Endophytic bacteria are ubiquitous in most plant species, residing latently or actively colonizing plant tissues locally as well as systemically. Several definitions have been proposed for endophytic bacteria; in this review endophytes will be defined as those bacteria that can be isolated from surface-disinfested plant tissue or extracted from within the plant, and that do not visibly harm the plant. While this definition does not include nonextractable endophytic bacteria, it is a practical definition based on experimental limitations and is inclusive of bacterial symbionts, as well as internal plant-colonizing nonpathogenic bacteria with no known beneficial or detrimental effects on colonized plants. Historically, endophytic bacteria have been thought to be weakly virulent plant pathogens but have recently been discovered to have several beneficial effects on host plants, such as plant growth promotion and increased resistance against plant pathogens and parasites. In general, endophytic bacteria originate from the epiphytic bacterial communities of the rhizosphere and phylloplane, as well as from endophyte-infested seeds or planting materials. Besides gaining entrance to plants through natural openings or wounds, endophytic bacteria appear to actively penetrate plant tissues using hydrolytic enzymes like cellulase and pectinase. Since these enzymes are also produced by pathogens, more knowledge on their regulation and expression is needed to distinguish endophytic bacteria from plant pathogens. In general, endophytic bacteria occur at lower population densities than pathogens, and at least some of them do not induce a hypersensitive response in the plant, indicating that they are not recognized by the plant as pathogens. Evolutionarily, endophytes appear to be intermediate between saprophytic bacteria and plant pathogens, but it can only be speculated as to whether they are saprophytes evolving toward pathogens, or are more highly evolved than plant pathogens and conserve protective shelter and nutrient supplies by not killing their host. Overall, the endophytic microfloral community is of dynamic structure and is influenced by biotic and abiotic factors, with the plant itself constituting one of the major influencing factors. Since endophytic bacteria rely on the nutritional supply offered by the plant, any parameter affecting the nutritional status of the plant could consequently affect the endophytic community. This review summarizes part of the work being done on endophytic bacteria, including their methodology, colonization, and establishment in the host plant, as well as their role in plant–microbe interactions. In addition, speculative conclusions are raised on some points to stimulate thought and research on endophytic bacteria.Key words: endophytic bacteria, methods, localization, diversity, biological control.

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The future use of genetically modified microorganisms in the environment will be dependent on the ability to asses potential or theoretical risks associated with their introduction into natural ecosystems. To assess potential risks, several ecological parameters must be examined, including the impact of the introduced genetically modified organism on the microbial communities associated with the environment into which the introduction will occur. A 2-year field study was established to examine whether the indigenous bacterial communities of the rhizosphere and endorhiza (internal root tissues) were affected differently by the introduction of an unaltered wild type and its genetically modified derivative. Treatments consisted of the wild-type strain Pseudomonas fluorescens 89B-27 and a bioluminescent derivative GEM-8 (89B-27::Tn4431). Cucumber root or seed samples were taken 0, 7, 14, 21, 35, and 70 days after planting (DAP) in 1994 and 0, 7, 14, 28, 42, and 70 DAP in 1995. Samples were processed to examine the bacterial communities of both the rhizosphere ad endorhiza. Over 7200 bacterial colonies were isolated from the rhizosphere Community structure at the genus level was assessed using genera richness and Hill's diversity numbers, N1 and N2. The aerobic-heterotrophic bacterial community structure at the genus level did not significantly vary between treatments but did differ temporally. The data indicate that the introduction of the genetically modified derivative of 89B-27 did not pose a greater environmental risk than its unaltered wild type with respect to aerobic-heterotrophic bacterial community structure.

Field releases of the wild-type plant growth-promoting rhizobacterium Pseudomonas fluorescens 89B-27, its bioluminescent derivative GEM-8 (89B-27::Tn4431), and a spontaneous rifampin-resistant variant (R34) were established on cucumber in 1994 and 1995 to examine the efficiency of these marker systems for estimating the wild-type population. Seed and root samples were taken 0, 7, 14, 21 or 28, 35 or 42, and 70 days after planting in each year and processed for enumeration by spiral plating or immunofluorescent colony staining (IFC). In both years, the populations of 89B-27, R34, and GEM-8, as measured by IFC, were not significantly different (P > 0.05) from each other at each sampling time. However, the populations of R34 and GEM-8, as measured by spiral plating and differentiation based on their respective phenotypes, were significantly lower (P < 0.05) than the wild-type populations and their IFC-determined populations. These data indicate that traditional marker systems may underestimate populations and hence the survival and colonization of genetically marked bacteria.

This review focuses on the induction of uncharacterized microbial communities through the use of compost teas. Due to the scarcity of peer-reviewed literature on this topic, this review will incorporate non-peer-reviewed literature and popular press accounts to indicate areas that show promise and are in need of scientific investigation. It is speculated that achieving consistent disease suppression with compost tea applications will require: modification of compost tea production steps (e.g. nutrient amendments) to ensure the growth of specific groups of microbes; the incorporation of spray adjuvants to enhance attachment and survival of microbes in soil or on plant surfaces, and the development of spray equipment that is more suitable to the delivery of living microorganisms. Compost teas represent only one tool in an arsenal of options that can be utilized within an integrated system that incorporates genetic disease resistance, fertility and water management, disease and pest forecasting, and other cultural approaches to enhance plant health.