L17: Intro to Microbio Flashcards
Normal flora (microbiota)
Tend to be commensal or mutual symbionts adapted to the special conditions found in various body locations. Normal flora tend to avoid injuring the host, and are often beneficial to the host.
Virulence
The ability of a microorganism to cause disease
Virulence factor
Factors (e.g. toxins) produced by organisms that enable it to infect, cause disease, and/or kill the host
Virus
Smallest infectious particle (0.03- 0.3 µm). Viruses are obligate intracellular pathogens; they require a host cell for replication.
Bacterium
Simple, unicellular, prokaryotes with no nuclear membrane, mitochondria, Golgi bodies or endoplasmic reticulum that reproduce by asexual division. 0.1- 10 µm.
Fungus
More complex cellular structure, eukaryotic organism with well-defined nucleus, mitochondria, Golgi bodies and endoplasmic reticulum. Fungi exist as unicellular yeast or in filamentous forms (mold). Fungi replicate sexually and asexually. 4- 10 µm.
Parasite
The most complex microbes. Eukaryotes that exist as single cellular (Leishmania) or multicellular (Shistosome). Wide range in size.
Differences between eukaryotes and prokaryotes
- -Lack nucleus
- -Typically have single, double-stranded, circular DNA chromosome
- -Haploid
- -Smaller ribosome (70S)
- -Mesh-like peptidoglycan cell wall
How can we detect bacteria in clinical samples?
- -Microscopy
- -Detection of bacterial antigens or nucleic acids
- -Culture
- -Detection of antibody response to bacteria (serology)
Microscopy
Looking at bacteria under a microscope to view morphology and Gram stain
Detection of bacterial antigens
Can be found in patient specimens; examples are H-antigen (flagella), K-antigen (capsule), O-antigen (LPS)
Detection of bacterial nucleic acids
PCR, sequencing
Culture
Using media, four types include:
- Enriched nonselective
- Selective
- Differential
- Specialized
How do we classify bacteria?
- -Visible features (size, shape, Gram stain)
- -Nutrition (media, aerobic vs. anaerobic)
- -End products
- -Surface molecules
General cytology of bacteria
- -Contain chromosome, mRNA, ribosomes, proteins, and metabolites in cytoplasm
- -Membrane is a lipid bilayer
- -Have a cell wall made of peptidoglycan
- -Have appendages such as pili or flagella
- -Capsule covers outside
Difference between Gram + and - cell walls
Gram + = layer of peptidoglycan covering outside with a plasma membrane underneath
Gram - = outer membrane surrounds a peptidoglycan layer with periplasmic space underneath and a cell membrane underneath that
General characteristics of peptidoglycan
- -Linear chain of alternating sugars (NAG and NAM) that are crosslinked by B-1,4 linkages
- -Provides essential protection for bacteria
- -Determines shape of bacteria
- -Good drug target because it’s found uniquely on bacteria
How does peptidoglycan affect our immune system (basic level)?
Proinflammatory; fixes complement, binds pattern recognition receptors and triggers TNF production; can be degraded by lysozyme found in tears and mucus
2 major components of Gram + envelope
- Peptidoglycan (any layers with extensive crosslinking)
2. Teichoic acid (along with LTA promotes attachment to other bacteria and host cells)
Basic elements of Gram - envelope
- -Greatly reduced peptidoglycan, which is located within periplasmic space
- -Surrounded by porin- and LPS-rich outer membrane that maintains structure and makes a permeability barrier
Lipopolysaccharide (LPS, endotoxin)
Comprised of 3 subunits:
- Lipid A – responsible for endotoxin activity of LPS (anchored into OM by fatty acids)
- Core polysaccharide – required for bacterial structure and viability (branched polysaccharide of 9-12 sugars)
- O antigen – long polysaccharide with much variation (not in Neisseria)
Acid-fast stain
Bacteria stain red by carbol fuchsin, destain by acid alcohol, then methylene blue used to counterstain – acid-fast organisms stay red/pink
Acid-fast bacterial structure
Similar to Gram + – LAM, arabinogalactans, and mycolic acid (waxy); grow VERY slowly (ex. mycobacteria)
Pili/fimbrae
Composed of protein subunits pilin which form tube with hollow core, hair-like and grow on both Gram +/-
2 types of pili
- Common (attachment to epithelial cells)
2. Sex (only one per cell, involved in gene transfer)
Flagella (H-antigen)
Rotating helical structures anchored to plasma membrane responsible for locomotion; important virulence factor for bacteria
Capsule (K-antigen)
Loose polysaccharide/protein layer surrounding Gram +/- bacteria that is hydrophillic and protects against immune system (antiphagocytic); important virulence factor
Biofilm
An organized community of microbial cells that has a capsule/slime layer over the entire population
Gram staining process
- -Crystal violet of Gram stain is precipitated by Gram iodine and is trapped in the thick peptidoglycan layer in gram-positive bacteria
- -The decolorizer disperses the gram-negative outer membrane and washes the crystal violet from the thin layer of peptidoglycan.
- -Gram-negative bacteria are visualized by the red counterstain.
Endospores
- -Only occur with Gram+ bacteria
- -Dehydrated, multishelled structures that are basically bacteria that are dormant
- -Contain a complete copy of the chromosome and a high concentration of calcium bound to dipicolinic acid
