Microbiology 1 Flashcards
What are the two main shapes of bacteria?
Bacillus and coccus
What colour is each gram stain?
Gram +ve: blue/purple
Gram -ve: red/pink
Describe a gram+ve and -ve cell wall.
+ve: a large outer peptidoglycan layer with a single cytoplasmic membrane.
-ve: inner membrane, thin peptidoglycan layer superficial to this, then a second outer membrane with a large lipopolysaccharide layer. Both may have capsules.
What is the function of a capsule and extracellular slime (biofilm)?
Capsule: avoid phagocytosis and aid attachment to host cells. Biofilm: can facilitate the attachment of bacteria onto artificial surfaces.
What do fimbriae do?
Aid in attachment to host cells and adhere to one another.
Describe the genetic machinery in a bacteria.
Single loop of double stranded, chromosomal DNA. Plasmids carrying extra-chromosomal DNA that confers resistance. 30s/50s ribosomes.
What are spores and when do they form?
Small, dormant cells with a thick coat that are highly resistant and can remain dormant for years. Formed in response to adverse conditions.
Give the 4 types of oxygen requirements for bacteria.
Strict aerobes, microaerophilic, strict anaerobes, facultative anaerobes.
Describe how Staph appear on a gram stain.
Purple cocci in clusters.
Is staph aureus a commensal?
Yes - about 30-50% in noses.
What is used to differentiate Staphs?
Coagulase test. +ve: s. aureus -ve: s.epidermidis and s. saprophyticus.
Give 4 things that contribute to s.aureus pathogenicity.
Coagulase (form a fibrin coat+avoid phagocytosis), toxic shock syndrome protein, adheres to cells and forms toxins that can make pores in cells.
Where does s. epidermidis colonise?
Skin.
What is s. epidermidis main virulence factor?
Ability to form a biofilm.
What does s. epidermidis commonly infect?
Prosthetics and catheters .
How do Strep appear on a gram film?
Purple cocci in chains.
What is alpha and beta haemolysis, and what is Lancefield typing?
Alpha: partial digestion of blood agar (appears green) Beta: complete digestion of blood agar. Lancefield is antigen testing to differentiate different types of B haemolytic Step. (catalase negative bacteria)
What type of haemolysis do strep pneumoniae and strep viridans show? How would you differentiate between them?
Alpha haemolysis. Optochin disk differentiates - S.pneumoniae is sensitive and S.viridans isn’t.
What infections is Strep viridans associated with?
Infective endocarditis and dental caries.
What haemolysis does Strep pyogenes show? What infection is it known for?
Beta. Tonsillitis and pharyngitis (i.e strep throat!), cellulitis.
How would you differentiate between Shigella sonnei, Salmonella enterica and E coli?
MacConkey agar: Red (lactose fermenting) is E coli, Pale (non-lactose fermenting) is either Shigella or Salmonella. XLD plate - Shigella red, salmonella red with black centres.
What are the 4 main strains of E.coli, and what infections do they cause?
Enterohaemorrhagic E. coli - dysentery. Enterotoxigenic E. coli - travellers diarrhoea. Enteropathogenic E. coli - chronic, watery diarrhoea. Enteroinvasive E. coli - dysentery.
What are the two main species of salmonella and what infections do they cause?
S. typhi (typhoid fever), S. enterica (salmonellosis)
What oxygen requirements does V cholerae have and does it cause dysentery? Why is a high dose needed to infect?
Facultative anaerobe and no. High dose due to sensitivity to stomach acid.
What is p.aeruginosa known for causing chronic infections in?
Cystic fibrosis and COPD patients.
What is the morphology of H. influenzae? What form is more virulent?
Gram -ve baccillococcus. The capsulated form is more virulent.
How would one grow H. influenzae on agar?
Adding factors X+V to chocolate agar.
Where is L. pneumophilia found?
Stagnant, warm, manmade water environments.
What bacterium causes pertussis (whooping cough)? Describe briefly.
Bordatella pertussis. Bacillus, low infective dose, transmitted in air.
What are the two important strains of Neisseria?
N. meningitidis and N. gonorrhoeae.
What shape are the two strains of Neisseria?
Cocci (only gram -ve cocci we need to know)
Give 2 virulence factors of N. meningitidis.
Capsule to avoid phagocytosis and endotoxin that causes inflammatory cytokine cascades.
What is the main cause of food poisoning in the U.K?
Camplyobacter jejuni and Camplyobacter coli.
How would one grow the main causes of food poisoning?
On CCDA-ve charcoal plate.
What shape is H.pylori?
Curved bacillus.
Outline the basic structure of a virus.
Contains either DNA/RNA, not both. No cell wall - instead have a ‘capsid’ which encloses core proteins, nucleic acid and enzymes. May have an envelope surrounding the capsid that presents glycoproteins to the host cells.
Describe the stages in viral replication.
Virus attaches to cell receptors. Viral core (containing genome and core proteins) are released. Virus uses cell’s genetic machinery for its replication and the virus is assembled within the cell. Then released by lysis or exocytosis.
How does influenza enter a cell?
Forms a vacuole and the virus remains intact. The virus is attached to the surface of the vacuole and the core is released.
What are the 5 mechanisms by which viruses can cause disease? (and examples)
Direct cell and tissue damage. (poliovirus)
Functional/physical modification of cells. (HIV)
Cell proliferation. (HPV)
Host overreacting to the infection. (Hep B)
Evading host defences.
What are the 2 cellular methods that viruses resist host defences?
Releasing proteins that influence CD4/CD8 cells and can be latent (hidden)
What are 3 genetic methods that viruses resist host defences?
Antigenic variability (influenza) Prevention of host cell defences (HIV) Downregulation of interferons (most viruses)
What are the 3 mechanisms of viral genetic variability?
Quazi-species being made continuously in the body
Frequent mutations
Sheer number of stable phenotypes
Are protozoa are eukaryotes?
Yes
What are the 3 main groups of eukaryotes?
Flagellates, amoebae and sporozoans.
Give 2 flagellates and the diseases they cause?
Giardia intestinalis (giardiasis) and trypanosome (African sleeping sickness)
Give an example of an amoeba and the infection it causes.
Entamoeba histolytica (ameobiasis)
What are the 2 main sporozoans?
Toxoplasma gondii and plasmodium
What is malaria caused and transmitted by?
Plasmodium. Transmitted by female anopheles mosquito.
Give 5 clinical features of malaria, with one of which being most indicative.
Fever Chills and sweats Headaches Fatigue Nausea Anaemia Jaundice
Why is P falciparum particularly dangerous?
Can cause cerebral malaria and haemoglobinuria (black urine)
How is a diagnosis of malaria made?
By 3 thick and thin blood film, each 12 hours apart. Thick film: confirm diagnosis, thin film: which species
How would you treat complicated P falciparum?
IV artesunate
What is chickenpox caused by?
Varicella zoster virus (VZV) a type of herpesvirus.
Where does the rash tend to divide?
On warmer areas of the body.
What is chickenpox pneumonitis?
A common complication: 15% of adults with chickenpox are at risk and it has a high mortality.
What is shingles caused by and what triggers it?
Caused by VZV, becoming latent and migrating into the dorsal root ganglia after chicken pox. Triggered to be reactivated by immune depression.
What clinical feature would you expect?
A dermatomal rash, can cause neuropathic pain lasting up to 6 months.
What red flags in singles imply immunodeficiency?
Child with shingles, more than 1 dermatome.
