Classification, Structure & Replication of Micro-Organisms Flashcards
What is the major problem when treating conditions caused by microorganisms?
they will evolve and adapt to preventative measures
they become resistant, particularly to antibiotics
What is globalisation and why is it a problem?
the spread of a pathogen from one location to another
antibiotic resistance needs to be tackled globally and not just in the UK, as antibiotic resistant bacteria may be brought in from another country and allowed to spread
In the future, what is another problem for treating infections caused by microorganisms?
we are encountering new pathogens frequently and there may not be drugs which work against these
Why is the domain ‘archaea’ clinically insignificant?
no members of this domain will cause infection
Why are viruses and prions not included in the tree of life?
These “infectious particles” do not satisfy the criteria for a living organism
What are the 9 stages in classification?
Which 2 are the most clinically significant?
Genus and species are the most clinically significant
What are the different components involved in this name?
What is meant by:
Escherichia spp.
spp. means species
this can be used if you are talking about multiple species of the same genus
Why is it important to state the species as well as the genus of an organism?
There are multiple species within a genus that cause different diseases with very variable symptoms
It is not enough just to state the genus as you need to understand the type of infection that is being dealt with
What is the difference between classification and identification?
Classification:
this involves separating organisms into classes by looking at similar traits
Identification:
this involves the identification of the individual organism
What method is used to decide where to place bacterial species in the tree of life?
Looking at the DNA that encodes the 16S ribosomal RNA sequence
This gene is present in all prokaryotes, but is different in different species of bacteria
How can organisms be identified by their 16S sequence?
there are hypervariable regions that differ between microorganisms
these act like “barcodes” allowing the individual species to be identified
What 6 characteristics are commonly used in identification, rather than classification?
- Gram stain
- cell shape
- endospore
- atmospheric preference
- fastidiousness
- key enzymes
What type of cells are bacteria?
Why are they classified this way?
Prokaryotes
They contain no membrane-bound organelles and are single-celled organisms
Where is the DNA found within a bacterial cell?
a single chromosome is condensed within the middle of the cell
this is the nucleoid
What are the following features of a bacterial cell?
What is the function of the inclusion bodies?
they act as a food source
What is the purpose of the flagellae?
they are responsible for locomotion (motility)
if a bacteria does not have a flagella, it cannot move about
How can the different types of flagellae be classified?
A - monotrichous
B -lophotrichous
C - amphitrichous
D - peritrichous
What is a virulence factor?
a component of a microorganism that contributes to its ability to cause an infection
How can flagellae act as a virulence factor?
E. coli and Proteus spp. need the flagella for propulsion up the urethra and into the bladder
They are incapable of causing disease without the flagella
How can the flagellae be used in identification of certain pathogens?
The flagella itself is the H antigen
You can raise antibodies against the H antigen, allowing the bacteria to be distinguished from other similar organisms
What is the purpose of pilli/fimbriae?
They are exposed to the extracellular space and can aid adhesion to host cells and colonisation
How can pilli/fimbriae act as virulence factors?
They allow the organism to hold on to specific molecules
e.g. in a UTI, E. coli will hold onto specific sugar receptors on the epithelium of the bladder
this means that the urine cannot flush the organism away
How do fimbriae/pilli contribute to tissue tropism?
this tells you where an organism can cause disease within the human body
they can only adhere to certain molecules that may only be located in certain regions
What does the capsule/slime layer protect the bacteria from?
- phagocytosis/immune attack
- dessication (drying out)
- antibiotics
What is the difference between a capsule and a slime layer?
a capsule is tightly adhered to the cell
the slime layer is loosely adhered to the cell
How can the capsule prevent phagocytosis?
It occludes the antigens that are expressed on the surface of the pathogenic cell
This subverts the immune system, allowing it to cause infection
What is meant by a biofilm?
a community of bacteria surrounded by a polysaccharide layer (an extension of the capsule/slime layer)
How can a biofilm contribute to antibiotic resistance?
it prevents a lot of antibiotics from diffusing through and reaching the microorganism
What is significant about the bacteria within a biofilm being metabolically inactive?
many antibiotics rely on bacteria being metabolically active, as they disrupt a process that is occurring within the bacteria
What is meant by an endospore?
A metabolically inert form of bacteria that is resistant to many forms of environmental stress
What are examples of environmental stresses that an endospore may be resistant to?
