Plant Pathogenic Bacteria

The interactions between fungi and plants encompass a spectrum of ecologies ranging from saprotrophy (growth on dead plant material) through pathogenesis (growth of the fungus accompanied by disease on the plant) to symbiosis (growth of the fungus with growth enhancement of the plant). We consider pathogenesis in this article and the key roles played by a range of pathogen-encoded molecules that have collectively become known as effectors.

The primary aim of culture preservation is to maintain the organism alive, uncontaminated, and without variation or mutation, that is, to preserve the culture in a condition that is as close as possible to the original isolate. The major disadvantages of the serial-transfer technique are the risks of contamination, transposition of strain numbers or designations (mislabeling), selection of variants or mutants, and possible loss of culture, as well as the required storage space. Commercially available strips impregnated with Bacillus subtilis spores are used to check the efficacy of the sterilization equipment and cycle. Many heterotrophic bacteria can be preserved in dried gelatin drops or disk. A variety of bacteria have been successfully preserved by drying on sterile silica gel granules. Freeze-drying (lyophilization) is one of the most economical and effective methods for long-term preservation of bacteria and other microorganisms. Two of the most common methods used are centrifugal freeze-drying and prefreezing. Furthermore, many culture collections serve as centers of expertise for preserving the microbial germplasm and are very useful in training others in the do's and don't's of culture preservation and maintenance.

This chapter focuses on the antibiotics and fungicides in agricultural use, problems of resistance, and effects on other organisms, particularly humans. The most common antibiotics used in plant agriculture are streptomycin and oxytetracycline, either in its hydrochloride or calcium form. Antibiotic resistance is of universal concern to human health. The antibiotics used in plant agriculture, except for kasugamycin, are also used in human clinical medicine. Azoles, used in human clinical medicine, are particularly popular in plant agriculture, because they are relatively cheap, have broad spectrum systemic action in plants for both preventive and curative effects, and are relatively stable. A potential approach to obtain new alternative antibiotics for plant agriculture might be a federal program analogous to the Orphan Drug Act. Whereas one could argue that antibiotics are of limited use in agriculture, fungicides are considered vital, necessary, and highly profitable for worldwide plant pathogen control. Data acquisition, usage, and interpretation for managing antimicrobial resistance in plant agriculture and human medicine should be coordinated. Federal and international organizations should do so as well. Such cooperation is essential to minimize and cope with microbial resistance in both plant and human pathogens.

Of the circa 12,000 known antibiotics, it has been estimated that some 160 are or have been in human clinical use. Streptomycetes, gram-positive filamentous bacteria, account for these production of about 55% of the commercially significant antibiotics. The Abs knockout leads to precocious production of all the Streptomyces coelicolor antibiotics in hours to days and in amounts up to 60-fold higher than normal, dependent on the culture conditions. The most extensive analysis of regulation of a specific antibiotic pathway, one step down from the Abs global regulators, is probably in Streptomyces virginiae in production of the two antibiotics virginiamycin M1 and virginiamycin S1. The hydrophobic side chain of the butyrolactones, known generically also as butaneolides, is in the same locus as the N-acyl moiety of the gram-negative quorum molecules. A schematic for regulation of biosynthetic gene expression for the major classes of streptomycete antibiotics (polyketides, nonribosomal peptides, and aminoglycosides) is beginning to take shape. Plant pathogenic bacteria often secrete enzymes (exoenzymes) with hydrolytic capacity to destroy the components of plant cell walls to release the nutrients that can then be utilized by the pathogens. The N-acylhomoserine lactones of Erwinia, Pseudomonas, and many other bacteria and the γ-butyrolactones of streptomycetes, serve equivalent purposes, as low-molecular-weight pheromones, for communicating population density-dependent signals between bacteria of the same species.