Microbiology Flashcards
Define pathogen.
Any type of microorganism that causes diseases.
Define colonisation.
Presence of microorganism in host but it isn’t causing disease.
Define infection.
Microorganism is present in the host, and it is causing disease.
What is an opportunistic infection?
Microorganisms that cause disease when the host defence is compromised.
When can host defence be compromised?
- Those on immunosuppressant drugs.2. Those who are immunocompromised.3. Those with a breakdown of host defence.
Define virulence.
The severity or harmfulness of a microorganism.
Which eight body sites are usually sterile? And which infection do they have if not sterile?
Blood (sepsis), CSF (meningitis), Pleural fluid (pericarditis, pleural effusion), Peritoneum (peritonitis), Joints (septic arthritis), Urinary tract (UTIs), Lower resp tract (TB + pneumonia).
What is the difference between gram negative and gram positive bacteria?
Gram positive: Peptidoglycan is in thick cell wall.Gram negative: Peptidoglycan is thin in cell wall.
How do gram positive and negative bacteria stain?
Gram positive - Stains purpleGram negative - Stains pink
Main shape of gram positive bacteria?
Cocci.
Main shape of gram negative bacteria?
Bacilli.
Which environment do gram positive bacteria prefer?
Dry and dusty environments.
Which environment do gram negative bacteria prefer?
Wet and damp environments.
Mnemonic for gram staining?
‘Come in and stain’1. Crystal violet2. Iodine3. Alcohol4. Safranin
What are the stages for gram staining?
- Stain with crystal violet (everything turns purple).2. Mordant application, iodine (crystal violet-iodine complex formed).3. Decolourisation with alcohol (distinguishes gram -ve and +ve)4. Counterstain, safranin (gram -ve goes pink)
Why do gram positive bacteria stain purple and gram negative bacteria stain pink?
Peptidoglycan layer soaks up the purple crystal violet.If it’s thin it will be decolourised by alcohol (gram -ve).If it’s thick it cannot be washed away and will remain purple (gram +ve).
What are the seven main types of agar and their uses?
- Blood - Non-selective agar.2. Chocolate - fastidious neisseria.3. MacConkey - lactose status.4. CLED - Stops motile proteus swimming/lactose status.5. XLD - Isolation of salmonella and shigella. (Both ferment and go red, salmonella also has black dots).6. Gonococcus - For neisseria.7. Sabourard - fungal.
What does the Ziehl-Neelsen test identify?
Acid-fast organisms, mainly mycobacteria (such as TB).
What does the catalase test differentiate and the results?
Differentiates between staphylococcus and streptococcus.Catalase +ve = StaphylococcusCatalase -ve = Streptococcus
What is the method for the catalase test?
Apply hydrogen peroxide 3% to a small sample of pure colony. Observe for bubbling.Any weak bubbling = POSITIVE (staph)
What does the coagulase test differentiate and what are the results?
Differentiates staphylococcus aureus from other staph (S. epidermis and S. saprophyticus).Coagulase +ve = Staph aureusCoagulase -ve = Staph epidermis and S. saprophyticus
What is the method for the coagulase test?
Apply rabbit plasma to a small sample of pure colony. Observe for fibrin clots.Clumps = POSTIVE (S. aureus)
What is used for bacteria that are catalase negative?
Alpha beta gamma haemolysis on blood agar.
What does alpha beta gamma haemolysis test for and what are the results?
Uses hydrogen peroxide to test reaction with haemoglobin.Alpha = PartialBeta = CompleteGamma = No change
What test is used when it’s alpha haemolysis to distinguish bacteria?
Optochin test.
What is the optochin test?
A chemical test used for after alpha haemolysis.S. pneumoniae is susceptible.All other streps are resistant (S. viridans).
What test is used for after beta and gamma haemolysis?
Lancefield grouping (A-G)
What test is used after gamma haemolysis?
Lancefield D - Enterococcus
What test is used after beta haemolysis?
Lancefield A, B, C.Lancefield A - Strep. pyogenesLancefield B - Strep. agalactiaeLancefield C - Strep. dysgalactiae
What are the types of gram positive bacteria?
