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 e
ssential to the survival of bdellovibrio populations, and to studies o
f the predator's role in nature, the number of bdellovibrio-susceptibl
e bacteria in environmental samples has not been investigated. This st
udy quantified bacteria that were susceptible to predation by the bdel
lovibrios. Bacteria recovered from water, sediment, and oyster-shell s
urface epibiota at various sites in the Chesapeake Bay system were tes
ted for their susceptibility to bdellovibrios collected from homologou
s sites. The mean number (log(10)) of susceptible bacterial colonies r
ecovered 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 isola
tes tested were susceptible to bdellovibrios. Considering the actual n
umber of bacteria in most environments is estimated to be 100 to 1000-
fold greater than measured by culturing, the number of bdellovibrio-su
sceptible bacteria in the three environments sampled is probably suffi
cient to support the growth of the bdellovibrios.