Bacterial structures

IMED2000

• free-living

• unicellular

• have distinctive structures to survive

• A small proportion of them cause diseases

• used to understand cause of disease and provide targets for chemotherapeutic agents

bacteria

• can be coccus (spherical) or bacillus (rod-shaped)

• spiral-shaped - spirillum and spirochetes

• coccobacilli - strecthed out cocci

• filamentous - bacilli in long threads

• vibrios - short and slightly curved rods

• fusiform bacteria with pointy ends

bacteria cell shape

• sometimes appear in groups when viewed under the microscope

• due to the plane of cell division

• can provide a clue to identifying an unknown sample

bacteria cell arrangements

• cytoplasmic components - cytoplasm, nucleoid, ribosomes

• cell envelope - cytoplasmic membrane, cell wall, glycocalyx (capsule)

• appendages - flagella, pili

major sub-cellular structures of bacteria (prokaryotes)

• no nucleus and no membrane bound organelles

• cytoplasm - gel matrix of water, proteins, vitamins, ions and nucleic acids, growth, metabolism and replication takes place

• nucleoid - irregular shape nucleoid which contains most of the genetic material - single circular chromosome

• cytoplasmic inclusions - structure for storing nutrients

prokaryotic

• machinery for protein translation

• ribosomal RNA and protein superstructures

• subunit structure is 30S +50S = 70S

• in cytoplasm or associated with mRNA

• good antimicrobial drug target

• very abundant

prokaryotic ribosomes

• when they bind they interlock on top of one another and decrease the surface area thus 70S

why 30S + 50S = 70S

• asymmetric - different structures on each side

• dynamic and adaptive

• in both prokaryote and eukaryote cells

• selectively permeable

plasma membrane

• determines bacterial shape

• allows for survival in many different environments,

• protection from osmotic pressure

• hypotonic solutions, chemicals and acid/alkali

• bacteria can be killed in hyperosmotic solutions

• complex semi-elastic lattice structure - porous

• rigid and elastic components

• unique composition to other bacterial species

• structural compound consist of peptidoglycan

prokaryotic cell wall

• supportive 3D scaffold like structure

• large polymer of interlocking chains of alternating monomers

• backbone of amino sugars, derived from glucose

• glycan polymers are connected by peptide bridge

• charged amino acids - hydrophilic

• sensitive to lysosome , ampicillin and derivates

• b-lactam antibiotics inhibit peptide cross-linking

peptidoglycan cell wall structure and composition

• gram-positive rods - thick cell wall layer

• gram-negative rods - addition outer membrane

classification of bacteria based on cell wall structure

• thick layer

• hydrophilic - helps to resist bile acids in gut

• teichoic acids - polysaccharides - polymers of glycerol or ribitol, covalently attached to NAM, only in gram positive

• lipoteichoic acids - anchor cell wall to membrane

• surface proteins - covalently attached to cell wall, vital in bacterial disease

gram-positive cell wall structure

• permeability barrier - protection

• hydrophilic and phobic components

• porins - allow diffusion of small hydrophilic molecules, nutrients, and ions across these membranes porins pump into periplasmic space

• peptidoglycan layer - thin, lipoproteins attach the other membrane to the peptidoglycan

• penicillins and cephalosporins intefere with peptidoglycan synthesis and cant get through outer membrane

gram-negative cell wall structure

• Lipopolysaccharide - outer layer of membrane

• lipid portion sits in outer membrane with sugar chain outside of cell

• endotoxin lipid A - effects immune system and organs

• antigenic - carbohydrate chain 0 antigens

• recognised by immune system and bacterial viruses

Lipopolysaccharide for gram cell wall

• a carbohydrate-rich, gel-like network covering the outer surface of bacterial and eukaryotic cells

• forms an extra layer outside the cell wall produced by some bacteria

• help bacteria to adhere to surfaces and evade immune system - virulence factor

• used as a vaccine target

• contains a capsule - secreted polysaccharides firmly attached to the cell wall and can protect bacteria from immune cells

• slime layer - polysaccharides or sometimes glycoproteins associated with the cell wall

• protects against desiccation and important in biofilms formation

glycocalyx

• a bacterial community living within an organic polymer matrix adhering to a surface

• helps to adhere bacteria to surface

• creates a benefical and protective

• inherently resistant to host defences since it blocks access by immune cells and antibodies

• acts as a filter to reduce diffusion of antibodies and antimicrobial agents

what is a bacterial biofilms and how does it help bacteria

• motility

• thousands of protein subunits

• propulsion towards environmental stimuli, receptors, flagella, chemotaxis

• almost all spirilla, half of all rod-shaped bacteria and only a few of the cocci are motile by flagella

• arrangement is dependent on species

• highly immunogenic—meaning our immune system easily "sees" them and creates antibodies to attack them.

• they are a virulence factor

flagella

• hair-like projections

• shorter and thinner than flagella

• hollow

• allow bacteria to attach to host surfaces is required for colonisation during infection or to initiate formation of a biofilm

• virulence factors- helps to cause disease

• cause phagocytosis resistance

• e.g. sex pilus - bacterial gene transfer

• antigenic targets for immune systems

pili

• dormant, highly resistant structure called spore - process of sporulation

• resistant to harsh environmental conditions,

• can survive in the lack of nutrients, radiation, temp, lysosyme

• e.g. endospores - commonly found in soil and water, long survival time

• important in sterilisation monitoring due to resistance

• serious infection and food poisoning can be caused by spores

• shape and position depend on species

spores

• increase in number of bacteria rather than size

• growth through binary fission, (1 cells divides into 2 )

• once bacteria divides into millions of copies, they aggregate to form a colony

• the time required for a population of microorganisms (such as bacteria) to double in number during the exponential growth phase

• DNA replication, formation of a septum, cell division

bacterial growth

• rapid increase in cell number, doubles in generation time

binary fission leads to exponential growth

• rate of bacterial growth depends partly on the nutritional status of environment

• When bacterial is introduced into a new environment is follows the pattern

• 1. lag phase - adapting to environment

• 2. log phase - maximum growth, no limiting conditons

• 3. stationary phase - nutrient depletion metabolites build up, growth balances cell death

• 4. death phase - cell death greater than growth

bacterial population growth curve