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microenvironments
- resources and physiochemical conditions determine growth of microorganisms in an environment
- growth rate is dependent on the microenvironment
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niche
for a particular organism is defined by the type and quantity of resources and its physiochemical coniditions
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prime niche
habitat in which the organism is most successful
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biofilms
microcolonies of bacterial cells attached to a surface and encased in adhesive polysaccharides excreted by the cells
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biofilm structure
- depends on flow rate
- low shear - mushrooms
- higher shear - streamers and ripples
- contains channels
- heterogeneous - growth rate, sensitivity to antibiotic, metabolic activity
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*biofilm formation and development
- attachment - adhesion of a few cells to a solid suitable surface
- colonization - intercellular communication, growth and polysaccharide formation
- development - more growth and polysaccharide
- dispersal
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in vitro assay for biofilm formation
- microtiter dishes
- inoculate wells with bacteria with growth medium
- wells are stained with crystal violet to stain just the bacteria
- rinse and see biofilm ring
- dye can be solubilized in ethanol
- can be used to isolate mutants defective in biofilm formation
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Type IV pili and biofilm formation
- important for formation
- *pilA mutant - defective in attachment and biofilm formation
- pili mediate twitching behavior - facilitate attachment
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*autoinducers
- important for biofilm formation
- small molecules that allow bacteria to communicate with each other (act as population counters or quorum sensors)
- once enough bacteria, autoinducers then control expression of genes involved in biofilm formation
- *intercellular
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*cyclic dimeric guanosine monophosphate (c-di-GMP)
- *intracellular second messenger
- controls transition to biofilm formation
- depending on species:
- 1. attachment via adhesin
- 2. decreases flagellar motility and increases extra polysaccharide production
autoinducer (intercellular communication) -> c-di-GMP (intracellular second messenger) -> decreased flagellar motility, increased extra polysaccharide production -> biofilm formation
GTP (diguanylate cyclases) -> c-di-GMP (phosphodiesterases) -> pGpG ->GMP
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dental plaque
- mixed biofilm
- 1. saliva provides glycoprotein film for attachment of bacteria
- 2. colonizationm by streptococcus species
- 3. then filamentous fusobacterium grow
- 4. then spirochetes, gram-positive rods, and gram-negative cocci
- 5. if very heavy plaque, obligate anaerobes may predominate
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Why do bacteria form biofilms?
- defense against: being swept away, phagocytosis by cells of immune system, toxic molecules such as antibiotics
- biofilms allow cells to remain in a favorable niche
- biofilms allow bacterial cells to live in close association (single species of mixed communities)
- may be the typical manner in which bacteria grow in nature where nutrients are typically limiting and there are diverse environmental exposures
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