Susceptibility of Bacteria in Estuarine Environments to Autochthonous Bdellovibrios T.D. Rice, 1 H.N. Williams, 2 B.F. Turng 2 1 Northwest Hospital Center, Randallstown, MD 21133, USA 2 University of Maryland at Baltimore Dental School/Microbiology, 666 West Baltimore Street, Baltimore, MD 21201, USA Received: 27 May 1997; Accepted: 14 October 1997 A B S T R A C T Members of the genus Bdellovibrio exist as obligate predators of other gram-negative bacilli. They are believed to require large numbers of prey bacteria (>10 4 ml -1 ) to survive. Although prey bacteria are essential to the survival of bdellovibrio populations, and to studies of the predator’s role in nature, the number of bdellovibrio-susceptible bacteria in environmental samples has not been investigated. This study quantified bacteria that were susceptible to predation by the bdellovibrios. Bacteria recovered from water, sediment, and oyster-shell surface epibiota at various sites in the Chesapeake Bay system were tested for their susceptibility to bdellovibrios collected from homolo- gous sites. The mean number (log 10 ) of susceptible bacterial colonies recovered by culture was 3.33 ml -1 in water, 4.14 ml -1 in sediment and 5.76 ml -1 from oyster shells. Seventy three to 85% of all isolates tested were susceptible to bdellovibrios. Considering the actual number of bacteria in most environments is estimated to be 100 to 1000-fold greater than measured by culturing, the number of bdellovibrio-susceptible bacteria in the three environments sampled is probably sufficient to support the growth of the bdellovibrios. Introduction Bdellovibrios are widely distributed in nature, having been isolated from soil, sewage, and aquatic environments [3, 4, 12]. The obligate requirement of the predatory bdellovibrios for prey bacteria, and their unique biphasic life cycle, pose special considerations for their survival in the environment. During intraperiplasmic growth, the bdellovibrios inhabit a physically protective and nutrient-favorable econiche; in the free-living state, they are directly exposed to the external environment, where they are subject to starvation and death unless suitable concentrations of prey bacteria are present [5]. The number of prey cells required to either survive or maintain a population in equilibrium has been the subject of some debate. Hespell et al. [5] calculated that a minimum density of 1.5 × 10 5 host cells ml -1 was required for bdel- lovibrios to have a 50% survival rate over a 10-h period. The investigators concluded that survival of the predators in un- polluted waters seemed unlikely. Correspondence to: Henry Williams; Fax: (410) 706-0193 MICROBIAL ECOLOGY Microb Ecol (1998) 35:256–264 © 1998 Springer-Verlag New York Inc.