Bacteria-General Flashcards
What are the different structures of bacteria?
Cocci- spheres
Bacilli- rods
Spirochetes- corkscrews
Vibrio- comma-shaped
Why are our microbiota helpful?
Help digest food, extract nutrients in gut, metabolize plant polysaccharides, synthesize and secrete vitamins K and B family.
Important in immune system and out-compete growth of pathogenic microbes.
Genome size of obligate intracellular pathogens?
Tend to have smaller genomes since they borrow some functions from host.
Cytoplasmic membrane
Phospholipid bilayer, does not contain sterols, site of energy production, anchoring site for proteins, selective transport.
Cell wall
Rigid peptidoglycan barrier linked by peptide bridges. Provides mechanical strength to prevent cell from lysing. Bacterial cell wall grows and remodels with growing cell.
Gram-positive bacteria
Stain blue with gram stain. Have thick cell walls.
Gram-negative bacteria
Stain pink with Gram Stain. Very thin cell wall surrounded by an outer membrane. Periplasm in between cytoplasmic membrane and outer membrane.
How does the Gram Stain work?
- Heat fixation of bacteria to slide
- Application of Crystal Violet
- Application of Iodine
- Wash with Alcohol (Decolorization)
- Counterstain with Safranin
Where are you more likely to find gram positive vs. gram negative bacteria?
Gram+ bacteria- dry environments, no outer membrane, not sensitive to dessication. Most bacteria found on skin is gram+.
Gram- bacteria- wet environments
How might the host respond to Gram- vs. Gram+ bacteria?
Gram+ bacteria- peptidoglycan and LTA are triggers of innate immune response
Gram- bacteria- LPS is a potent trigger of host response
Why do penicillin-type antibiotics work best on Gram+ bacteria?
Peptidoglycan chains are linked together by peptide bridges to form a mesh wall that surrounds the cell. Penicillins block trans-peptidation step that links sugar strands together. Mainly affect growing bacteria that are actively dividing.
What do bacteria use their surface for?
- Motility
- Adherence
- Sensing the environment
- Acquiring nutrients
- Avoiding immune defenses
How are bacteria motile?
Flagellum- long, thin appendage rotated clockwise or counterclockwise by a motor. Can be polar or all over.
Chemotaxis- can move toward an attractant or away from a repellant. Sample the environment over time.
Some pathogenic bacteria need to swim in order to reach mucosal sites (small intestine, bladder).
How do flagella trigger innate immunity of the host?
Flagellin (major protein in flagella) can trigger Toll-like receptor 5. So some bacteria down regulate expression of flagellin once inside human host. BUT some bacteria need motility to survive, so evolved strategy to avoid detection. Flagellin of these bacteria contain amino substitutions in region of flagellin recognized by TLR5. Helicobacter remains motile while avoiding TLR5 detection.
How do bacteria attach to the host?
Pili or fimbriae- shorter surface appendages important for attachment of bacteria to host cells
Pili- made up of repeating subunits of one protein
Bacterial adhesions- individual surface proteins that bind to specific receptors on target host cell
Interaction btw bacterial adhesion molecule and host cell receptor determine where bacteria will colonize
How do bacteria sense the enivronment?
Bacteria able to respond quickly to changing environments: Histidine kinase protein (HK)- spans the bacterial membrane, auto-phosphorylates when it binds stimulus on outside of membrane. Response regulator protein (RR)- located in bacterial cytoplasm, a transcription factor active only when phosphorylated Phosphorylated HK (HK-P) phosphorylates RR inside cell. RESULT: change in environmental condition outside of bacterial cell results in change in gene expression of bacterium.
How do bacteria avoid the host immune response?
Some bacteria decorate their surface with host-like molecules or host-derived molecules as a disguise. Other bacteria produce thick polysaccharide layer (capsule) that make it difficult for phagocytes to recognize and destroy encapsulated bacteria. Capsule is an important virulence factor that can be produced by both Gram+ and Gram- bacteria.
Describe a Type III Secretion System.
Type Three Secretion System (TTSS): found only in Gram- bacteria. Secrete proteins (effector molecules) from inside bacterial cell and transport them directly into cytosol of mammalian host cell, crossing 3 biological membranes (bacterial inner membrane, bacterial outer membrane, mammalian plasma membrane). Very similar in structure to bacterial flagellum. Effector molecules often mimic host protein function and capable of manipulating host cell’s behavior. By directly introducing bacterial effector molecules into mammalian cell, no chance of antibodies to neutralize bacterial “toxins”. Ex. manipulation of host actin cytoskeleton to promote or prevent uptake of bacterial cell.
What is an example of a bacterial multi-cellular structure?
Quorum sensing- bacteria living cooperatively, communicating via chemical signals
Biofilm- multi-cellular structure that works together
Fruiting bodies- at tips of fruiting bodies, specialized bacterial cells terminally differentiate to form endospores (stable, highly resistant, dormant cells). Think Anthrax.
Describe how a biofilm functions?
Cells attach to solid surface and start to secrete a sticky goo/slime called extracellular polymeric substance (EPS). EPS is a mixture of polysaccharides and proteins. Bacterial cells communicate with one another, differentiate, and produce more slime. Biofilm grows by cell division (slow cell growth) and bacterial cell recruitment. Biofilms can contain one or many species of bacteria and are very prevalent in nature. Biofilms resistant to antibiotics and host anti-microbial peptides. Especially hard to treat and like to grow on medical devices.
How do bacterial pathogens evolve and acquire virulence factors?
Mutations- less likely, since most mutations detrimental
Horizontal/lateral transfer-more likely, passing on a functional gene, allows whole sets of genes with related functions to be transferred together. Many virulence genes and antibiotic resistance genes carried on mobile genetic elements.
What is a plasmid?
A small, circular, mobile DNA element in the cytoplasm of the bacterium that can replicate independently of the bacterial chromosome. Antibiotic resistance and virulence factor genes often carried on plasmids.
How are plasmids transferred from one bacterium to another?
Bacterial conjugation- a plasmid-encoded mechanism, plasmid encodes a “sex pilus”, which the donor cell produces, mediating contact between donor and recipient. Plasmid DNA transferred into donor cell as it is being replicated. After conjugation, both donor and recipient cell contain copy of plasmid. Very efficient, especially in the presence of selective pressure for a plasmid-carried trait.
How do temperate bacteriophages integrate genomes into bacterial cell chromosome?
Temperate phages have the ability to integrate their genomes into the bacterial cell chromosomes (lysogenic mode). Resulting bacterial cell is a lysogen. Phage conversion. Phage genome is replicated as part of the bacterial chromosome and passed on to all daughter cells. Bacteriophages are viruses that infect and replicate in bacteria.
