Microbiology Flashcards
Define pathogen.
An organism capable of causing disease.
Define commensal.
An organism that colonises the host but causes no disease.
Define opportunist pathogen.
An organism that only causes disease if host defences are compromised.
Define virulence.
The degree to which a given organism is pathogenic.
What would gram positive bacteria look like down the microscope?
Purple/blue.
What would gram negative bacteria look like down the microscope?
Red/pink.
What type of organism would you stain with Ziehl Neelsen?
Mycobacteria e.g. TB.
How do you apply the gram stain?
Apply crystal violet to heat fixed bacteria. Treat with iodine. Decolourise the sample and then counterstain.
Describe the characteristic features of gram positive bacteria?
- Single membrane.
- Large peptidoglycan area.
Describe the characteristic features of gram negative bacteria?
- Double membrane.
- Small peptidoglycan area.
- LPS (endotoxin area).
Between what temperatures and what pH range can bacteria grow?
Between -80 to +80°C. And from a pH of 4 to 9.
What are the 3 phases of bacterial growth?
- Lag phase.
- Exponential phase.
- Stationary phase.
Give an example of a slow growing bacteria.
TB.
Give an example of a fast growing bacteria.
E.coli and S.aureus.
Give 2 functions of pili.
- Help adhere to cell surfaces.
- Plasmid exchange.
What is the primary function of flagelli?
Locomotion.
What is the primary function of the polysaccharide capsule?
Protection; prevents MAC or opsonisation molecules attacking.
What types of bacteria release endotoxin?
Gram negative.
What types of bacteria release exotoxin?
Gram positive and gram negative.
Describe endotoxins.
Endotoxin (LPS) is an outer membrane component released when bacteria are damaged. They are less specific and are toxic to the host. They are heat stable.
Describe exotoxins.
Proteins secreted from gram positive and gram negative bacteria. They are specific and heat labile.
What are endotoxins made from?
Lipopolysaccharides (LPS).
What are exotoxins made from?
Proteins.
What are plasmids?
Circular pieces of DNA that often carry genes for antibiotic resistance.
Give an example of a gram negative diplococci?
Neisseria e.g. N.meningitidis and N.gonorrhoeae.
Give an example of a gram positive cocci?
Staphylococcus and streptococcus.
What test can be done to distinguish between staphylococcus and streptococcus?
The catalase test; detects the presence of catalase enzyme using hydrogen peroxide. Staph = catalase + ve.
Strep = catalase - ve.
How would you describe the arrangement of staphylococci?
Clusters of cocci.
How would you describe the arrangement of streptococci?
Chains of cocci.
What test could be done to further distinguish between staphylococci bacteria.
Coagulase test; looks at whether a fibrin clot is produced.
What bacteria would be coagulase positive?
Staphylococci aureus.
What bacteria would be coagulase negative?
All others e.g. staphylococci epidermidis.
What test could be done to distinguish between different streptococci?
Blood agar haemolysis.
What further test can be done for those streptococci in the β haemolysis group?
Serogrouping; detecting surface antigens. e.g. lancefield grouping.
What would you see on the agar plate in α haemolysis and give an example of a bacteria in this group.
α haemolysis is partial erythrocyte lysis; you see a green colour. Streptococcus pneumoniae falls in this group.
What would you see on the agar plate in β haemolysis and give an example of a bacteria in this group.
β haemolysis is complete erythrocyte lysis; you see a clear area. Streptococcus pyogenes and streptococcus agalactiae fall in this group.
What would you see on the agar plate in γ haemolysis and give an example of a bacteria in this group.
γ haemolysis is when there is no haemolysis. Streptococcus bovis falls in this group.
Give examples of gram negative bacilli.
Shigella, salmonella, E.coli etc.
Give examples of gram positive bacilli.
Clostridium, bacillus, cornyebacterium etc.
What kind of bacteria is MacConkey agar used with?
Gram negative bacilli.
What is MacConkey agar?
MacConkey agar contains bile salts, lactose and pH indicator. If an organism ferments lactose, lactic acid will be produced and the agar will appear a red/pink colour.
