1. Introduction

The number of human infections caused by opportunistic pathogens has increased dramatically in recent years. Each year, roughly 1 in 6 people gets sick from eating contaminated food. Growing awareness that contaminated plants, fresh fruits and vegetables are responsible for a significant proportion of food poisoning with pathogenic microorganisms endorses the demand for understanding the interactions between plants and human pathogensand for determining the specific interventions needed to reduce illness and to measure progress toward public health goals resulting from food safety policies and interventions.

One natural reservoir of opportunistic pathogens is the rhizosphere.While many members of the rhizosphere microbiome are beneficial to plant growth, plant pathogenic microorganisms also colonize the rhizosphere, striving to break through the protective microbial shield and to overcome the innate plant defense mechanisms in order to cause disease. A third group of microorganisms that can be found in the rhizosphere are the true and opportunistic human pathogenic bacteria.

Human pathogens are studied almost exclusively for their detrimental impact on human health. Pathogens possess a diverse set of genetic factors that enable pathogenicity, ranging from specialized secretion systems to toxins and adhesins, all of which are involved in manipulating or circumventing the human immune system. We often consider these human pathogenic bacteria to be dedicated animal pathogens that cause disease and epidemics; yet, the constant interaction of human carriers with their environment predisposes these pathogens to alternative niches that include non-animal hosts. Not surprisingly, evidence is being accumulated that some animal pathogens which are known to cause serious human diseases can also act as plant pathogens. The epidemiology and disease strategies of these pathogens, whether considered primarily a plant or a human pathogen, are of particular interest from the perspectives of both the biology and evolution of cross-kingdom pathogenesis.

The disease incidence related to Escherichia coli, Salmonella enterica, and Listeria monocytogenesinfections caused by consumption of (fresh) vegetables, sprouts, and occasionally fruits made clear that these pathogens are not only transmitted to humans via the “classical” routes of meat, eggs, and dairy products, but can also be transmitted to humans via plants or products derived from plants. It is now of major concern that these human pathogensbecomeadapted to environmental habitats without losing their virulence to humans.Pathogen contamination of fresh produce may originate before or after harvest and once contaminated, the produce is difficult to sanitize. This problem is compounded if the bacteria invade the vascular system of the plants, as these bacteria are then protected from any conventional surface sanitation treatments applied. Such treatments reduce the overall microbial load and reduce the levels of surface contamination, but do not penetrate into the plant tissue.Adaptation to the plant environment would lead to longer persistence in plants, increasing their chances of transmission to humans via consumption of plant-derived food.

Numerous pathogenic bacteria appear to have a fairlybroad spectrum of host organisms.These cross-infective microorganisms are more insidious than those simply transmitted to humans by contact or consumption of infected plants. The cross-kingdom human pathogens, and their potential plant reservoirs, have important implications for the emergence of infectious diseases.

Management of cross-infective diseases of both plants and animals will require more interdisciplinary research and cooperative agency interactions.Estimates of foodborne illness source attribution are used for many purposes, including informing strategic planning, informing risk-based decision-making, estimating benefits of interventions, and evaluating the impact of interventions. The strategies for reducing the contamination from farm to table are high on the agenda of food safety agencies.

This lecture describes the fitness of human enteric pathogens on plants which have implications for food safety. The growth and survival ofenteric pathogens on plants are discussed in the light of concepts in plant microbial ecology. Informationregarding the various factors that affect the survival ofenteric pathogens provides a good foundation forassessing their role in the infectious dose of the pathogens and constitutesa basis for development of a Food Plant Pathogen Awareness Program.

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