Bacteria can secrete a wide array of antibacterial compounds when
competing with other bacteria for the same resources. Some of
these compounds, such as bacteriocins, can affect bacteria of
similar or closely related strains. In some cases, these secretions
have been found to kill sibling cells that belong to the same colony.
Here, we present experimental observations of competition between
2 sibling colonies of Paenibacillus dendritiformis grown on
a low-nutrient agar gel. We find that neighboring colonies (growing
from droplet inoculation) mutually inhibit growth through
secretions that become lethal if the level exceeds a well-defined
threshold. In contrast, within a single colony developing from a
droplet inoculation, no growth inhibition is observed. However,
growth inhibition and cell death are observed if material extracted
from the agar between 2 growing colonies is introduced outside a
growing single colony. To interpret the observations, we devised
a simple mathematical model for the secretion of an antibacterial
compound. Simulations of this model illustrate how secretions
from neighboring colonies can be deadly, whereas secretions from
a single colony growing from a droplet are not.