Multicellular bacteria deploy the type VI secretion system to preemptively strike neighboring cells

Area of Science:

• Microbiology
• Bacterial cell-cell interactions
• Molecular mechanisms of bacterial warfare

Background:

• The Type VI Secretion System (T6SS) is a bacterial nanomachine delivering toxic effectors, but its natural function and regulation remain poorly understood.
• Proteus mirabilis exhibits coordinated multicellular swarming behavior, forming visible boundaries (Dienes lines) when encountering other isolates.

Purpose of the Study:

• To investigate the role of T6SS in bacterial social recognition and cooperative behavior.
• To elucidate the mechanisms underlying the formation of Dienes lines and inter-bacterial discrimination.

Main Methods:

• Transposon mutagenesis to identify genes involved in the recognition phenomenon.
• Characterization of T6SS and effector operons through killing assays, mutant construction, and complementation.
• Real-time live-cell microscopy to observe T6SS activity during swarming.

Main Results:

• The Dienes line formation and bacterial recognition are dependent on the lethal action of the T6SS.
• Mutants with defects in a 33.5-kb T6SS region failed to discriminate against other Proteus isolates.
• T6SS-dependent killing and immunity require cell-cell contact and are mediated by specific genes.

Conclusions:

• Bacterial recognition, including killing and immunity, is mediated by the T6SS and requires direct cell contact.
• The T6SS functions as a preemptive mechanism for cooperative growth, rather than solely an offensive or defensive weapon.
• This study reveals a novel role for T6SS in social recognition and population dynamics during bacterial swarming.