Anaerobe or aerobic.
What is an example of a gram positive anaerobic bacilli?
Clostridium - C. difficile.
What is an example of a gram positive aerobic bacilli?
Bacillus - B. cereus.Corynebacterium - C. diptheriae (diphtheria).
What is the appearance of gram-negative bacteria on MacConkey agar (or CLED/XLD)? What do these results mean?
Pink - Lactose fermentingWhite - Non-lactose fermenting
What are examples of lactose fermenting and non-lactose fermenting gram-negative bacteria?
Fermenting - E. coli, klebsiella.Non-fermenting - Shigella, salmonella, proteus, pseudomonas.
Why would you perform an oxidase test on non-lactose fermenting gram negative bacteria?
To distinguish pseudomonas from the others (shigella, salmonella and proteus).Positive = pseudomonas
What are the three main mechanism of actions of antibiotics?
- Inhibit cell wall2. Inhibit protein synthesis3. Inhibit nucleic acid synthesis
Which classes of antibiotics inhibit cell wall synthesis? Give examples.
Glycopeptides (vancomycin) and beta lactams (penicillins)
Which classes of antibiotics inhibit protein synthesis? Give examples.
- Chloramphenicol2. Macrolides (clarythromycin)3. Tetracyclines (doxycycline)4. Aminoglycosides (gentamicin)
Which class of antibiotics is used for people with a penicillin allergy?
Macrolides
What are the 4 mechanisms that inhibit nucleic acid synthesis?
- Inhibit folate synthesis2. Inhibit DNA gyrase3. Bind to RNA polymerase4. DNA strand breaks
Which antibiotic breaks DNA strands?
Metronidazole
Which antibiotic binds to RNA polyermase?
Rifampicin
Which antibiotic inhibits DNA gyrase?
Ciprofloxacin
Which antibiotics inhibit folate synthesis?
Sulphonamides (sulphamethoxazole) and trimethoprim.
Give a description of mycobacteria.
Atypical bacillus, slow growing and cause gradual onset diseases. Acid fast.
What is a characteristic feature of all mycobacteria?
Acid-fastness.
What is an example of mycobacteria?
TB.
What are some symptoms of TB?
Night sweats, cough, haemoptysis, weight loss, malaise.
What are the treatments for active TB?
4 antibiotics:-For the first 6 months: isoniazid, rifampicin.-For first two months: ethambutamol, pyrazinamide.
What are the treatments for TB and their side effects?
4 antibiotics:-For the first 6 months: isoniazid (peripheral neuropathy), rifampicin (orange/red urine).-For first two months: ethambutol (ocular effects), pyrazinamide (joint pain).
What is used to stain TB and why?
Ziehl-Neelsen stain. This involves heating a sample with carbol fuschin.Acid-fastness is a characteristic property of mycobacteria.Heat makes bacteria resistant to decolourisation by acid. When flooded with acid, mycobacteria stay pink while everything else decolourises to blue.
What are protozoa?
Microscopic unicellular eukaryotes (living or parasitic).
What is an example of protozoa?
Malaria.
How is malaria spread?
Spread through bites from female anopheles mosquitos that are vectors.
What does malaria do once in the bloodstream?
Infects and destroys mainly liver cells and RBCs.
What symptoms does malaria cause?
Non-specific: Fever, sweats, headache, myalgia, N+V.Specific: Pallor due to anaemia, hepatosplenomegaly, jaundice.
Typical history of someone with malaria?
History of recent travel, history of recent bite, non-compliance with prophylaxis.
What is the prophylaxis for malaria?
Be aware of high-risk location.Use mosquito spray, nets and barriers.Taking antimalaria medication.
What are the species that cause malaria?
Plasmiodum falciparum.P. malariaeP. vivaxP. ovaleP. knowlesi
Explain the life cycle of malaria.
- Spread by female anapholes mosquitos.2. Malaria in blood reproduces in mosquito gut and produces sporozoites (malaria spores).3. Sporozoites spread to hosts and travel to liver.4. Mature into merozoites which enter blood and infect RBCs.5. In RBCs, merozoites reproduce over 48 hours causing them to rupture and spread more merozoites in blood - causing haemolytic anaemia.6. This is why people infected with malaria have high fever spikes every 48 hours.
