9. Introduction to microbiology Flashcards
Which bacteria do the hard surfaces of the teeth carry in large numbers?
Streptococcus mutans
Which bacteria do the soft tissues in the mouth and the tongue carry?
Streptococcus salivarius
Although considered a commensal bacterium, streptococcus mutans can also cause disease. Which disease?
Dental caries
particularly in people with diets rich in sugar
How are the microorganisms which cause disease described?
‘virulent’ or ‘pathogenic’
Give an example of how our gut commensals are beneficial?
Without them, we would require a continuous supply of vitamin K
Bacteria are responsible for supplying most of the ‘fixed’ nitrogen
Who set out the criteria used to decide if a microorganism caused a disease?
Robert Koch
Koch’s postulates
What do Koch’s postulates determine?
Whether a microorganism causes a disease
What are the 4 Koch’s postulates?
- the causative organism must be isolated from every individual suffering from the disease
- the causative organism must be cultivated artificially in pure culture
- when the causative organism is inoculated from pure culture, the typical symptoms of the infection must result
- the causative organism must be recoverable from individuals who are infected experimentally
In addition to Koch’s postulates, what would one also look for these days to determine whether a microorganism causes a disease?
Antibodies against the causative organism in natural cases and in individuals infected experimentally
What are the first 3 Koch’s postulates for genes?
Genes encode virulence factors:
- the gene encoding the trait of interest should be present and transcribed/translated in a virulent strain
- the gene encoding the trait of interest should NOT be present or should be silent in a strain that does not cause the disease
- disruption of the gene in a virulent strain should result int he formation of a strain that is incapable of causing disease
What are the last 3 Koch’s postulates for genes?
- introduction of the gene into a strain that previously did not cause disease should transform the strain into one that does cause disease
- the gene must be expressed during infection
- antibodies raised against the gene product or the appropriate cell-mediated immunity should protect experimental subjects against disease
What are the problems with Koch’s postulates?
- difficulty isolating the causative agent
- impossible to grow some pathogens in artificial culture
- ethical objections
- animal models not sufficient
Give a specific causative bacterium which is difficult to isolate from people who are infected with particular pathogens
Myobacterium tuberculosis
the cause of tuberculosis
Give a specific causative bacterium which cannot grow in artificial culture
- Myobacterium leprae
(the cause of leprosy)
- Treponema pallidum
(the cause of syphilis)
Why might there be ethical objections relating to Koch’s postulates?
Ethical issues with infecting subjects with infectious agents
(animal models not possible for all infections)
Give an example of where an animal model is not sufficient for testing the effect of a causative agent?
Salmonella typhi in mice causes gastroenteritis
Salmonella typhi in humans causes typhoid fever
(vice versa with salmonella typhimurium)
Are viruses living organisms?
This is debatable - they entirely rely upon other cells for their replication - they are obligate intracellular parasites
What is an ‘obligate intracellular parasite’?
Something that requires to live within a cell in its host
Viruses are obligate intracellular parasites
Who is affected by viruses?
Every class of organism, not just humans
What are viruses comprised of?
A nucleic acid core wrapped in a protein coat
Some viruses have an envelope, others are naked
What is the envelope of a virus made out of?
Lipid and usually derived from the cell in which they grow
What protects the nucleic acid core in the centre of a virus?
The protein coat
And sometimes the lipid envelope
Which nucleic acid do viruses contain?
Either DNA or RNA (but not both)
How are retroviruses unusual and different from normal viruses?
The virion carries an RNA copy of the genome but upon infection of a host cell, a cDNA copy of the virus genome is made using the enzyme reverse transcriptase
When a retrovirus infects a host cell, a cDNA copy of the virus genome is made from RNA. Which enzyme is required for this?
Reverse transcriptase
Viruses have a nucleic acid core wrapped in a protein coat. What is this protein coat made of?
Units called capsomeres
What are capsomeres?
They are the units that make up the protein coat of a virus
What is a bacteriophage?
A special class of virus that attacks bacteria
Infects and replicates within the bacterium
What are some examples of shapes of viruses?
- round
- rod-shaped
- icosahedral
- brick-shaped
- bullet-shaped
What are viroids?
Naked, infectious RNA molecules that are not associated with any proteins (the smallest infectious pathogens known)
What do viroids infect?
Plants
Give some spongiform encephalopathies
- Creutzfeldt-Jakob disease
- Srapie (sheep)
- BSE (mad cow disease, cattle)
What are spongiform encephalopathies caused by?
Infectious proteins known as PRIONS
Are fungi prokaryotic or eukaryotic?
Eukaryotic
What is the cell wall of fungi made from?
Chitin
What is chitin?
A polymer of N-acetyl glucosamine
Makes up the cell wall of fungi
A major component of the exoskeleton of arthropods such as insects
While the cell wall of fungal cells are made up of chitin and other polymers, what is the cell wall of plants made from?
Cellulose
What is mould?
