular matrix through mechanical force rather than enzymatic attack.

Key findings

• Bdellovibrio bacteriovorus successfully penetrated E. coli K29 capsules without enzymatic degradation of the polysaccharide matrix
• Capsulated and non-capsulated E. coli prey showed identical susceptibility to predation by B. bacteriovorus 109J
• Capsules remained intact during bdelloplast formation and lysis, demonstrating capsular stability despite predation
• Multiple bdellovibrios could penetrate a single capsule, and capsulated bdelloplasts were observed for the first time
• Active flagellar propulsion, not enzymatic attack, enables bdellovibrios to penetrate the gel-like capsular barrier