Name 2 gram negative bacilli that will give a positive result with MacConkey agar.
- E.Coli.
- Klebsiella pneumoniae.
Where in the body might you find staphylococci?
Nose and skin.
How is staphylococcus aureus spread?
Aerosol and touch.
Give 4 virulence factors of staphylococci?
- Toxins.
- Proteases.
- Toxic shock syndrome toxin.
- Protein A.
How is c.diptheriae spread?
Droplet spread.
Does shigella have a H antigen?
Shigella is non motile and doesn’t have flagellum. It therefore doesn’t have a H antigen.
What category of bacteria does shigella fall under?
Gram negative bacilli.
Does shigella give a positive result with MacConkey agar?
No. Shigella does not ferment lactose and so gives a negative result.
Does salmonella have a H antigen?
Salmonella is motile and has a flagellum; it therefore does have a H antigen.
Does salmonella give a positive result with MacConkey agar?
No. Salmonella does not ferment lactose and so gives a negative result.
Does e.coli have a H antigen?
E.coli is motile and has a flagellum; it therefore does have a H antigen.
Does e.coli give a positive result with MacConkey agar?
Yes. E.coli does ferment lactose and so you would see a red/pink colour indicating a positive result.
How can you distinguish between gram negative bacilli (salmonella, shigella and e.coli)?
Use MacConkey agar and use serology to detect the presence of the H antigen.
Why are there pathogenic strains of e.coli?
Due to the acquisition of genes from other bacteria.
Which type of e.coli would you associate with causing travellers diarrhoea?
Enterotoxigenic e.coli (ETEC).
Describe the process by which enterotoxigenic e.coli (ETEC) causes traveller’s diarrhoea?
Heat labile ETEC toxin modifies Gs protein, it is in a ‘locked on’ state. Adenylate cyclase is activated and there is increased production of cAMP. This leads to increased secretion of Cl- into the intestinal lumen, H2O follows this down an osmotic gradient and this subsequently results in traveller’s diarrhoea.
What effect do enteropathogenic and enterohaemorrhagic e.coli (EPEC and EHEC) have on the mucosa?
They adhere to microvilli, rearrange actin, and lead to pedestal formation.
What are the symptoms of enteropathogenic e.coli infection?
Chronic watery diarrhoea.
What are the symptoms of enterohaemorrhagic e.coli infection?
Bloody diarrhoea.
What are the symptoms of shigella infection?
Severe bloody diarrhoea and frequent passage.
How is shigella passed on?
Via contaminated food/water or from person to person.
Shigella is acid tolerant. Why is this advantageous for shigella?
It means shigella can pass through the stomach without being destroyed by the low gastric pH. It can then move onto the intestine.
What is the action of shigella in the intestine?
In the intestine it induces self uptake and leads to macrophage apoptosis. Cytokines are released and neutrophils are attracted = inflammation. Shigella spread to adjacent cells.
What bacteria is responsible for salmonellosis?
S.enterica.
Name the 3 forms of salmonellosis.
- Gastroenteritis.
- Enteric fever.
- Bacteraemia.
What is gastroenteritis?
Frequent cause of food poisoning, 24 hour incubation period. Highly infective dose.
What is enteric fever?
Enteric fever: typhoid fever. Systemic disease.
What are the symptoms of v.cholerae?
Huge volumes of watery stools (no blood or pus).
Why is v.cholerae so dangerous?
You’re losing huge amounts of water which can result in hypovolemic shock and severe dehydration, this can lead to death.
Why is v.cholerae not killed if you have a fever?
It grows at 18 - 42°C.
Why would you need to be infected with a large amount of v.cholerae to show symptoms of the disease?
The optimum pH for v.cholerae growth is 8; alkaline. It is therefore very sensitive to the pH of the stomach.
How would you grow haemophilus influenzae?
On chocolate agar as it requires haem and NAD.
What diseases can haemophilus influenzae cause?
Meningitis and pneumonia.
Describe the pathogenesis of gastro-enteritis.
- Endocytosis.
- Chemokine release.
