Use the link below to share a full-text version of this article with your friends and colleagues. Learn more. During the last years, the number of human infections caused by opportunistic pathogens has increased dramatically. One natural reservoir of opportunistic pathogens is the rhizosphere, the zone around roots that is influenced by the plant.
Mechanisms responsible for colonization of the rhizosphere and antagonistic activity against plant pathogens are similar to those responsible for colonization of human organs and tissues, and pathogenicity. Multiple resistances against antibiotics are not only found with clinical strains but also with strains isolated from the rhizosphere.
High competition, the occurrence of diverse antibiotics in the rhizosphere, and enhanced horizontal gene transfer rates in this microenvironment appear to contribute to the high levels of natural resistances.
OPPORTUNISTIC YEAST-LIKE FUNGI
While opportunistic bacteria from the rhizosphere have some properties in common, each of these emerging pathogens has its own features, which are discussed in detail for Burkholderia , Ochrobactrum and Stenotrophomonas.
Opportunistic [Lat. In the last two decades, the impact of opportunistic infections on human health has increased dramatically. Despite this fact, little is known about the ecology and pathogenesis of these emerging pathogens. While some opportunists live as human commensals or in aquatic habitats, other bacteria originate from terrestrial ecosystems like the rhizosphere, which is the zone around plant roots.
In this review we will focus on indigenous rhizobacteria that bear the potential to be risky to human health. Several members of these genera show plant growth promoting as well as excellent antagonistic properties against plant pathogens and were therefore utilized for the development of biopesticides Weller, ; Whipps, However, many strains also successfully colonize human organs and tissues and thus cause diseases.
The development of biotechnological applications in the biological control of plant pathogens and bioremediation of xenobiotics is now gaining momentum but more risk assessment needs to be carried out on these products to ensure that they do not inadvertently pose a threat to human health. This review discusses key aspects of rhizobacteria as possible facultative pathogenesis to humans: i the rhizosphere as an environment for opportunistic bacteria, ii the occurrence and diversity of opportunistic bacteria in the rhizosphere, iii characteristics of the opportunistic pathogens essential for infection and patient factors affecting predisposition, and iv general and specific characteristics on examples such as Burkholderia, Ochrobactrum and Stenotrophomonas.
The rhizosphere, a term introduced by Hiltner already in , is defined as the layer of soil influenced by root metabolism.
The rhizosphere and its inhabiting microorganisms fulfil important ecological functions, e. The rhizosphere effect describes the phenomenon that in rhizospheres, in comparison to bulk soil, the biomass and activity of microorganisms is enhanced as a result of the exudation of compounds. However, the nutrient content is not constant and the different amounts of nutrients lead to changing osmolarities in the rhizosphere Miller and Wood, In comparison to bulk soil, the high content of nutrients in the rhizosphere is an outstanding parameter of the rhizosphere.
Bacteria, which are highly competitive, for example due to the production of antibiotic substances, can colonize the rhizosphere.
The occurrence and production of diverse antibiotic substances in the rhizosphere explains the frequent detection of bacteria with multiple antibiotic resistances in this microenvironment. They found that soil samples harbour antibiotic resistance genes in a greater genetic diversity than has been accounted for by previous studies based on cultivation dependent studies.
So far, whenever antibiotics have been detected in aquatic or terrestrial habitats before, it has been in material obtained from these microhabitats, which are localized areas of intense microbial interaction. Indeed, highly complex interactions that can occur in the rhizosphere as has been reviewed by Whipps Like a protection shield, they are able to prevent infections by plant pathogens Weller, Some bacterial species with antagonistic activity towards other microorganisms including eukaryotic ones can also cause opportunistic infections in humans.
Hence, the number of opportunistic pathogens may also be increased in the rhizosphere in comparison to bulk soil. Due to horizontal gene transfer HGT , the composition of bacterial genomes can be changed rapidly. Evidence for the conjugative transfer of chromosomal genes of the biocontrol strain Pseudomonas fluorescens CHA0 and the clinical strain Pseudomonas aeruginosa PAO1 in the rhizosphere of wheat in contrast to bulk soil was presented by Troxler and colleagues Most of these genes are isoforms of genes previously found on Staphylococcus aureus plasmids.
In conclusion, due to the high microbial population density and the rapidly changing conditions, the rhizosphere forms a unique habitat in terrestrial ecosystems, which may select for opportunistic pathogens, as these organisms are fiercely competitive for nutrients and produce many antimicrobial metabolites. For grouping of bacteria into risk groups the public databases, e.
These examples demonstrate that opportunistic pathogenic bacteria belong to many different species.
Altogether, 27 different genera with 36 species were reported until now. Some species, e. Burkholderia cepacia and Stenotrophomonas maltophilia were detected in most of the investigated rhizospheres whereas others, e.
Many bacteria closely related to important nosocomial pathogens like P. Furthermore, the rhizosphere harbours bacterial species that have been reported to cause skin and wound infections Bacillus cereus, Proteus vulgaris, Pseudomonas spp.
Although these data provide evidence for the occurrence of opportunistic pathogens in the rhizosphere very little information on their virulence relative to the one of their clinical counterparts is available.
Therefore, the risk assessment for each strain is necessary and will be a very important objective for microbiologists in the future. For example, strains could be analysed for the occurrence of specific virulence determinants.