Fungi that grow in mats of tiny filaments known as hyphae or mycelia
What does hyphae mean?
Greek for thread
What does mycelia mean?
Greek for mushroom
Moulds may or may not be subdivided into separate compartments. By what?
Cross walls known as septa
Are moulds multicellular or unicellular?
Multicellular.
Unicellular fungi = yeasts
What is yeast?
Unicellular fungi
Which is the most familiar yeast?
Saccharomyces cerevisiae
How do yeasts grow and multiply?
By budding daughter cells off from a mother cell
What common, superficial infections do moulds cause?
- ringworm
- athlete’s foot
What is the most common yeast infection?
Thrush, caused by candida albicans
What is thrush caused by?
Candida albicans
A yeast
What is tinea pedum?
Athlete’s foot
A mould infection
What do some yeasts have the ability to develop under certain conditions?
Pseudomycelia
Cells cling together in chains resembling small true mycelia
What are protists?
Unicellular eukaryotes
What are the 4 classes of protista?
- apicomplexa
- flagellate protista
- ciliate protista
- amoebae
What were apicomplexa protists formerly known as?
Sporozoa
Give examples of how protists are spread
- by insects
- by human sexual contact
- producing cysts to survive outside the body
Give examples of infections caused by protists
- toxoplasmosis
- amoebic meningitis
- malaria
- trypanosomiasis
- leishmaniasis (Kala-Azar)
- amoebic dysentery
- diarrhoea
Which protist causes diarrhoea?
Cryptosporidium
Which protist causes vaginal infections?
Trichomonas vaginalis - causes a foul-smelling vaginal discharge
What effect does trichomonas vaginalis have in men?
Men can be asymptomatic carriers
But this protist can also cause balanitis
Which fungus was previously thought to be a protist?
Pneumocystis carinii/jiroveci
Pneumocystic carinii/jiroveci is a fungus, previously thought for many years to be a protist. What does it do?
Important for patients suffering from AIDS
Causes a type of pneumonia
Are bacteria prokaryotes or eukaryotes?
Bacteria are prokaryotes, they lack a membrane-bound nucleus
What proportion of bacteria causes disease?
Only a small minority of bacteria causes disease
Which type of micro-organisms have an enormous range of metabolic capacities and can be found in some of the most extreme environments on earth?
Bacteria, despite their simplicity
Which shape are most bacteria?
- round (cocci)
- rod-shaped (bacilli)
A few are comma-shaped or spiral
In the Gram reaction, what is being tested?
The cell’s ability to retain a crystal violet-iodine dye complex when the cell is treated with acetone or alcohol
Which dye is used in the Gram reaction?
Crystal violet-iodine dye
What colour will Gram-positive bacteria appear?
Violet
What colour will Gram-negative bacteria appear?
Red/pink
What is the counterstain used in Gram staining?
Safrinin or fuchsine (making all the Gram-ngeative bacteria red/pink)
What does the envelope of Gram-positive bacteria consist of?
30-40 layers of a uniquely bacterial polymer “peptidoglycan”, interlinked sugar molecules and short peptides containing both D- and L- amino acids
What does the envelope of a Gram-negative bacteria consist of?
Very little peptidoglycan
It posses an extra, complex “outer membrane”
Rather than a peptidoglycan envelope, gram negative bacteria have an outer membrane. What is the structure of the outer membrane of Gram negative bacteria?
An outer leaflet containing lipopolysaccharides
What is responsible for the symptoms of gram-negative shock?
- sugars from the surface antigen of the bacterium
- complex lipid, known as lipid A
(acts as endotoxin)
Many bacteria are motile. What allows them to be motile?
Flagella
either surrounding the cell or clumped at one end of at both ends
Which classification of bacteria contains fimbriae?
Gram-negative bacteria
What are Gram-negative bacteria are surrounded by?
Hair-like structures known as fimbriae
Gram-negative bacteria are surrounded by hair-like structures called fimbriae. What do these do?
Aid adhesion onto particular surfaces
What are sex pili?
Tubes that join two cells together during conjugation
Used by gram-negative bacteria to exchange genetic material
Which classification of bacteria have sex pili?
Gram-negative bacteria
What are some bacteria enclosed by which they use to protect themselves?
A capsule
What is the function of a capsule that some bacteria possess?
Protects the bacterium, even within phagocytes
Helps prevent the cell from being killed
How does a capsule around a bacterium appear?
Revealed by negative staining
Appear as a halo around the producer cell
What can some bacteria produce to help them stick to surfaces?
Slime
What does the slime produced by Streptococcus mutans do?
Enables the bacterium to stick to the surface of teeth, where it helps to form plaque, leading eventually to dental caries
What is the slime produces by bacteria usually made up from?
Polysaccharides
Which bacteria causes infections associated with implanted plastic medical devices?
‘Coagulase-negative’ staphylococi
‘Coagulase-negative’ staphylococci is associated with implanted plastic medical devices. Why?