- dessication
- UV radiation
- extreme temperatures
- certain antiseptic agents e.g. alcohol handgel
What usually triggers endospore production?
How long does it last for?
a lack of available nutrients
endospores allow bacteria to lie dormant for extended periods - even centuries
What are the 2 most common genera that contain endospore-producing bacteria?
bacillus and clostridium
How can endospores be used in identification?
What are the 4 types of cell wall encountered in bacteria?
- Gram-positive
- Gram-negative
- Mycobacterial (“acid-fast”)
- no peptidoglycan
What is the difference between the cell walls of Gram positive and Gram negative bacteria?
Gram-positive has a thick layer of peptidoglycan polymer and a single cell membrane
Gram-negative has a thin layer of peptidoglycan and both an inner and an outer cell membrane
What are examples of Gram-positive bacteria?
- staphylococcus
- streptococcus
- bacillus
- clostridium
What are examples of Gram-negative bacteria?
- E. coli
- Acinetobacter
- Klebsiella
- Neisseria
What are examples of mycobacterial bacteria and “atypical” bacteria?
Mycobacterial:
mycobacterium, rhodococcus
No peptidoglycan (“atypical”):
chlamydia, mycoplasma
What is significant about Gram-positive bacteria having a thick cell wall and the location in which they are found?
They are found on the outside of the body where there is a drying environment
They need a thick cell wall to survive
What is the role of the outer membrane in a Gram-negative bacteria?
it acts as a permeability barrier
hydrophilic compounds over a certain size cannot get through the outer membrane (this includes some antibiotics)
Why may a bacterium delete its porin?
porin allows substrates to pass through the outer membrane
it can delete its own porin to prevent antibiotics from passing through
What is the composition of a cell wall of a mycobacterial bacterium?
there is a thin peptidoglycan layer and no second membrane
there is a waxy layer with many mycolic acids that many antibiotics cannot penetrate
Where are “atypical” bacteria usually found?
Which type of antibiotics cannot be used against them?
Usually intracellular organisms
Any antibiotics that target the synthesis of peptidoglycans will not work against these organisms
How is the Gram reaction used to divide bacteria into 3 groups?
it is based on their ability to retain a crystal violet-iodone dye complex following acetone/alcohol treatment
How will Gram-positive and Gram-negative bacteria appear following the Gram reaction and why?
Gram-positive:
retain the crystal violet-iodine complex and appear blue/purple
Gram-negative:
cannot retain the crystal violet-iodine complex and appear red/pink
How can the Gram reaction be used to identify mycobacteria?
the stain is unable to penetrate the mycolic acid layer, so the bacteria remain colourless
Identify the bacteria following the Gram reaction
What are the 6 different shapes of bacteria?
How can cocci and bacilli (rods) be identified by clustering?
By which mechanisms do bacteria replicate and acquire DNA?
Replication is by vertical gene transfer (binary fission)
Acquisition of DNA is by horizontal gene transfer
What are the 3 types of horizontal gene transfer?
- conjugation
- transformation
- transduction
What is clinically significant about horizontal gene transfer?
it is important in acquisition of virulence determinants and antibiotic resistance determinants
What happens in binary fission?
- DNA doubles in size
- FtsZ proteins form a contractile ring within the septum
- This pinches to 2 cells off from each other to form 2 daughter cells
What is meant by ‘generation time’?
How does it affect diagnostic tests?
the time it takes for a bacterial cell to divide
a lot of diagnostic tests rely on the time it takes for an organism to grow and whether it can be cultivated in the lab
How is growth of a bacterial population described?
What is a determining factor in this?
Growth of a bacterial population is exponential
As long as sufficient nutrients are availablw
What is the mechanism behind conjugation?
A plasmid or piece of DNA travels along the sex pilus from one organism to another
What is the mechanism behind transformation?
The bacteria takes up DNA from its surrounding environment
e.g. if a cell has lysed and released its contents into the surrounding environment
What is the mechanism behind transduction?
sometimes, when it replicates, a bacteriophage takes up DNA from the bacterial host
it then transfers this genetic material into another bacterial cell
What is a fungi cell wall made from?
chitin
It provides structural support and rigidity
What are the 3 types of disease-causing fungi?
- yeasts
- filamentous (moulds)
- dimorphic (yeast & mould)
What is a dimorphic fungi?
It is a yeast and mould
It can switch between 2 forms
What does candida albicans cause?
