microorganism diversity & biology

microbiome

community of bacteria found on and in different parts of the body

what functions do bacteria regulate

• development of the intestinal immune system

• homeostasis

• prevents colonisation/infection by bacterial pathogens

• effects on extra-intestinal autoimmune diseases

nucleoid

• double-stranded DNA

• circular chromosomes

• spatial organisation - supercoiled to keep compact

• full of ribosomes

ribosome

2 subunits:

30S + 50S = 70S ribosome

• fast production of protein

• several RNA messengers produced at one time

major targets for antibacterial drugs

• targets different steps of protein production using different compounds

• different antibiotics can block or prevent certain paths

cell membrane

• lipid bilayer structure - similar to the eukaryotic membranes

• difference from our membranes is that they don’t have cholesterol

• osmotic barrier

• transport of solutes

• respiratory electron transport

cell wall

• rigid layer around the cytoplasmic membrane

• peptidoglycan (mucopeptide or murein): polysaccharide chains with peptide cross-links - contains other macromolecules

• resists osmotic pressure and determines cell shape

• signal to the innate immune system of bacterial presence

cell wall - gram stain

• bacteria are classified according to their cell wall

• positive or negative depending on colour

• violet - positive

• pink - negative

• different characteristics justified by difference of cell wall composition

• peptidoglycan in both

• outer membrane modification of cell membrane absent in positive and present in negative

cell wall - acid-fast cell develop

• large amounts of “wax” - mycolic acids

• mycolic acids - long, branched and complex fatty acids

• hydrophobic layer - impenetrable to many harsh chemicals, disinfectants, strong acids

• reduced rate of nutrient uptake

• neutral

• resistant in environment

• difficult to treat

cell wall - peptidoglycan

• complex polymer of sugars with side chains of amino acids

• mechanical strength

• counteract osmotic pressure of the cytoplasm

• role in bacterial replication

• target for some antibiotics

• glucose interlinked with chain of polypeptides

• glucose is the same in different species, how the peptides link to glucose is different

• positive - thick (20-25 layers), negative - thin (1-3 layers)

glucose derivatives:

• N-acetylglucosamine (NAG)

• N-acetlymuramic acid (NAM)

cross-linked to one another by a tetrapeptide

cell wall - peptidoglycan inflammatory response

• trigger inflammatory responses in immune cells - recognition via NOD1 and NOD2 receptors

• lysozyme - antimicrobial protein, critical in host defence, break bacteria and release DNA

• play a key role in determining the overall inflammatory response during infection

• the immune system detects peptidoglycan fragments in the absence of obvious infection - necessary for proper immune cell development and homeostasis

• if bacteria doesn’t have peptidoglycan it is susceptible to osmotic pressure and cytosol is concentrated with nutrients so results in death

• many bacteria under stress modify their cell wall peptidoglycan (sugar backbone and amino acid side chains) so immune system can’t recognise it, to enhance pathogenicity

cell wall - gram positive bacteria

teichoic acids (TA):

• chains of glycerol phosphate/ribitol phosphate

• bound covalently to the peptidoglycan

• provide rigidity

• role in cell morphology and division

• major surface antigen

• recruit outside peptidoglycan

• promote interaction with host cells and biofilm formation

• promote adherence to host cells

• proinflammatory responses through Toll-like receptor 2

• help evade microbial peptides

• modulate immune response

cell wall - gram negative bacteria

• more complex than gram-positive cell wall

• thinner peptidoglycan than gram-positive

• absence of teichoic and lipoteichoic acids

outer membrane (OM):

• inner leaflet: phospholipids

• outer leaflet: lipopolysaccharide (LPS)

periplasmic space:

• transport systems for iron, proteins, sugars and other metabolites

• hydrolytic enzymes - breakdown of large macromolecules (proteases, phosphatases, lipases, nucleases, carbohydrate-degrading enzymes etc)

• virulence factors, such as collagenases, hyaluronidases, proteases, and beta-lactamase

lipopolysaccharide (LPS, endotoxin)

• outer part of gram negative membrane

• endotoxin released when infection cleared abruptly

lipid A - 4 to 6 3-hydroxy-fatty acids attached to dimer of glucosamine phosphate

core polysaccharide - fairly conserved linker: 8, 7 and 6-carbon sugars

O-polysaccharide - 4 to 5 sugars repeated 10 to 20 times; highly strain-specific

LPS (especially lipid A) causes fever - somatic antigen endotoxin

• essential for bacterial viability

• hydrophilic O-polysaccharide repels hydrophobic molecules which could otherwise penetrate the cell membrane (including many antibiotics)

• small hydrophilic molecules can enter freely through porins

• LPS structure is used to classify bacteria

binary fission

• bacterial growth

• increase in the number of cells, not the size of a single bacterium

• when the bacteria is split in two, two to four

• exponential growth

binary fission - how is it done

1. elongation of cell wall, cell membrane and overall volume start chromosome duplication

2. septum wall grows inward chromosomes are pulled toward opposite ends

3. septum is synthesised and cell membrane start to separate cell chambers

Fts proteins

Fts [filamentous temperature sensitive]

• interacts to form the divisome

• recognise the middle of the cells and accumulate there and form an FtsZ ring, where the constriction starts to separate the two cells

• FtsZ is from tubulin-like GTPase

MinCDE proteins

• cytoskeletal-like coiled in the poles

• bi-polar gradient that help to localise the ring

• these cells move around the inner part of the cell membrane to prevent the FtsZ form the ring before the DNA is completely divided and processed into different copies

• involves tight regulation

bacterial cell cycle