Microbial Metabolites in Fruits and Vegetables 519
et al., 2002). Indeed, the rhizosphere of plants is typically dominated by antibiotic-
producing strains that can become dominant in such a competitive environment.
Pyoluteorin, pyrrolnitrin, phenazines, and 2,4-diacetylphloroglucinol are the most
significant classes of antibiotics produced by rhizosphere pseudomonads (Shanaham
et al., 1992). Antibiotics produced by plant-associated microbes have a wide spec-
trum of antagonism toward viruses, bacteria, and fungi (Table 17.2). Among the
soil-borne fungi, Gaeumannomyces graminis var. tritici, Pythium ultimum, and
Thielaviopsis basicola can be controlled by Pseudomonas strains producing
2,4-diacetylphloroglucinol. Some strains produce a single type of antibiotic whereas
others such as P. fluorescens CHA0 produce multiple types, thereby extending the
antagonistic spectrum of the bacterium.
Antibiotic production by Pseudomonas is complex and dependent on a wide
range of factors. For example, plant stress, nutrient availability, temperature, and
signal molecules released in plant root exudates all affect antibiotic production. A
most striking example of the latter is antibiotic production in Baciilus cereus, which
increases over 300% in the presence of alfalfa extracts (Milner et al., 1996). In general,
the production of antibiotics is favored in low-stress, nutrient-rich environments.
The application of antibiotic-producing strains in the field is a contentious issue
since it is well established that overexploitation in the medical field has led to multi-
drug-resistant strains of human pathogenic bacteria. To deliberately add antibiotic-
producing strains to fresh-cut produce or fruit would probably work in the short
term to extend shelf life but eventually resistant strains would likely evolve. Attempts
to introduce antibiotic-producing strains into the rhizosphere have to date met with
limited success. Many strains show variable levels of antibiotic production in the
field environment. To be successful as a biocontrol agent the introduced bacterium
must become established and express its antibiotic phenotype. Typically an intro-
duced bacterium competes relatively poorly in the presence of an established com-
petitive microflora and cannot colonize the roots in sufficient numbers to exert its
biocontrol effect.
TABLE 17.2
Antibiotics Produced by Plant-Associated Bacteria
Antibiotic Species/Strain Origins Ref.
DAPG Psuedomonas spp.
Q2-87
CHAO
F113
Wheat
Tobacco
Sugar beetVincent et al., 1991
Keel et al., 1992
Shanaham et al., 1992
DDR Pseudomonas borealis Wheat Hokeberg et al., 1998
Phenazines Pseudomonas spp.
PNA1
30-84
Chickpea
WheatBruhlmann et al., 2000
Pierson and Thomashow, 1992
Kanosamine Bacillus cereus Alfalfa Milner et al., 1996
AFC-BC11 Burkholderia cepacia BC11 Cotton Kang et al., 1998
Oomycin A Pseudomonas fluorescens Hv37a Barley Gutterson et al., 1986