Microbial Ecology of Spoilage 401
cell-to-cell communication. Bacteria secrete chemicals into the surrounding envi-
ronment, and the concentration of the chemicals that accumulates is dependent on
population density. By detecting and reacting to these chemicals, individual cells
can sense how many cells surround them, and whether there are enough bacteria (a
“quorum”) to initiate the change toward acting in a multicellular fashion. This is
known as quorum sensing [146].
Changes in behavior result from switching on specific genes in response to the
signal. The purpose of the change in behavior is that a population of bacteria can
cooperate to exploit their environment in ways that individual cells cannot. For
example, a single pathogenic bacterium attempting to invade its host has little chance
of overcoming the plant’s defense systems. Such a pathogen benefits from delaying
expression of virulence factors until there are sufficient numbers of bacteria present
to ensure success. Similarly, beneficial bacteria responsible for nitrogen fixation may
use quorum sensing to optimize nodule formation on plant roots. Many bacteria
produce substances that kill, or inhibit the proliferation of other, disease-causing
microorganisms, and can therefore be useful as “biocontrol” agents. However, these
substances may only be produced when the bacteria reach a critical population
density.
One set of signals that these diverse types of bacteria use in quorum sensing is
a family of structurally related chemicals. These chemicals are based on a modified
amino acid (homoserine lactone) carrying a variable acyl chain substituent and are
called acyl homoserine lactones (AHLs). The diversity in the acyl chain (chain
length, degree of oxidation and saturation) can confer some specificity on the
communications system. Nevertheless, it is likely that there is some cross-talk
between bacterial genera: plant-growth-promoting bacteria can influence the quorum
sensing systems of plant pathogens. Some of this cross-talk may represent a way
for bacteria to acquire information on the total bacterial population. This could
permit a response to competitors, or potential associates, or a method of direct
competition if a particular AHL has a detrimental effect on other species. Optimizing
the beneficial traits of plant-growth-promoting bacteria, therefore, requires an under-
standing of the cell-to-cell communication that occurs between members of one
species, as well as cross-talk with other bacterial types [141,147].
The production of AHLs and regulation of different phenotypic traits have been
reported in many Gram-negative bacteria [146]. Classical examples include regula-
tion of symbiotic behavior (e.g., bioluminescence in V. fischeri) or virulence factors
(e.g., elastase in Pseudomonas aeruginosa [148], antibiotic production in Erwinia
carotovora [149], and Ti-plasmid transfer in Agrobacterium tumefaciens [150], but
also more complex behavior such as surface motility and colonization of S. lique-
faciens [151,152] and biofilm formation of P. aeruginosa [147] and Burkholderia
cepacia [153] have been noted. Cell density-dependent regulations of gene expres-
sion probably reflect the need for the invading pathogen to reach a critical population
density sufficient to overwhelm host defenses and thus establish infection. For
example, transgenic plants producing N-oxoacyl-honoserine lactone (OHL) showed
increased resistance to infecting E. carotovora. The plant-originating signal mole-
cules force the E. carotovora to switch on production of virulence factors at low
bacterial population density. Production of virulence factors elicits a plant defense