Bdellovibrio’s Appetite for Metabolites
by Henry N. Williams
Not long ago, Elio said in this blog that predation, a major force in evolution, is somewhat neglected in microbiological circles. The full implications of predation are just beginning to be uncovered as more becomes known about the ecology, physiology, and genomics of predators against microbes and their interactions with their prey. The collective knowledge of how they influence each other will naturally feed curiosity of scientists regarding how some cellular characteristics evolved. Here, I address a new metabolic feature of a predatory bacterium that will certainly lend itself to evolutionary considerations.
Since the discovery of Bdellovibrio and Bdellovibrio-like organisms (BALOs), it has been known that these obligate predatory bacteria have an appetite for some but not all Gram-negative bacteria. Most BALOs can feed on E. coli but show no craving for the likes of the Flavobacteria. The reasons for the discriminatory taste are not known and could indeed be that the predators actually hunger for the Flavobacteria but these have attributes that enable them to resist the predators. Beyond these broad observations, little is known about the details of the BALOs nutritional preferences. Do specific nutrients confer advantages to these predators? Recent reports have only begun to scratch the surface of this important topic, which is what makes a report by Martinez and others particularly interesting.
B. bacteriovorus preying on P. putida accumulating PHA.A. Phase-contrast microscopy of B. bacteriovorus growing on P. putida for 24 h. Attack phase Bdellovibrios and extracellular mcl-PHA granules can be observed. White bar: 2 μm. B. B. bacteriovorus growing on a lawn of P. putida accumulating mcl-PHA after 48 h of incubation. C. Transmission EM showing B. bacteriovorus inside P. putida prey cells forming the bdelloplast (the structure of the prey converted upon BALO predation).
You may have heard that certain bacteria make a class of polymers called polyhydroxyalkanoates (PHAs) that are used as storage of carbon and energy. Some make these compounds in large amounts, which is of commercial interest, as they are biodegradable plastics. In some bacteria, PHAs undergoes a cycle of synthesis and degradation. PHAs contribute to fitness, survivability, and adaptation to various environmental stresses that allow PHA producing bacteria to successfully compete in soil, water, and the rhizosphere. The authors skillfully designed experiments using as prey Pseudomonas putida, a PHA producing strain and various mutants, to investigate the degradation of their medium chain length (mcl) PHA by Bdellovibrio bacteriovorus. What advantages may the predator gain from this carbon and energy source?
When exposed to PHA-accumulating and non PHA-accumulating P. putida strains, Bdellovibrio’s appetite was clearly for the former. After 24 h incubation, the numbers of Bdellovibrio were 6-fold higher in the PHA making culture than in non-PHA mutants. Indeed some benefits such as increased ATP levels, did accrue to the predators during growth on the PHA producing cells. Analyses of the Bdellovibrio genome revealed the potential to convert fatty acids such as PHA monomers into acetyl-CoA, which enters the tricarboxylic acid cycle that leads to ATP production. Motility was also substantially greater than in predators grown on a PHA minus mutant. Since BALO survival is dependent on its motility, anything that diminishes their speed decreases the chances of locating and successfully attaching to and penetrating a prey cell. This makes a demand on the predator’s endurance, as its only source of food and energy is its prey. Thus, PHA enhancement of Bdellovibrio’s motility is likely to enhance its survival. It is apparent from these results that Bdellovibrio preying on PHA producing prey cells gains an ecological advantage in its predation and survival.
The findings that a specific advantage to Bdellovibrio can be traced to a specific compound found in some prey bacteria provides a new perspective on the role of metabolites in the predatory cycle. It is particularly important in that it opens the door for future investigations of other nutrients on BALO predation, which will ultimately lead to a greater understanding of the factors which influence the predator’s appetite. Over several decades now Jurkevitch and collaborators have made important contributions to advancing the body of knowledge on the BALOs. This reports ranks high among those contributions.
Henry Neal Williams is Professor at the School of the Environment, Florida A&M University.