A Tad-like apparatus is required for contact-dependent prey killing in predatory social bacteria

Abstract

Myxococcus xanthus, a soil bacterium, predates collectively using motility to invade prey colonies. Prey lysis is mostly thought to rely on secreted factors, cocktails of antibiotics and enzymes, and direct contac with Myxococcus cells. In this study, we show that on surfaces the coupling of A-motility and contact-dependent killing is the central predatory mechanism driving effective prey colony invasion and consumption. At the molecular level, contact-dependent killing involves a newly discovered type IV filament-like machinery (Kil) that both promotes motility arrest and prey cell plasmolysis. In this process, Kil proteins assemble at the predator-prey contact site, suggesting that they allow tight contact with prey cells for their intoxication. Kil-like systems form a new class of Tad-like machineries in predatory bacteria, suggesting a conserved function in predator-prey interactions. This study further reveals a novel cell-cell interaction function for bacterial pili-like assemblages.

The predatory soil bacterium Myxococcus xanthus combines a Tad- and an atypical type 3-like protein secretion system to kill bacterial cells

“…The bright-field image shows the E. coli cell visibly deflating within 15 min after cell death, while the M. xanthus cell continued its movement (Figure 2A). In our exper-iments, 91% of E. coli cells were killed after contact with M. xanthus WT (Figure 2G), in line with previous reports (Arend et al, 2021;Seef et al, 2021;Zhang et al, 2020).…”

“…Mutation of the M. xanthus T6SS did not affect M. xanthus predation of E. coli: the mutant strain cleared E. coli colonies on agar plates indistinguishably from WT (Figure 1B) and reduced the number of viable E. coli at a similar rate as WT (Figure 1C). This confirms that T6SS has no major function in interspecies bacterial killing during predation (Seef et al, 2021), although it mediates cell killing in the context of M. xanthus intraspecies competition (Troselj et al, 2018a(Troselj et al, , 2018b.…”

“…The domain structure of the OM secretin, MXAN_3106, suggests classification of this system as a Tad (tight adherence) transport system, which is found in many bacteria and also archaea with a putative function in fibril formation and surface colonialization (Figure S2; Abby and Rocha, 2012;Tomich et al, 2007). In a parallel study, Seef et al (2021) reported an additional gene cluster (MXAN_4648-4661), which encodes protein components that, together with proteins encoded by MXAN_3104-3108, form a Tad-like secretion complex, named the "Kil complex" (Figure S1; Seef et al, 2021). Tad mutants (D3106 (kilC), U3107 (kilF), U3108 (kilD)) expanded into E. coli colonies on agar plates, but caused visibly smaller clearing zones (Figure 1B).…”

“…These secreted lytic factors can accumulate within large multicellular groups of M. xanthus and increase their predation efficiency, which prompted the comparison of preying M. xanthus with hunting "wolf packs" (Berleman and Kirby, 2009;Mun ˜oz-Dorado et al, 2016;Rosenberg et al, 1977). On the other hand, it became evident that individual M. xanthus cells kill prey bacteria "one-on-one" within minutes, employing an unknown mechanism that requires cell contact (Arend et al, 2021;Mcbride and Zusman, 1996;Seef et al, 2021;Zhang et al, 2020).…”

Bacterial outer-membrane polysaccharide export (OPX) proteins occupy three structural classes with selective β-barrel porin requirements for polymer secretion

“…Myxococcus xanthus is a Gram-negative bacterium that exhibits an intricate social multicellular lifestyle (Muñoz-Dorado et al ., 2016). This motile soil bacterium (Faure et al ., 2016, Islam & Mignot, 2015) is able to predate other bacteria (Seef et al ., 2021) and saprophytically feed on the degradation products. Under nutrient deprivation, cells in a swarm biofilm form myxospore- filled fruiting bodies through a developmental program resulting in a differentiated cell community (Muñoz-Dorado et al ., 2016).…”