Microbial Communities and Interactions

Midterm 2

Ecosystem

the microbial community along with all other biotic and abiotic components of an environment

Community

all of the guilds present in a specific environment

guild

populations of different organisms that compete for the same resources

population

a collection of individual microorganisms that are genetically related and perform the same function in a proximate location

individual

one organisms/entity

Species Richness

the total number of different species present

Species Abundance

the proportion of each species in an ecosystem

Species Diversity

amount of genetic morphological and functional variation in microbial populations occupying a given environment and the distribution of that variation

Alpha

how many different species are there in each sample/community and their abundance

Beta

how much overlap is there, count all the species that do not occur in both communities

Gamma

All the unique species in the ecosystem

Soil Type Influences Microbial Diversity

texture, pH, organic matter, and nutrient availability determine bacteria, archaea and fungi thrive

Forest Soils

often support diverse fungi and bacteria that decompose matter

Agricultural Soils

Microbial communities are shaped by crop type, tillage, fertilizer use, pesticides, and crop rotations

Desert and Wetland Soils

drought-adapted microbes thrive in dry soils, while waterlogged soils favour anaerobes involved in denitrification and methane production

Biofilms

assemblages of bacterial cells adhered to a surface and enclosed in an adhesive matrix excreted by the cells, matrix is typically a mixture of polysaccharides and proteins

Biofilm Formation

1) attachment of pili

2) formation of microcolonies

3) formation of extracellular matrix

4) biofilm development and maturation

5) recruitment of additional species/organisms, dispersal

Rhizosphere

a narrow zone of soil (mm) that surrounds and is influenced by plant roots, high numbers of microbes due to release of organic and inorganic compounds from the root and is determinant of plant health and crop yield

Rhizoplane

soil that is in direct contact with root

Bacterial Signalling

Coordination of gene expression within populations and inhibition of a competitor’s gene expression/stimulation of host colonization

Intraspecies Signalling

Signaling within a single population

interspecies signaling

signaling between bacterial populations

interkingdom

exploitation of signal transduction pathways to alter gene transcription, and thus behavior of an organism from a different kingdom

Signal Transuction

external signal can be detected by a sensor and transmitted to regulatory machinery, most systems are two-component regulatory systems

Quorum Sensing

mechanism by which bacteria assess their population density, ensures sufficient number of cells are present before initiating a response that requires a certain cell density to have an effect

How Quorum Sensing Works

1) each species of bacterium produces a specific autoinducer molecule

2) diffuses freely across the cell envelope

3) reaches high concentrations inside cell only if many cells are near

4) binds to specific activator protein and triggers transcription of specific genes

5) acyl homoserine lactone was the first autoinducer to be indentified

Quorum Quenching

Breaking down signal molecules or otherwise interfering with signaling, several plants produce signal mimics that activate or inhibit bacterial quorum sensing

Quorum Sensing in Gram-Positive Bacteria

Do not use N-acyl homoserine lactones because they lack porous outer membrane, and have thick peptidoglycan layer may restrict diffusion of AHL signals, use small peptides as primary communication

positive cross talk

interspecies signaling, rhizosphere isolates that restore ability of a p. chlororaphis mutant to make phenazines via production of diffusible AHL signals

negative cross-talk

interspecies signaling, rhizosphere isolates that inhibit the ability of wild the p.chlororaphis to make phenazines by production of diffusible signals

Competitive Exclusion

competition between microorganisms for carbon, nitrogen, o2, iron and other nutrients

Antibiosis

inhibition of one organism by another as a result of diffusion of an antibiotic

Biological Control

supressess pests and plant pathogens by producing antibiotics that inhibit the growth of competing microorganisms for nutrients and space