L13 & 14 overview and bacterial structure Flashcards
consequences of pathogen adherence
- immune system activation
- uptake into a specialized double membrane compartment
- Cytoskeletal rearrangement, formation of a specialized lesion and entry into the host cell
- Cytoskeletal rearrangement, formation of a specialized lesion and some degree of entry into the host cell (pedestal)
2 functions of capsules
adherence or avoiding the immune system
bacterium size
.1-10 um
major exterior organelles
capsule, the flagella and the pili
capsule
Capsule- A thick, sometimes amorphous layer, often (but not always) largely composed of polysaccharide. The capsule is not required for the non-pathogenic life style of the bacterium, and is often produced only by virulent strains of a given bacterium, or only under certain conditions.
The capsule can be a potent determinant of virulence by providing protection from host defenses, the ability to attach to host tissues or by being harmful to the host on its own.
Flagella
The organelles of motility. A helical propeller that spins to allow swimming. Because motility can be important to pathogenesis, flagella are sometimes virulence factors. They are extremely strong antigens (they strongly
stimulate the immune system) and antibodies against them can be useful diagnostic tools. Can be organelles of attachment (cap is replaced w/ an adhesion)
pili (fimbrae)
Pili- Long, thin filament-like projections that cover the surface of some Gram - bacteria.
The large variety of pili, as well as the variety of specialized tip molecules allows for attachment to many different sites. Pili have adhesive molecules at their tips, which promote specific interactions with host cells. Some bacteria can switch these tip molecules to facilitate adherence to diverse host tissues.
The envelope
two types of bacterial envelope, called Gram - and Gram +.
The envelope consists of an inner membrane, a cell wall (the peptidoglycan) and, in the case of Gram - envelopes, an outer membrane.
inner membrane of the envelope
is common to both Gram - and Gram + envelopes. It
consists of a lipid bilayer comprising phospholipids and proteins.
The inner membrane serves to selectively transport molecules in and out of the cell, and serves as the site of many biochemical reactions.
cell wall
cell wall or peptidoglycan is found in both types of envelope and is unique to bacteria. It is composed of sugar chains with attached short peptides. The chains are cross-linked together by bonds between the peptides. The result is a strong meshwork that surrounds the cell and gives it shape.
Antibiotics can act to disrupt cell wall biosynthesis resulting in cell lysis. Peptidoglycan of some bacteria is toxic.
Gram + vs Gram - cell wall
Gram + cell wall: Thick, multiple layers of peptidoglycan, contains additional molecules called teichoic and lipoteichoic acids that are important antigenic determinants.
Gram - cell wall: Thin, one or a small number of peptidoglycan layers.
outer membrane
is unique to the Gram - envelope.
Like the inner membrane, it is a bilipid layer, but the outer leaflet has lipopolysaccharide (LPS) molecules instead of phospholipids. The outer membrane is a strong barrier to most molecules that might enter the cell.
Porins are structures unique to the outer membrane that allow passive entry and exit of small molecules.
LPS (lipopolysaccarides) three major components:
A lipid portion (lipid A),
a polysaccharide portion (core polysaccharide) and
the O antigen polysaccharide side chain.
Lipid A
makes LPS highly toxic resulting in endotoxic shock, anchors LPS to the membrane
interior of the cell
This is crowded, poorly described region of the cell. The cytosol contains ribosomes and the nucleoid, and often plasmids.
Ribosomes
are visible as dark granules in the electron microscope. They are macromolecular machines that synthesize proteins from amino acids, using mRNA as templates. They are frequently the targets of antibiotics.
nucleoid
bacterial chromosome. It is not enclosed in a
membrane, unlike the chromosomes of the eukaryotes. Often, additional genetic information is encoded on pieces of DNA that are separate from and autonomous of the chromosome called plasmids.
plasmids
encode determinants of virulence such as
antibiotic resistance factors and the molecules that comprise the pilus.
They can be easily transmitted between cells, and their spread is facilitated by over use of antibiotics.
Spores
A special dormant cell type formed by certain bacteria as a response to stress. They are highly resistant, making disinfection very difficult, and easily dispersed.
In some cases, the production of a spore by a stressed cell will directly result in disease (as in Clostridium perfringens- toxin produced simultaneously with sporulation) and in other cases, the spores must convert back to a normal growing cell (germination- vegetative cells produce toxin) before they cause disease symptoms (as in Bacillus anthracis).
Mycoplasma
A very simple cell type that has only an inner membrane, of special composition. Because it lacks the peptidoglycan, it can have a variety of shapes.
Sugars on the mycobacterial cell surface are thought to mimic host sugars and, as a result, engage macrophage receptors that facilitate uptake and sequestration into a safe intracellular compartment. These sugars may also stimulate anti-inflammatory responses on macrophage and dendritic cells.
Biofilms
organized multicellular bacterial communities
form when bacteria sense the correct density of their cohort via pheromones
Biofilm characteristics
Adherence, controlled release of bacteria from the biofilm, immune system avoidance, alteration of bacterial growth kinetics, activation of bacterial stress and defense responses, alteration of drug pharmacokinetics. Many if not most bacteria can form biofilms.
A strategy to defeat biofilms
A new approach: first treat the biofilm, then treat the bacterium
Apply a drug that blocks pheromone communication among bacteria in the biofilm.
Cell-cell communication blocked by a therapeutic molecule, biofilm cannot continue to form.
Then give antibiotic
biofilm distribution in the GI tract
most prominent in the proximal colon and appendix and decrease toward the distal colon
possibly due to differences in IgA. If some feature of IgA allows it to better bind bacteria in the distal colon, this could inhibit bacterial deposition at that site.
multi-drug efflux system
Clinically significant multidrug resistance is usually associated with a relatively impermeable outer membrane. Strongly selected for in hospitals
trans-envelope channel, not a porin. That transports drugs back out of the cell (very rapidly). Transport can either happen when the molecules are in the periplasm or cytoplasm.
may have evolved from a mutated channel
