Microbial Metabolites in Fruits and Vegetables 523
antimicrobial peptides such as bacteriocins has also been demonstrated to be under
the control of quorum sensing (Strume et al., 2002). The key difference in Gram-
positive quorum sensing is that peptides (specifically, autoinducing peptides) rather
than AHLs act as the signaling molecule. Peptides implicated in quorum sensing
are small and diverse and undergo posttransitional modification to enhance stability
and functionality. Interestingly, nisin can act as an autoinducing peptide, in addition
to having antimicrobial effects.
The role of quorum sensing in food spoilage is only at the early stages of
investigation. It is known that several pectinolytic Erwinia and Pseudomonas, which
spoil ready-to-eat vegetables such as bean sprouts, produce AHLs. Storage trials
with bean sprouts have demonstrated that inoculation of sprouts with AHL-produc-
ing, pectinolytic E. carotovora results in faster spoilage. If quorum sensing does
indeed have a significant role in regulating the spoilage capacity of bacteria associ-
ated with fruits and vegetables it would provide a novel preservation strategy. An
obvious method would be to introduce AHL-degrading enzymes (or bacterial cells
expressing the enzyme) onto fruits or vegetables to disrupt the cell-to-cell commu-
nication. To date such an enzyme has been recovered from Bacillus species and
P. aureofaciens. An alternative approach would be to add chemicals that could block
or inhibit the quorum-sensing response. One promising group of quorum-sensing
inhibitors (QSI) are the halogenated furanones produced by the Australian red algae,
Delisea pulchra. These furanones interfere with the receptor proteins and release
the AHL signal. In certain examples, such as the increased production of antimicro-
bial peptides, the quorum-sensing stimuli could be amplified or disconnected from
cell density dependency. In this way lactic acid bacteria, for example, could be made
to produce bacterocins constitutively. However, as refered to previously, this would
not enhance produce’s shelf life.
17.4.2 MICROBIAL PHYTOHORMONES
Phytohormones (auxin, cytokinin, and gibberellins) play a major role in regulating
plant growth, development, and reproduction. Phytohormones also play a role in the
communication between microbes and the plant that can either be beneficial (sym-
biotic) or detrimental. Fungi are known to produce a wide range of phytohormones
that affect plant development. Indeed, many of the phytohormones were first dis-
covered in fungi, suggesting coevolution with plants. The production of phytohor-
mones by microbes is considered beneficial in terms of enhancing plant development
and yield. However, many bacteria associated with plants produce the plant regulator
indole 3-acetic acid (IAA). IAA promotes cell wall loosening and, along with auxin,
can facilitate nutrient leakage from plant cells. This not only facilitates survival of
the bacterium on the plant’s surface in the field but can also accelerate the degradation
process.
In addition to chemicals, genetic material can also be transferred to plants from
bacteria. For example, the Ti plasmid found in the soil-borne bacterium Agrobacte-
rium tumefaciens can be transferred to plants, resulting in tumor induction (crown-
gall). Upon infection a 30,000-kb segment separates from the plasmid and integrates
into the plant’s chromosomes. The inserted genes encode for proteins that stimulate