Microorganisms Flashcards
What are the steps in a Gram stain?
1) Stain with crystal violet
2) Add iodine (binds to crystal violet)
3) Wash with acetone
4) Stain with safranin
Why can’t we use an H&E stain to detect bacteria?
Insufficient magnification (have to use oil immersion)
Tissue preparation deliberately removes microbes in order to isolate human cells
What is the difference between Gram +ve and -ve results?
Gram +ve = crystal violet and iodine stain not removed (purple)
Gram -ve = acetone washes away violet and iodine stain, safranin stain remains
What is the difference between Gram +ve and Gram -ve bacteria?
Gram +ve = thick peptidoglycan cell wall
Gram -ve = lipopolysaccharide layer + periplasmic space (some peptidoglycan)
What would you use to stain mycobacteria?
Acid Fast stain e.g. Ziehl-Neelsen, fluorescent auramine
What features define a microbial reservoir?
- specific energy source
- specific building blocks
- specific atmosphere
Anything containing a living infectious agent
What is the definition for infection?
Multiplication of a pathogenic microbe in a susceptible host with associated dysfunction or damage
What are the Henle-Koch postulates?
1) Microorganism must be present in every case of the disease
2) Suspected microorganism must be isolated and grown in pure culture
3) Same disease must result when the isolated microorganism is inoculated into a healthy host
4) Same microorganism must be isolated again from the diseased host
What is the definition of a virus?
Obligate intracellular parasite = incapable of growth outside of host cell
What are the cytopathic effects of viruses?
Destructive e.g. herpes
Production of syncytiol (multinucleated mass of cytoplasm not separated into individual cells)
What is the difference between +ssRNA and -ssRNA?
+ve = same direction as genomic RNA (can be directly translated into protein)
-ve = opposite direction as genomic RNA (cannot be directly translated into protein)
What is the definition of a virion?
Particle enclosing nucleic acid (composed of capsomeres)
What is the definition of a nucleocapsid?
Capsid + nucleic acid
What are the functions of a capsid?
Protects genomic material
Attachment to host cell
What are the types of viral infections?
- lytic (host cell lyses to release viruses)
- latent (virus present, but no symptoms and no transmission) e.g. herpes
- chronic (low level replication over time) e.g. HIV
What are the requirements for a host cell for viral replication?
Viral receptor which can bind to cell membrane
Contains required cell machinery for viral replication
Permissive cell -> PRODUCTIVE INFECTION
How are viruses classified?
Baltimore classification system
e.g. type of genome, enveloped or not
Outline the steps in viral infection.
1) Attachment
2) Entry (receptor-mediated endocytosis/crosses cell membrane via coated pits/fusion with cell membrane)
3) Uncoating
4) Viral proteins synthesised
5) Integration of viral genome in host cell (provirus) OR formation of a circular, non-integrated genome (episome)
6) Release: cell lysis (non-enveloped) or budding (enveloped)
What are some examples of Gram +ve bacteria?
~ Staphylococcus aureus (impetigo) ~ Staphylococcus pyogenes (scarlet fever) ~ Group B Streptococci (meningitis) ~ Streptococcus pneumoniae ~ Clostridium difficile (diarrhoea) ~ Clostridium perfringens (gas gangrene)
What are some examples of Gram -ve bacteria?
~ Neisseria meningitidis ~ Neisseria gonnorrhoeae ~ Escherichia coli (diarrhoea) ~ Salmonella spp. (enteritis + diarrhoea) ~ Shigella (colitis + diarrhoea) ~ Helicobacter pylori (duodenal ulcers)
What are some causes of meningitis?
E. coli & Haemophilus influenzae (newborn)
Neisseria meningitidis (young adult)
Group B Streptococci + Streptococcus pneumoniae (adult)
Give some examples of DNA viruses.
Enveloped: Hepatitis B, Herpes, Smallpox
Non-enveloped: Papillomavirus e.g. HPV
Give some examples of RNA viruses.
Enveloped: Rubella, Rotavirus (diarrhoea), HIV, Myxoviruses (Influenza, Measles, Mumps), Coronaviruses (SARS, Common cold)
Non-enveloped: Picornaviruses (Polio, Hepatitis A, Common cold)
What are the routes of transmission of microbes?
DIRECT: sexual, vertical transmission (mother -> child)
INDIRECT: aerosols, faecal-oral contamination, transcutaneous (inoculation)
What does R0 refer to regarding infection control? What does it mean when is greater than/less than 1?
No. of secondary cases produced by one infected individual (basic reproductive rate of infection)
R0 > 1 infection propagates
R0 < 1 infection dies out
What are some factors affecting the spread of infection?
- time period of infectivity
- rate of mixing between infectious and susceptible individuals
- capacity for transmission to take place
What are the revised Henle-Koch postulates?
Necessary = disease cannot occur in the absence of the cause
Sufficient = a cause alone can lead to the disease
Specific = the cause is absent in other diseases
What is the definition of a source of microbes?
Readily available form of infectious agent.
What is the definition of commensal microbes?
Bacteria that is found normally in our body (usually non-pathogenic; symbiotic relationship with host)
Give an example of an antibiotic that targets bacterial cell wall synthesis, and explain how it works.
Penicillins
Inserts into peptidoglycan cell wall, inhibits penicillin-binding protein, prevents cell wall synthesis - cell lyses
Give an example of an antibiotic that targets bacterial transcription, and explain how it works.
Rifampicin (TB)
Inhibits RNA polymerases, preventing RNA chain from extending.
Give an example of an antibiotic that targets bacterial protein synthesis, and explain how it works.
Erythromycin (macrolides)
Blocks P site of ribosome, tRNA cannot bind & peptidyl transferase is inhibited, prevents translocation in protein synthesis
Give an example of a drug that targets cancer cell DNA replication, and explain how it works.
Anti-folates e.g. Methotrexate
Competitive inhibitor of enzyme in S-phase which stimulates nucleotide synthesis, preventing human cell synthesis.
How can folinic acid help with chemotherapy?
Helps non-cancerous cells to recover (note: cancer cells divide faster, so chemo has a greater effect on cancerous cells)
What are some different antibiotic-resistance mechanisms of bacteria?
- enzymes which inactivate antibiotics (enzymatic cleavage)
- alteration of target of antibiotics/lack of expression of target
- efflux pump
- prevention of entry into cell
- methylation of target
