The Bdellovibrio are miniature "living antibiotic" predatory bacteria which invade, reseal, and digest other larger Gram-negative bacteria, including pathogens. Nutrients for the replication of Bdellovibrio bacteria come entirely from the digestion of the single invaded bacterium, now called a bdelloplast, which is bound by the original prey outer membrane. Bdellovibrio bacteria are efficient digesters of prey cells, yielding on average 4 to 6 progeny from digestion of a single prey cell of a genome size similar to that of the Bdellovibrio cell itself. The developmental intrabacterial cycle of Bdellovibrio is largely unknown and has never been visualized "live." Using the latest motorized xy stage with a very defined z-axis control and engineered periplasmically fluorescent prey allows, for the first time, accurate return and visualization without prey bleaching of developing Bdellovibrio cells using solely the inner resources of a prey cell over several hours. We show that Bdellovibrio bacteria do not follow the familiar pattern of bacterial cell division by binary fission. Instead, they septate synchronously to produce both odd and even numbers of progeny, even when two separate Bdellovibrio cells have invaded and develop within a single prey bacterium, producing two different amounts of progeny. Evolution of this novel septation pattern, allowing odd progeny yields, allows optimal use of the finite prey cell resources to produce maximal replicated, predatory bacteria. When replication is complete, Bdellovibrio cells exit the exhausted prey and are seen leaving via discrete pores rather than by breakdown of the entire outer membrane of the prey.The predatory bacterium Bdellovibrio bacteriovorus invades and grows within the periplasmic space of another prey bacterium, hydrolyzing the interior of that prey bacterium to provide a quantized meal, growing into long elongated cells, and using those resources and not external nutrients (18). Although Bdellovibrio bacteria were discovered in 1962, their small size (0.25 by 1 m, compared to the more usual 1-by 3-m dimensions of a typical Escherichia coli cell) and the very nature of their growth within the periplasm of another bacterium has made their growth and development recalcitrant to live microscopic studies. (25) Thus, we have not been able to observe how exactly a single predatory Bdellovibrio cell makes use of the finite resources of a single prey cell (called a bdelloplast, once invaded) to grow and then manages to coordinate the departure of its progeny from that bdelloplast once prey resources are exhausted. The conundrum of predatory, intrabacterial growth by Bdellovibrio bacteria, which seems at odds with the conventions of typical binary fission of simple, nonpredatory bacteria in limitless culture media, has interested microbiologists since the 1960s (12,18,21).Early electron microscopic (EM) studies at time points throughout a predatory infection showed "attack-phase" Bdellovibrio cells entering prey by squeezing through a pore made in the outer membra...
