VIEW ARTICLE

Postharvest Pathology and Mycotoxins

Identification of Bacteria Associated with Postharvest Diseases of Fruits and Vegetables by Cellular Fatty Acid Composition: An Expert System for Personal Computers. J. M. Wells,U. S. Department of Agriculture, ARS, Eastern Regional Research Center, 600 E. Mermaid Lane, Philadelphia, PA 19118; J. E. Butterfield, and L. G. Revear. U. S. Department of Agriculture, ARS, Eastern Regional Research Center, 600 E. Mermaid Lane, Philadelphia, PA 19118. Phytopathology 83:445-455. Accepted for publication 15 December 1992. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1993. DOI: 10.1094/Phyto-83-445.

The cellular fatty acid composition of 190 bacterial strains representing six genera associated with postharvest diseases of fruits and vegetables was statistically analyzed and was used as the basis of an expert system of identification. The expert system was built with off-the-shelf hardware and software, i.e., a commercially available, database management program and personal computer. The database included fatty acid profiles of Bacillus, Clostridium, Cytophaga, Xanthomonas, and the species: Erwinia amylovora, E. ananas, E. herbicola, E. carotovora subsp. carotovora and E. carotovora subsp. atroseptica, E. chrysanthemi, E. rhapontici, Pseudomonas cepacia, P. gladioli, P. aeruginosa, P. cichorii, P. fluorescens, P. putida, P. syringae, and P. viridiflava. A total of 78 fatty acids were detected by gas-liquid chromatography, and mean percentages (of the total) were analyzed statistically for each fatty acid and chemical class. Genera were differentiated by a class analysis. Clostridium had the highest mean percentage (63.90%) of saturated, straight-chain, even-carbon fatty acids (class A), significantly different from all genera except Erwinia (40.89%). Cytophaga was unique, with a high mean percentage (9.44%) of saturated, straight-chain, odd-carbon fatty acids (class B). Mean percentages for class C, saturated, straight-chain fatty acids, were significantly higher for Erwinia (42.11%) and Pseudomonas (fluorescent, 53.10%, and nonfluorescent, 35.33%) than for Bacillus (8.91%), Clostridium (16.31%), and Cytophaga (19.98%). In class D, hydroxy-substituted acids, mean percentages for Bacillus (1.06%) and Clostridium (2.28%) were significantly lower than for Cytophaga (11.00%) and the nonfluorescent pseudomonads (13.97%). In class E, saturated, branched-chain fatty acids, mean percentages for Erwinia and Pseudomonas were less than 1.5% compared to over 11% in other genera. In class F, unsaturated, branched-chain fatty acids, mean percentages for the pseudomonads were less than 0.2%, significantly lower than in any other genus. The ratio of class C to class D was useful in differentiating over 90% of the fluorescent pseudomonads (<3.5) from the nonfluorescent pseudomonads (>3.5). Of 61 fatty acids identified in Erwinia and Pseudomonas, the mean percentages of nine differed significantly in Erwinia species, and 23 differed significantly in the pseudomonads. “Rules” based on a profile of percentage ranges for each fatty acid and each class total differentiated each genus and species in the computer expert system. Fatty acid data from analyzed samples were compared with profile rules by a series of “if/then” (true/false) statements. The expert system correctly identified all strains in the database, with the exception of one strain of P. viridiflava that also matched the profile of P. syringae and one strain of E. rhapontici that also matched the profile of E. herbicola. The system also calculated a covariance factor for each strain, measuring its similarity to profiles of any selected group.