- Neutrophil recruitment and migration.
- Neutrophil induced tissue injury.
- Fluid and electrolyte loss -> diarrhoea.
Describe the pathogenesis of enteric fever.
- Endocytosis.
- Migration to the basolateral membrane.
- Survival in macrophage -> systemic spread.
Name the bacteria that can cause legionnaires disease?
Legionella.
Who might be susceptible to infection by legionella?
Immunocompromised individuals.
What type of bacteria are Neisseria?
Gram negative diplococci.
What are the two medically important species of neisseria?
N.meningitidis and N.gonorrhoeae.
How is N.meningitidis transmitted?
Aerosol transmission. High risk in colonised people e.g. university, Haj.
Describe the pathogenesis of N.meningitidis.
Crosses nasopharyngeal epithelium and enters blood stream. Can cause asymptomatic bacteraemia or septicaemia. If the bacteria crosses the BBB it can cause meningitis.
What are the virulence determinants of N.meningitidis?
- Capsule; anti-phagocytic.
- Pili; adherence to host cell.
- LPS.
What are the consequences of N.gonorrhoeae infection?
STI - rectal, vaginal or oral inflammation.
Describe bacteroides.
Opportunistic, obligate anaerobes.
Can you grow chlamydia on agar?
No, chlamydia is an obligate intracellular parasite.
How can you detect chlamydia?
Serum antibodies or PCR.
What are the 2 developmental stages of chlamydia’s unique growth cycle?
- Elementary bodies (infective).
- Reticulate bodies (intracellular multiplication).
- Reticulate bodies are converted back into elementary bodies and are released. The cycle continues.
What can c.trachomatis cause?
Chlamydia; the most common STI.
Name 2 bacteria in the chlamydophila genre.
- C.pneumoniae - respiratory tract infection.
- C.psittaci - associated with birds.
Describe the flagellum of a spirochaete.
Spirochaete’s have an endoflagellum, it lies between the inner and outer membrane.
Name the spirochaete that is responsible for causing lyme disease.
B.burgdorferi.
Name the spirochaete that is responsible for causing syphilis.
T.pallidum.
Describe the three stages of syphilis.
- Primary stage: localised infection.
- Secondary stage: systemic - skin, lymph nodes etc.
- Tertiary stage: CV syphilis and neuro syphilis.
Describe yeast.
Single celled organism. Asexual reproduction.
Describe mould.
Multicellular organism. Reproduce by spore formation.
What are dimorphic fungi?
Fungi that can exist as both yeast and mould; they are yeast in tissues but mould in vitro.
Give an example of a dimorphic fungi.
Coccidioides immitis.
Why are severe fungal infections rare?
Fungi are unable to grow at 37°C and are often killed by the innate and adaptive immune response.
Name 3 common fungal infections.
- Nappy rash.
- Tinea pedis.
- Onychomycosis (fungal nail infection).
Name a drug that is good for treating onychomycosis.
Terbinafine - it reaches poorly perfused sites e.g. nails.
What is the fundamental principle behind antifungal treatments?
Selective toxicity!
What can antifungal treatments target?
- Fungal cells walls; they contain polysaccharides and chitin.
- Ergosterol containing plasma membrane.
Antifungal treatments: how does amphotericin work?
It targets ergosterol in the plasma membrane and causes pore formation, this leads to cell death.
Antifungal treatments: how do azoles work?
They affect the ergosterol synthetic pathway.
Give 4 disadvantages of azoles.
- High first pass metabolism, bioavailability = 45%.
- ADR’s, can cause hepatitis.
- Drug interactions due to CYP450.
- Resistance can develop e.g. in candida.
What is candida?
A yeast. It grows in warm, moist areas and has high levels of β-D-Glucan.
What test can be done to identify fungal antigens?
β-D-Glucan test.
What fungal infection can often be a presenting factor for HIV?
Pneumocystis pneumonia; opportunistic infection, can cause lung infection in immunocompromised people.
Why is pneumocystis pneumonia not commonly found in the lungs of healthy people?
It is opportunistic and so can cause disease in immunocompromised individuals.