What happens when it causes infection?
oral candidiasis (thrush)
when it causes infection, it goes from being a yeast-type to a mould-type and produces pseudohyphae that allows it to anchor into the tissues
What are the following structures of a yeast?
How do yeasts divide?
Either by mitosis or meiosis (sexual or non-sexual)
Mitosis involves budding where a bud occurs on the side of the mother cell
How do moulds replicate?
it can be sexual or non-sexual and is based on spores
spores are enclosed within a sporangium
What types of infections are caused by moulds?
- common, superficial infections such as ringworm and athletes foot
- uncommon, severe infections such as aspergillosis and mucormycosis
What are the following features of a mould (filamentous fungi)?
What are the stages in the reproductive cycle of a mould?What are the 2 different groups of parasites and the main difference between them?
Protozoa:
these are unicellular eukaryotes
Helminths (worms):
multicellular eukaryotes
What are examples of intestinal protozoa?
- entamoeba
- giardia
- cryptosporidium
What are examples of non-intestinal protozoa?
- plasmodium
- leishmania
- trypansoma
What are examples of intestinal and non-intestinal helminths?
Intestinal:
- enterobius (pinworm)
- taenia (tapeworm)
- ascaris (roundworm)
Non-intestinal:
- schistosoma
How are most protozoa transmitted into a cell?
through some form of vector
What are the 3 characterisitics of parasitic replication?
- it can be sexual or asexual
- it may require multiple hosts to complete a lifecycle
- it may include cyst formation as a mode of survival in harsh environments
What are viruses?
What types of organisms suffer from virus infections?
obligate intracellular parasites
every class of organisms suffers from virus infection
What is the structure of a virus like?
it has a nucleic acid core wrapped in a protein coat
a naked virus does NOT have a lipid envelope
some viruses DO have a lipid envelope
What is the protein coat and the lipid envelop of a virus made out of?
capsid is made out of balls of protein called capsomeres
the lipid coat is part of the cell membrane of the host cell, that forms after blebbing
What is the benefit of a lipid coat to a virus?
Being coated in host lipid bilayer allows the virus to hide from the immune system as no foreign antigens are presented
What is the nucleic acid core of a virus made up of?
DNA or RNA
Retroviruses contain RNA and reverse transcriptase
Why do retroviruses contain reverse transcriptase?
the reverse transcriptase converts negative sense RNA into cDNA so it is then translated into postitive sense RNA
Only positive sense RNA can enter a ribosome and be converted into protein
What are the 6 stages in the lifecycle of a virus?
- attachment
- penetration
- uncoating
- biosynthesis
- assembly
- release
What happens during the attachment stage of the lifecycle of a virus?
the virus attaches to the host cell via specific receptors
this is tissue tropism as there is recognition between the virus and the host cell
What happens during the penetration stage of the viral lifecycle?
the cell engulfs the virus by receptor-mediated endocytosis or membrane fusion
What happens during the uncoating stage of the viral lifecycle?
viral contents are released and the genetic material is targeted to the nucleus
What happens during the biosynthesis stage of the viral lifecycle?
viral RNA enters the nucleus where it is replicated by the viral RNA polymerase
early viral proteins are produced - polymerase and integrase - which allow for genome replication
What happens during the assembly phase of the viral lifecycle?
new phage particles are assembled
late viral proteins are produced - structural proteins, capsid and surface proteins
What happens during the release stage of the viral lifecycle?
the new viral particles are released via budding or cell lysis
What is the role of the viral protein integrase?
it helps with the integration into the host chromosome
this allows the virus to subvert the host cell functions so that RNA can be converted into protein
What factors determine which family a virus is placed into?
- nature of the nucleic acid - RNA or DNA
- symmetry of the capsid
- presence or absence of an envelope
- dimensions of the virion and capsid
What are prions?
they are proteinacious infectious agents
they are misfolded proteins that contain NO genetic material
How do prions cause disease?
the misfolded prion can induce misfolding in other proteins
this leads to accumulation of abnormal misfolded proteins (aggregate)
the amyloid aggregate disrupts cellular function
What are examples of 4 diseases caused by prion proteins?
- creutzfelt-jakob disease
- bovine spongiform encephalopathy
- scrapie
- kuru
How does someone contract a prion disease?
- can be inherited
- can be spread via contaminated material
- can occur spontaneously
