TEMs of thin sections of infected V. vulnificus prey
cells. (A) Infected by phage only. (B) Infected by
BALO only. (C and D) Co-infection by both BALO
and phage. In (D) BALO did not change the shape
of the prey cell. B, C, and D show the bdelloplast,
the post-BALO infection structure with the predator
residing inside the prey cell. Bar = 500 nm. Source.
What fun, unless you are the one being preyed upon. Two predators, a phage and a “bacteriophagous” bacterium, simultaneously dine and reproduce in one host cell.
Doesn't this open a whole lot of questions, such as: How often does co-infection take place in the wild? Do the two predators “talk” to each other? Does this matter for the geochemical cycles? And, most important, who has first dibs?
Huan Chen and Henry Williams of Florida A&M University ask just such questions, using the marine bacterium Vibrio vulnificus as the prey, with both a phage and a BALO, Bdellovibrio-like organism (actually something called Cluster IX of Bacteriovorax) as predators. They mixed the participants in a predator/prey ratio of 1:1:1, let them work on each other, then looked at what was happening in thin sections under the electron microscope. The conclusion? The two predators do not seem to compete with one another, rather they both feast on the prey together. Both the invading bacterium and the phage multiply within the same host cell. This is a first.
Bdellovibrios and phages reproduce by very different mechanisms, but both require an intact host cell. Specifically, bdellovibrios penetrate the cell wall, reside in the periplasmic space, and shut off the host metabolic functions. They induce cell lysis after about 15 minutes, which is less than the time most phages require for their cycle. But the phage, it appears, is speedy enough to replicate in the time available. No doubt, there are some complex dynamics afoot.
This is a complicated business. Whereas each of the two predators has its own lifestyle with regards to cellular localization, reproductive strategy, and kinetics, co-infection makes special demands on each. Just how infection by BALO affects that of the phage and vice versa remains a fertile topic for investigation. Who says that things are simple in the oceans’ microbiome? The authors point out: Rather than the viruses and Bdellovibrio and like organisms competing for a single prey cell, both can survive in the same cell and successfully reproduce themselves. This is an especially valuable mechanism when the prey is in short supply, and the survival of the predators may be at stake.
The august (and relatively new) ASM journal mBio published this work in a section entitled Observations. This is a nice innovation for which the editors should be saluted.