Give an example of a mould.
Aspergillus fumigatus.
Aspergillus niger.
What are moulds composed of?
Branched filamentous filaments called hyphae.
Describe mycobacteria.
- Aerobic.
- Non-motile.
- Non spore forming.
- Bacilli.
Give an example of mycobacteria.
M.tuberculosis (TB).
M.leprae (leprosy).
Why is it hard to use the gram stain on mycobacteria?
The cell wall is very thick and has a high lipid content.
Why is it hard to use therapeutic antibodies against mycobacteria?
Mycobacteria grow very slowly and so treatment with antibodies is difficult. (This also makes them hard to culture).
How would you stain mycobacteria?
Using Ziehl-Neelsen stain for acid fast bacili.
How could you detect whether an individual has had previous exposure to TB?
- Tuberculin skin test (mantoux).
- Interferon gamma release assays.
Name 6 sterile sites in the body.
- Urinary tract.
- CSF.
- Pleural fluid.
- Peritoneal cavity.
- Blood.
- Lower respiratory tract.
Where in the body would you find normal flora (commensals)?
- Mouth.
- Skin.
- Vagina.
- Urethra.
- Large intestine.
Give an advantage and a disadvantage of the slide coagulase test as opposed to the tube test.
Advantage: quicker and easier.
Disadvantage: less sensitive as it only detects bound coagulase and not free coagulase too.
What colour do staphylococcus aureus colonies appear on blood agar?
Creamy/yellow.
Which Lancefield groups are associated with tonsilitis and skin infection?
A , C and G.
Which Lancefield groups are associated with neonatal sepsis and meningitis?
B.
Which Lancefield groups are associated with UTI’s?
D.
What is the oxidase test?
Detects the presence of cytochrome oxidase in bacteria. A positive test is indicated by the disk turning blue.
Which group of streptococci can cause infective endocarditis?
Alpha haemolytic streptococci.
How can you differentiate streptococci pneumoniae from other streptococci?
The optochin test can differentiate streptococci pneumoniae from other streptococci. Pneumococci are sensitive and so a clear area would be seen.
Describe chocolate agar and explain why it might be used.
Chocolate agar is blood agar that has been heated so as to release nutrients. Chocolate agar is often used for growing fastidious bacteria.
What is the function of bile salts in MacConkey agar?
They inhibit gram positive bacteria growth.
What is CLED agar used for?
It is used to differentiate micro-organisms in urine and can classify lactose fermenters and non-lactose fermenters.
What is Gonococcus agar used for?
It contains growth factors to promote the growth of Neisseria. It also contains antibiotics and antifungal agents to inhibit growth of other organisms.
What is XLD agar used for?
It is a very selective growth medium used to isolate salmonella and shigella. Salmonella shows black dots.
What is Sabouraud’s agar used for?
Used to culture fungi.
Why do bacteria produce coagulase?
They use it as a defence mechanism by clotting the areas of plasma around them, thereby resisting phagocytosis.
What are protozoa?
Single celled eukaryotic organisms.
What genus of parasitic protozoa is responsible for causing malaria?
Plasmodia spp.
How is malaria transmitted?
Via the bite of female mosquitos from dusk till dawn.
Why are there different clinical manifestations of malaria?
The difference in clinical manifestation can be due to variation in the plasmodia life cycle. The plasmodia life cycle has stages in the human and the mosquito.
What are the stages of the plasmodia life cycle in the human called?
Exo-erythrocytic and endo-erythrocytic stages.
What happens in the stages of the plasmodia life cycle that occur inside the human?
Exo-erythrocytic: Hepatcoytes become infected by sporozoites, the cells mature and develop and are released as tropozites.
Endo-erythrocytic: tropozites invade RBC’s. Parasite numbers expand rapidly with a sustained cycling of the parasite population.
What 2 species of the plasmodia genus lie dormant and cause late relapse of malaria?
P.ovale and p.vivax.
What signs and symptoms might you see in someone who has been infected with malaria?
Fever, haemolysis, chills, sweats, headaches etc.