BK - Biofilms on the phylloplane Flashcards
What are the key terms for plant-associated microbial habitats? (3)
Rhizosphere: The below‐ground habitat colonized by microbes.
Phyllosphere: The aerial plant habitat colonized by microbes.
Phylloplane: The leaf surface where microbes reside; these inhabitants are called epiphytes.
What are key plant–microbe interactions occurring in the rhizosphere? (3)
- Formation of nodules and nitrogen fixation regulated by flavonoids and nod genes.
- Agrobacterium utilizes Ca²⁺-dependent adhesins, att genes, EPS, and cellulose fibres for attachment.
- E. coli and Salmonella attach to roots via curli structures and, in the case of Salmonella, may internalize through lesions.
How do plant–microbe interactions in the phyllosphere differ from those in the rhizosphere? (2)
- The phyllosphere mainly involves plant pathogens, with a lesser focus on zoonotic pathogens.
- Research in this area often targets the economic impact of plant pathogens and spoilage on crops.
What observations have been made when E. coli and Salmonella enterica Newport interact with plants? (2)
- E. coli shows poor attachment to roots.
- Salmonella Newport exhibits abundant attachment, with evidence of biofilm colony formation on root hairs and seed coats.
What challenges and research focuses characterize the Phylloplane as a Hostile Ecosystem? (3)
- Faces harsh, fluctuating environmental stresses (e.g. UV radiation, temperature changes, desiccation).
- Despite the stress, supports diverse microbial communities including plant pathogens and zoonotic pathogens.
- Current research targets the economic impacts, limited biofilm literature, and detection of VBNC zoonotic pathogens.
What does Scanning Electron Microscopy (SEM) analysis reveal about bacterial colonization on leaves? (2)
- Bacteria form microcolonies and produce extracellular polymeric substances (EPS) on the leaf surface.
- They tend to cluster near features such as stomata, suggesting possible interactions with these structures.
Which bacteria are the principal colonisers on the leaves of perennial rye (Lolium perenne)? (3)
- Pseudomonas fluorescens (~20.12%).
- Xanthomonas campestris (~19.64%).
Oddity:
- Listeria spp. (~4.02%).
What are the principal bacterial colonisers on olive (Olea europea) leaves? (2)
- Pseudomonas syringae (51.0%).
- Xanthomonas campestris (6.7%).
What human gut pathogens have been associated with fresh produce items? (5)
E. coli O157:H7, Salmonella, Campylobacter spp, Listeria monocytogenes and Shigella have been associated with fresh produce e.g. lettuce
How are pathogens typically transferred onto the leaf surface of crops like lettuce? (3)
- Through contaminated manure from animal farming (manure reapplied to fields).
- Via nematodes that climb from soil to leaves.
- From feral animals (e.g. pigs) and overflying birds that deposit pathogens.
What methods are used to detect bacterial contamination on leaves and what challenges do they face? (2)
- Ex situ detection: Involves sample preparation and plating on selective culture media.
- Challenges: Non-culturable (VBNC) species and stressed cells may not grow; efficient recovery (e.g. using a Pulsifier) is critical.
Which in situ detection techniques are used for studying phylloplane microbial communities? (2)
- EDIC/EF microscopy: Uses non-contact lenses for real-time scanning without stains, providing views of epidermal cells, stomata, and vein cells.
What structural features of the phylloplane can be observed using EDIC/EF microscopy? (3)
Epidermal cells: Form a continuous layer covering the entire leaf surface
Stoma: Serve as air pores for gas diffusion
Leaf vein cells: Transport water and nutrients
How do unwashed leaves differ from chlorine-washed leaves in phylloplane studies?
Unwashed leaves: Contain bacteria and biofilms on the surface
Chlorine-washed leaves: Stomata close after washing, yet biofilms persist
What are GFP bacteria and why are they useful in phylloplane research? (2)
- GFP bacteria: Engineered to produce Green Fluorescent Protein, causing each cell to fluoresce bright green
- Usefulness: They can be tracked and counted using fluorescence microscopy to monitor colonization patterns
What have GFP tracking studies revealed about Salmonella colonization on leaves? (3)
- Immediately after inoculation: Bacteria are widely distributed on the leaf surface.
- Within 2 hours: They predominantly locate at cell margins and within stomatal apertures.
- Over time: Microcolonies form along leaf veins, with increasing density and co-aggregation with other bacteria (e.g. Pantoea agglomerans).
How do fimbriae and flagella contribute to Salmonella attachment on surfaces? (4)
- Fimbriae (curli): Crucial virulence factors that mediate adhesion to host cells, abiotic surfaces, and plant tissues.
- Mutant strains lacking curli (e.g. ΔcrlΔcsgB, ΔrpoS) and partly lacking curli (Δcrl, ΔcsgB) show reduced biofilm formation on abiotic surfaces.
- Although differences on the phylloplane can be less marked.
- flhC mutant strains (lacking flagella) saw a significant reduction of biofilm formation at 37º on spinach phylloplane
Pili also potentially contribute to attachment mechanisms to the phylloplane
Pili are typically longer and fewer in number than fimbriae
What conclusions are drawn from studies on Salmonella attachment mutants? (3)
- Genes such as agfB, agfD, and rpoS are essential for both animal virulence and plant colonization.
- Curli fimbriae, particularly through the action of agfB, are critical for attachment.
- Plants can serve as vectors for the transmission of Salmonella enterica.
agfA, agfB and agfD are direct homologs of csgA, csgB and csgD
What roles do CsgD and its regulatory network play in Salmonella biofilm formation? (4)
- CsgD: A LuxR quorum-sensing transcription regulator that upregulates CsgB (necessary for curli formation), bapA, and AdrA.
- AdrA: Acts as a diguanylate cyclase that promotes cellulose production via the bcs operon.
- BapA: Biofilm associated protein 386 kD folds with Ca2+
- Crl and RpoS: Work together to activate genes involved in biofilm formation, including those regulated by CsgD (Crl activates rpos).