It lives on the skin and some strains produce a slime that enables them to stick to plastics
A few bacteria have the ability to produce highly resistant structures. What are these known as?
Endospores (or simply, spores)
Endospores are produced by some bacteria and they are highly resistant. What do they resist?
A range of hazardous environments, and protect against heat, radiation, and desiccation (extreme dryness)
How may an infection spread person-person?
- inhalation of infectious droplets
- faecal-oral route
- sexual transmission
- direct inoculation
- animals
- fomites
How are airborne infections spread?
Through inhalation of infectious droplets
Give an example of an airborne infection that is spread through inhalation of infectious droplets
Many viruses
eg. the common cold viruses
“coughs and sneezes spread diseases”
What is the faecal-oral route a common means of spreading?
Gastrointestinal infections
- typhoid
- cholera
- dysentery
- hepatitis A
- poliomyelitis
What is dysentery
Infection of the intestines resulting in severe diarrhoea with the presence of blood and mucus in the faeces
Many gastrointestinal infections are commonly spread through the faecal-oral route. What does this mean?
Acquired through drinking water contaminated with human faeces for example
The pathogens that cause sexually transmissible infections are vulnerable. What does this mean?
They rapidly die when exposed to conditions outside the body
In order to spread, these pathogens require the most intimate of human contact
Which type of pathogens are ‘vulnerable’
Pathogens that cause sexually transmissible infections - they cannot survive outside of the body
Who is most at risk from acquiring infections by direct inoculation?
Intravenous drug users who share dirty needles
Give examples of infections that can spread by direct inoculation
Hepatitis and HIV
Why is donated blood screened?
It is screened for blood-borne infections to minimise risk of infection following transfusion
Give an example of an infection that is spread by animals. What animal is it spread by?
Malaria
Malaria is spread by the female anopheles mosquito
Malaria is spread by the female anopheles mosquito. What is malaria caused by?
Protists of the genus Plasmodium
Animals can be the reservoir for bacteria. What bacteria can they hold and what are such infections referred to as?
Bacteria such as Salmonella enterica
Such infections are referred to as “zoonoses”
What is a zoonosis?
An infection which can be transmitted from animals to humans
The animals act as a reservoir for the bacteria
As well as humans, animals and microorganisms, what else may act as the vector for infection?
Inanimate objects such as paper, pens, surgical instruments etc.
What are “fomites”
Any object capable of carrying infectious organisms and hence transferring them.
Skin cells, hair, clothing, and bedding are common hospital sources of contamination
What does controlling infection depend upon?
- identifying the mode of spread
- interrupting the cycle of infection, replication, and spread
What does the cycle of infection require?
- pathogenic microorganisms to encounter and adhere to a host
- to multiply within the host
- to be dispersed to encounter other hosts
What does a pathogen need more of, the deeper it penetrates into the body?
The deeper pathogens penetrate into the body, the greater the defences they need to protect them from our immune defences
What are the traits that pathogens use to complete the cycle of infection referred to as?
Virulence factors
What is unusual about cases of intoxicating illnesses such as tetanus, botulism and ergotism?
The victim does not need to encounter live microorganisms as the disease results from exposure to a toxin (exotoxin) rather than a living micro-organism
“Lockjaw” is an alternative name for which disease?
Tetanus
Since the toxin causes muscles to go rigid, firstly those of the face
By what mechanisms do bacteria causes disease?
- production of structures that enable the microorganism to attach to the surface at which they cause the disease
- production of toxins
- production of aggressins
- initiating undesirable consequences of the host defences
In what way do microorganisms produce structures that enable the microorganism to attach to the surface at which they cause disease?
- pili and fimbriae (hair-like structures that enable bacteria to stick to surfaces through a specific mechanism)
- slime (eg. produced by streptococcus mutans that allows it to stick firmly to the enamel of the tooth)
What two types of toxins may bacteria produce?
- exotoxins (such as that responsible for tetanus)
- endotoxins (the cause of Gram-negative shock)
What is a circulating immune complex?
A combination of soluble antigen produced by bacteria and antibodies.
How can circulating immune complexes cause problems?
They can become trapped in vessels and compromise their function - an example = glomerulonephritis, kidney damage following streptococcus pyogenes infection
How does the autoimmune disease rheumatic fever result?
Following streptococcal pyogenes infection, antibodies raised against the bacterial antigens cross-react with the antigens produced by human tissue, leading to autoimmune disease
What is an aggressin?
Any substance, produced in the body by a pathogenic bacterium, that enhances the virulence of the bacteria
What is the characteristic lesion of tuberculosis called?
The tubercle
How does mycobacterium tuberculosis produce disease?
By over-stimulation of our immune response to infection - the tubercle consists of giant cells and activated macrophages and lymphocytes
What does the tubercle in tuberculosis consist of?
Mixture of giant cells formed by the fusion of several macrophages, as well as activated macrophages and lymphocytes