Bacteria living in the rhizosphere can have a neutral, pathogenic or beneficial interaction with their host plant. In healthy plants, the occurrence of pathogenic bacteria is low and controlled by the plant defence system and plant beneficial bacteria enriched by the rhizosphere conditions. Antagonists are naturally occurring organisms that express traits that enable them to interfere with pathogen growth, survival and infection.
Bacteria antagonistic to plant pathogens represent an important part of the rhizosphere communities. The majority of rhizobacterial species which emerged as pathogens belongs to the group of antagonistic bacteria, e.
To our knowledge, only for Ochrobactrum spp. Rhizobacteria with antagonistic activity against eukaryotes are able to interact with their hosts by various mechanisms. Other factors that contribute to rhizosphere fitness include the ability to use seed and root exudates as carbon sources or, more in general, ecological and nutritional versatility. In addition, synthesis of compatible solutes by bacteria contributes to survival under changing osmolarities, which occur in the rhizosphere Miller and Wood, Pathogenic bacteria can cause damage to the host.
Steps of pathogenesis include invasion, colonization and growth, and several strategies to establish virulence, the relative ability of a pathogen to cause disease in the host. In addition, recognition and adherence to human cells is necessary to establish pathogenicity. Berg, unpublished results. Concerning the induction of resistance in plants by rhizosphere bacteria, the stimulation of the plant's innate immunity by bacterial components like flagella or lipopolysaccharide resembles in many ways the response of the mammalian innate immune system towards pathogens.
Identification of virulence factors is particularly difficult in many opportunistic human pathogens because of the lack of adequate animal models.
Several studies provided evidence that similar or even identical functions are responsible for beneficial interactions with plants and virulence in humans. BH72 are responsible for the adhesion on plant and fungal cells.
In a study published by Alonso and colleagues it was shown that clinical and environmental isolates of P. While a remarkable conservation of genes including those encoding nearly all known virulence factors was observed, preferential sites for the integration of novel genetic material were also detected.
Data of recent analyses revealed that the P. Restriction fragment length polymorphism based on 14 single nucleotide polymophisms SNPs of conserved loci in P. Interestingly, the highly virulent clinical strain CHA shared their SNP genotype with two environmental strains, which again supports the view that P.
Furthermore, mutations in gac A and gac S are the basis for phase variation and different regulation of secondary metabolites and enzymes in Pseudomonas sp.
Types of Fungal Diseases
A further hint that similar traits are involved in the interaction between bacteria and eukaryotes was obtained from studies which analyse the rhizosphere competence of human pathogenic bacteria.
While B. Troxler and colleagues also reported that the human pathogen P. An important mechanism by which harmless bacteria can behave as pathogens is change of host or host niche, upon which their virulence potential is frequently revealed to its full extend.
This mechanism is clearly relevant for opportunistic pathogens from the rhizosphere. Genes responsible for pathogenicity or fitness of bacteria often occur as genomic islands, which are blocks of DNA with signatures of mobile genetic elements Hacker and Carniel, Patients with CF are particularly susceptible to infections, because they have sticky, dehydrated mucus that lines epithelial cells of the digestive tract and the lungs, leading to nutritional disorders and chronic lung infections caused by a succession of microorganisms.
Pulmonary infections typically begin in infancy with S.
SUMMARY AND CONCLUSIONS
These facts underline the importance of a sensitive balance between the pathogen and the host in its impact on disease severity. The genus Burkholderia comprises more than 30 species, many of which are important pathogens of plants, animals and humans Coenye and Vandamme, The highly pathogenic species Burkholderia pseudomallei and Burkholderia mallei are the causative agents of melioidosis and glanders, respectively, and are deemed potential agents of bioterrorism.
In addition to the primary human pathogens, several other Burkholderia species have emerged as important opportunistic pathogens. Cystic fibrosis patients usually acquire Bcc strains late in the course of the disease when the patients are already chronically colonized with P. Even though all Bcc strains have been isolated from CF patients, B. In contrast to P. At present, the factors that are required for the colonization of plants are unknown.
Evidence that has accumulated over the past few years showed that the same Burkholderia clonal lines that occur in the environment are also capable of infecting CF patients. For example, the B.
Likewise, B. In conclusion, while B.
List four opportunistic human pathogenic fungi pdf
Further work will be required to identify virulence factors of these bacteria to eventually link the pathogenic potential of a strain to its genetic repertoire. The genus Ochrobactrum belongs to the family Rhizobiaceae in the alpha 2 subclass proteobacteria and was first described by Holmes and colleagues Using the monoclonal antibody mAb 2. As no specific enrichment medium was available for Ochrobactrum , the monoclonal antibody mAb 2. However, it was not possible to clearly separate the isolates according to their habitat place of isolation within the Ochrobactrum spp.
Classification and Structure of Fungi (Fungal Infections - Lesson 1)
Lebuhn, unpublished results. However, it is possible to clearly separate Brucella from O. In their diverse environmental niches rhizosphere, soils, sediments and activated sludge Ochrobactrum spp. Some Ochrobactrum spp. The medical relevance of O.
Clinical problems after transplantations related to O. Stenotrophomonas maltophilia is associated with a broad spectrum of clinical syndromes, e.