Contamination of Plant Raw Materials
Contamination of plant-derived materials occurs through two primary pathways.
Primary (natural) infection involves the presence of normal microflora or phytopathogens on the plant prior to harvesting. Secondary (exogenous) contamination occurs during harvesting and processing. Transmission vectors include water, indoor production air, personnel hands, and non-sterile containers. Improper harvesting creates an ideal nutrient medium for bacterial proliferation. To prevent microbial growth, the bulk of plant raw material must be thoroughly dried.
Normal Microflora
Natural microbiota is traditionally divided into two major functional groups based on localization:
- Epiphytic microflora — inhabitants of aerial plant parts (leaves, stems, seeds). They do not harm the host, feeding instead on organic debris and plant exudates. Epiphytes perform a critical protective function: they act as antagonists to phytopathogens, preventing them from penetrating tissues and stimulating a primitive form of "immunity." The dominant species is Pantoea agglomerans (an antagonist of soft rot). Pseudomonas fluorescens is also common, whereas spore-forming rods (Bacillus mesentericus) and fungi are found in the minority.
- Rhizosphere microflora — microorganisms in the soil layer directly adjacent to the roots. These include pseudomonads, mycobacteria, actinomycetes, and fungi. They metabolize soil substrates into forms accessible to the roots and synthesize biologically active substances (vitamins, antibiotics). Representatives of the genus Pseudomonas predominantly colonize the root surface itself (rhizoplane).
Phytopathogens and Bacterial Infections
Plant diseases are caused by bacteria, viruses, and microscopic fungi. Bacterial diseases are referred to as bacterioses. They penetrate plant tissues through mechanical damage or natural openings (such as stomata) and are transmitted via infected seeds, soil, water, or insect vectors. Based on their clinical course, bacterioses are classified as general (affecting vascular systems or roots, killing the entire plant) and local (confined to parenchyma regions).
| Pathogen Genus | Type of Lesion | Representative (Disease) |
|---|---|---|
| Erwinia | Blights, soft rot, wilt | E. amylovora (fire blight of apples) |
| Pseudomonas | Bacterial leaf spot | P. syringae (leaf spots) |
| Xanthomonas | Vascular pathologies | X. campestris (black rot) |
Pathogenesis of Tissue Damage
Destruction of plant tissues occurs through several mechanisms. In the parenchymatous type, bacteria spread intercellularly, causing softening (maceration) and cellular detachment. In the vascular type, Gram-positive non-spore-forming rods (e.g., Corynebacterium) release glycopeptides that damage vascular membranes. Bacterial masses occlude vascular channels, which, combined with toxin action, leads to rapid wilting.
A unique mechanism is demonstrated by the genus Agrobacterium. The bacterium transfers its genetic material (via an oncogenic plasmid) into the plant cell, triggering uncontrolled cell proliferation and tumor formation (such as crown gall). Notably, once the tumor is established, the bacteria themselves are typically no longer detectable within the affected tissues.
Medical Significance and Human Risks
Plants grown in cultivated soils are always more heavily contaminated than wild-grown plants (forest or meadow flora). The maximum microbial load accumulates in the root zone. High-risk zones include wastewater irrigation fields, pastures, and manure storage sites, where crops can be contaminated with pathogens hazardous to humans.
Phytopathogenic fungi present a major threat. Consumption of grain infected with these fungi causes severe food poisonings known as mycotoxicoses.
- Cereals are frequently infected by ergot (Claviceps purpurea), which forms characteristic sclerotia ("horns") and causes ergotism.
- Fungi of the genera Fusarium, Penicillium, and Aspergillus actively multiply on harvested plants under high humidity and low temperatures, producing toxins that are lethal to humans.