Which malaria causing species lie dormant in the liver and in what form?
P. vivax and P. ovale.Lie dormant as hypnozoites.
What is fungi?
A eukaryote with a cell wall made of chitin and glucans.
How do fungi move?
By growing across or through structures by dispersion in air or water.
What are the two forms of fungi?
Yeast - Single cell that divide via budding.Moulds - Form multicellular hyphae or spores.
What are some examples of fungal infections in humans?
Only a few cause infection in humans - nappy rash, athlete’s foot.
What is the treatment for fungal infections?
Antifungals - target cell wall/plasma membrane.However, these don’t work that well and are generally difficult to treat. (-azole drugs).
Are worm infections common?
Common worldwide but rare in the UK.
What are the three groups of worms?
Roundworms, flatworms, tapeworms.
What is the pre-patent period in relation to worms?
Interval between infection and appearance of eggs/larvae in stool.
What is the immune response for worms?
Poor immune response. IgG and IgE mediated.
What is an example of group A strep?
S. pyogenes.
Which antibiotic is used to treat MRSA?
Vancoymycin.
What are the colours of gram negative bacteria on MacConkey agar? What is their significance?
Pink (lactose fermenting) and white (non lactose fermenting).
Give 2 examples of gram negative lactose fermenting bacteria.
E. coli, klebisella.
Give 4 examples of gram negative non-lactose fermenting bacteria.
Shigella, salmonella, proteus, pseudomonas.
What are viruses dependent on?
The host cells.
Briefly describe the structure of viruses.
No organelles or cell wall.
How severe are viral infections in general?
Usually self limiting, antivirals can be given if severe.
Can viruses be identified on light microscopy or cultures?
They can’t be seen because they’re too small.-They need host cells to live so can’t be cultured.
What are the four main methods to identify viruses?
PCR, serology and NAAT (nucleic acid amplification test), electron microscopy.
Which three HIV markers are tested in the blood?
p24 antigen, HIV Ig’s, HIV RNA.
If meningitis is suspected, what is taken?
A CSF sample.
What does the VZV virus cause?
Shingles - painful red rash.
What is the most common cause of infective endocarditis?
Staph aureus.
Which infective endocarditis causing bacteria is associated with poor dental hygiene?
S. viridans
What is a cause of infective endocarditis in those who are immunocompromised?
Fungal - candida albicans.
How is staph aureus treated in infective endocarditis?
Flucloxacillin.Vancomycin if MRSA.
How is strep viridans treated in infective endocarditis?
IV benzylpenicillin 4-6w.+/- gentamicin 2w.
Once a gram bacteria is identified, what is done and why?
MacConkey/CLED/XLD agar.-To find out lactose fermenting status.
Is electron microscopy used for viral infections?
No, too long.
Which four antibiotics cause C. diff infections?
Cephalosporins, co-amoxiclav, clindamycin, ciprofloxacin.
What are the symptoms of C. diff associated disease?
Recent use of antibiotics with abdominal pain, diarrhoea and fever.
How are C. diff infections treated?
Vancomycin/metronidazole.
Which agar is alpha/beta/gamma haemolysis done on?
Blood agar.
How does s. aureus appear?
Gold and round.
What is a virus?
Non-living infectious intracellular parasite with genetic material and protein coat. They depend on the host cell.
What are three common virus shapes?
Helical, icosahedral and complex.
What are the eight steps of viral life cycle?
- Attachment.2. Entry.3. Genome released.4. Transcription.5. Translation.6. Genome replication.7. Assembly.8. Exit.
In which five ways do viruses cause disease? (with examples).
- Direct destruction (poliovirus).2. Host cell modification (rotavirus).3. Overreactive immune system (HBV).4. Damage through cell proliferation (HPV).5. Evasion of host defences (VZV).
How are worms spread?
Faeco-oral transmission.
What is schistosomiasis?
Worm infection contracted by freshwater swimming or drinking in endemic areas.-Can cause bladder cancer.
