Prof Barer Flashcards
What are the two main ways of staining bacteria?
1). Acid fast stain
Used for mycobacterium to detect and classify
2). Gram stain
Those with thick peptidoglycan walls retain the dye (positive) whereas those without/with only thin walls will not (negative)
Why are bacteria difficult to stain and visualise?
Insufficient magnification - oil is needed to exclude air between lense and object at 100x
Stain poorly with H&E
Bacteria are removed during slide preparation
What are the 4 shapes of bacteria and the 3 variations of these?
Shapes: Cocci Bacilli/rods Streptococci/coccobacilli Diplococci
Variations:
Curved
Spiral
Filamentous
What clinical importance do cell walls have in bacteria?
They can be exploited by antibiotics
Allow detection via gram/acid fast staining
Have endotoxin effects (e.g. Lipopolysaccharide is a toxin released only upon destruction of the bacterial cell wall)
What are 4 external features of bacteria?
Flagella - aid mobility
Pilli - allow attachment
Capsule - protect against phagocytosis
Outer membrane – may have a LPS layer = toxic
Which are the gram positive cocci bacteria and the conditions they cause?
Staphylococcus aureus - meningitis, impetigo
Streptococcus pyogenes - impetigo, TSS
Streptococcus pneumonaie - pneumonia
Group B streptococcus - GBS (complications e.g. Meningitis)
What are the gram positive bacilli bacteria and the conditions they cause?
Clostridium difficile - AAD, pseudomembranous colitis
Clostridium perfringen - food poisoning, gas gangrene
What are the gram negative diplococci bacteria and the conditions they cause?
Neisseria gonnorrhoea - gonnorrhoea
Neisseria meningitidis - meningitis
What are the gram negative bacilli?
Salmonella spp - salmonella
Escherichia coli - gastroenteritis
Shigella - dysentery
What are the bacteria which don’t stain with gram staining?
Mycobacterium TB - TB
Mycobacterium leprae - leprosy
Chlamydia
How do bacteria replicate?
Binary fission
What is the clinical importance of biofilm, broth turbidityand colonisation?
Biofilm - bacteria adhere to surfaces e.g. IV lines
Broth turbidity - allows sensitive detection of infection, especially in fluid filled cavities
Colonisation - allows us to count, identify and work out the speed at which the bacteria grows.
What is the cell envelope of bacteria?
The plasma membrane, cell wall and outer membrane if the bacteria has one.
In acid fast bacteria, this is neither gram positive or negative.
Why do mycobacterium have no gram stain?
Cell wall has a high lipid content = retain no stain (gram negative often retain a second stain added)
What is the structure of viruses?
DNA is surrounded by a protein capsid. This may then be enveloped. The outermost layer (capsid or envelope) contains ligands.
What forms can the genetic n formation in a virus take?
ssRNA --> + sense = can be used to form mRNA and therefore be translated --> - sense = can't form mRNA and can't be translated ssDNA dsRNA dsDNA Linear Circular
What is an infection?
Multiplication of a pathogenic microbe within a susceptible host with associated damage or dysfunction
How do viruses infect cells?
1) . Host cell must contain a receptor to the viruses ligand = virus binds
2) . Virus is taken into the cell via receptor mediated endocytosis, a coated pit or by fusing with the membrane.
3) . Virus can then translate its DNA into viral proteins.
How are viral proteins released?
1) . Cell lysis
2) . Budding - capsid and viral proteins push into cell membrane, and part is pinched off.
What are the 4 categories of viral classification and examples for each?
1) . DNA enveloped
- Hep B
- Herpes
2) . DNA non enveloped
- Papilloma virus
3) . RNA enveloped
- Rubella
- HIV
4) . RNA non enveloped
- Hep A
- Polio
What is the Baltimore scheme and how does it work?
It is a way of classifying viruses based on the relationship between their viral genome and mRNA used for translation during expression of this genome.
How big are viruses?
18-350nm
How can viruses be diagnosed?
Culturing
Detection of viral antigens e.g. In blood
Detection of viral nucleus acids e.g. Via PCR
How do viruses replicate?
1) . They enter the host cell, are uncoated and after making a few viral proteins, may inject their DNA into the host DNA, or form an episome. The host cell then replicates this.
2) . They enter the cell, are uncoated and replicate their own nuclei acids. New viruses are then reassembled for this DNA before being released.
How can infection spread?
From animals
Water borne
Food contamination
Person to person
What are the two types of infection?
Exogenous - arises from the invasion of pathogenic bacteria
Endogenous - the bodies natural micro iota become pathogenic
How can you control infection?
Eliminate the reservoir
Isolate or protect the susceptible host
Eliminate the mode of transmission
How are Henle-Kochs postulates carried out?
Isolate the microorganism from the diseased animal.
Propagate the microorganism in vitro.
Insert the culture into a healthy animal.
Cultivate the microorganism again from the healthy animal which has then turned diseased to prove it is the same agent.
What are the issues with performing Henle-Kochs postulates?
Unethical to give a creature a disease
Some microbes don’t grow in labs
Some diseases require more than one microbe to manifest
What is the reservoir and immediate source of an infection?
Reservoir - where the microbe is normally found
Immediate source - where it can be found in contact with humans
What is the equation for the incidence of colonisation in patients?
R0 - 1
Y0 = ——–
R0 + Rn
What is R0?
If R0 is greater than 1 the infection takes hold, if it is less than 1, it doesn’t.
R0 = (time period of infectivity) x (no. Infected people the infected individual had contact with during time period of infectivity) x (transmission coefficient)
