Zoomorphism and the danger of analogy

Myxobacteria are soil-dwelling microbes that live by killing and consuming other members of the soil microbial community. They aren’t fussy, eating all varieties of bacteria and also fungi, making them of considerable interest as potential sources of new antimicrobials.
Myxobacterial feeding has been described as employing a ‘wolf-pack’ strategy, as it seems to be more efficient when performed by larger by numbers of cells. Myxobacterial cells secrete toxic molecules of death (Figure 1) into the extracellular milieu, killing surrounding prey cells, and releasing nutrients which can then be taken up for biomass assimilation.
Wolf-pack predation conjures up images of sophisticated pack behaviour, with several organisms orchestrating their attacks. And so it seems to be with myxobacterial predation. There are reports of myxobacteria luring prey towards themselves, probably through the secretion of prey food (perhaps more akin to a fishing trip than a pack hunt). We recently found that the signals prey send to each other (acyl-homoserine lactones – AHLs) can be eavesdropped on by myxobacteria, who then up-regulate their predatory behaviour in response (Lloyd and Whitworth, 2017).

Figure 1. Myxobacteria (yellow rods) secrete toxic molecules of death (black circles) which kill neighbouring prey cells. Nutrients released from prey are taken up by the myxobacteria, allowing them to reproduce.

In order to identify the genes involved in co-ordinating the response to prey, we undertook a transcriptomics experiment, assessing gene expression of predator and prey during predation (Livingstone et al., 2018). Myxobacteria have large genomes (with ~7,000 genes) and shifting them between nutrient-rich and nutrient-poor substantially changes the expression of around 1500 genes. When myxobacteria are exposed to prey, only 3 genes are switched on (and their role seems to deal with the indirect osmotic consequence of prey presence). The transcriptional response of myxobacteria to prey is a resounding ‘who cares?’.
Behavioural changes in bacteria can be modulated by post-translational mechanisms without recourse to switching genes on or off, but it is hard to imagine a process that involves the secretion of ~100 proteins to be unregulated transcriptionally. To explain this, we invoked a model of constitutive feeding, with regulated nutrient assimilation triggered by the presence of decaying prey (adding dead prey to myxobacteria causes them to up-regulate a much more respectable 124 genes). This was certainly not wolf-pack behaviour and we were reminded more of spiders – creating external secretions (a web) which captures/kills prey, while signalling to the predator that prey is present.
So are myxobacteria wolves or spiders? Obviously they are neither. Nor are they fishermen. We often feel the need to anthropomorphise, and when doing so we often unconsciously taint our analogies with extraneous associations. Zoomorphising is no better.

Post by Dave Whitworth.

Livingstone, P.G., Millard, A.D., Swain, M.T. and Whitworth, D.E. (2018) Transcriptional changes when Myxococcus xanthus preys on Escherichia coli suggest myxobacterial predators are constitutively toxic but regulate their feeding. Microbial Genomics. 2018: 5.
Lloyd, D.G. and Whitworth, D.E. (2017) The myxobacterium Myxococcus xanthus can sense and respond to the quorum signals secreted by potential prey organisms. Frontiers in Microbiology. 8: 439.