Microbiology Flashcards
What is the most common cause of UTI’s?
E.coli
What is a commensal?
Organism that colonises host but causes no disease normally
What is an opportunistic pathogen?
Microbe that only causes disease if the host defence is commpromised
What is virulence?
The degree to which an organism is pathogenic
What are the stages of gram staining?
- Fixation of material to microscope slide
- Application of primary stain
- Application of iodine
- Decolourisation step
- Application of counterstian
What agent is used as a primary stain and what colour does it stain cells?
Crystal violet- turns all cells purple
What is applied after crystal violet and what does it form?
Iodine- crystal violet iodine complex formed
What is added in the decolourisatoin step?
Acetone or ethanol
What differentiated in the decolourisation step (4)?
Gram +ve and Gram -ve bacteria
What colour do gram positive bacteria stain?
Purple
What colour to gram negative bacteria stain?
Pik
What is added as a counterstain to make gram -ve’s pink?
Safranin
Pneumonic to remember order of stains?
Come In And Stain Crystal violet Iodine Alcohol Safranin
What is the differene between gram positive and gram negative bacteria?
Gram +ve: Thick cell wall (thick layer of peptigoglycan)
Gram -ve: Thin layer of peptidoglycan (thin cell wall), high lipid content
What are the two types of gram positive cocci?
Staphylococcus
Streptococcus
What are the types of gram positive rods?
Corneybacteria, mycobacteria, isteria, bacillus, nocardia
What test can be done to differentiate staphylococcus?
Coagulase test
What organism would a positive coagulase test indicate?
Staph, aureus
What would a negative coagulase test indicate?
Coagulase negative staphylococcus (epidermis, saprophiticus)
What happens in a coagulase test and what is it used for?
Used to differentiate staph. aureus from other types of staph.
Coagulase is an enzyme produced by S. aureus that converts soluble fibrinogen to insoluble fibrin, causing clumping.
What test is done to differentiate gram positive cocci?
Catalase test
What bacteria are present if it is catalase positive?
Staphylococcus
What bacteria are present if it is catalase negative?
Streptococcus
What is a catalase test?
Catalase is an enzyme that converts hydrogen peroxide to water and oxygen gas. Bacterial isolate is added to hydrogen peroxide and if positive, bubbles of oxygen are observed
How can streptococcus be differentiated?
Haemolysis on blood agar
What is blood agar and what does it test?
Agar plate that contains sheeps blood:
Tests the ability of an organism to produce hemolysins, enzymes that break down red blood cells
What are the three outcomes of haemolysis on blood agar?
Alpha
Beta
Gamma
What does alpha haemolysis look like?
Partial lysis–> Green on red plate
What does beta haemolysis look like on a blood agar plate?
Complete lysis–> Clear
What does gamma haemolysis look like on a blood agar plate?
No lysis–> Remains red.
What organism is present is there is greening of the BAP?
Alpha haemolytic strep
How are alpha haemolytic streps differentiated?
Optochin test
What is the optochin test?
Optochin antibiotic disk is placed on plate, which will inhibit growth of susceptible bacteria
What bacteria are optochin resistant?
Viridans strep
What bacteria are optochin sensitive?
S. Pneumonia
What bacteria are present if there is complete lysis (clear) BAP?
S. pyogens
Lancefield groups A, B, C, G
What bacteria are present if there is no lysis (red BAP)?
Lancefield D: S. bovis, enterococcus.
What kind of bacteria is S. aureus and what kind of infections does it usually cause?
Gram +ve, Coagulase +, catalase + Staphylococcus (cluster)
Skin infection
What kind of bacertia is MRSA?
Gram +ve, Coagulase +ve Staphylococcus
What kind of bacteria is S.pneumonia and where does it usually affect?
Gram +ve, Catalase -ve, alpha haem Streptococcus
Affects ENT
What kind of bacteria is S. pygones and where does it usually affect?
Group A streptococcus
Throat + skin
How are gram negative bacilli differentiated?
By their appearance on MacConkey agar
What is MacConkey agar and how does it differentiate gram negative bacteria?
Inhibits Gram-positive organisms and differentiates Gram-negative organisms by lactose fermentation.
What colour will lactose fermenting bacteria turn MacConkey agar?
Pink
What colour will non-lactose fermenting bacteria stain MacConkey agar?
White/ clear
What are the main lactose fermenting bacteria?
E.Coli
Klebsiella
What are the main non lactose fermenting bacteria?
Shigella
Salmonella
Pseudomonas
Proteus
How are lactose fermenting bacteria further differentiated?
Biochemical identification (API strip) + Sensitivity tests
How are non lactose fermenting bacteria further differentiated?
Oxidase test
What bacteria tests positive in the oxidase test?
Pseudomonas
What non lactose fermenting bacteria tests negative in an oxidase test?
Shigella, Salmonella, Proteus
What type of bacteria is E.Coli and what are the main infections it causes?
Lactose fermenting Gram -ve bacteria:
UTI’s
Travellers diarrhoea
What type of bacteria is Klebsiella and what are the main infections it causes?
Lactose fermenting gram -ve bacilli:
Pneumonia, meningitis
What type of bacteria is pseudomonas and what infections does it commonly cause?
Non-lactose fermenting, Oxidasae +ve gram negative bacilli:
Pneumonia
What type of bacteria is Shigella and what infections does it commonly cause?
Non-lactose fermenting, Oxidasae -ve gram negative bacilli:
Shigellosis (diarrhoea/ fever)
What type of bacteria is Salmonella and what infections does it commonly cause?
Non-lactose fermenting, Oxidasae -ve gram negative bacilli:
Salmonellosis (diarrhoea)
What type of bacteria is Neisseria and what infections does it commonly cause?
Gram negative cocci:
Gonorrhoea, meningitis
What are the most common gram positive rods?
Corneybacteria, Mycobacteria, Listeria, Bacillus, Nocardia
What are the most common gram negative cocci?
Neisseria (gonorrhoea), Moraxella
What are the two most common causes of meningitis and what type of bacteria are they?
N. meningitidis (meningococcus)–> Gram negative cocci
S. pneumonia–> Gram positive cocci
(Diplococci)
Which Hepatis is the only DNA virus (instead of RNA)?
Hep B
Which forms of hepatitis are spread by faecal-oral transmission?
Hep A & E
Which forms of hepatitis are spread by blood-blood transmission?
B, C, D
How does acute non-inflammatory diarrhoea present?
Watery and non-bloody with no fever
How does acute inflammatory diarrhoea present?
Bloody, possibly with pus and fever
What is bloody diarrhoea also known as?
Dysentry
What are the two ways bacterial toxins can enter the body?
Secrete exotoxins directly into food
Enter the body and secrete enterotoxins
What bacteria produce preformed exotoxins?
Staph. aureus
Bacillus cereus
What are the main pathogens that cause noninflammatory acute diarrhoea?
E.coli (ETEC) Vibrio cholerae Staph. aureus Bacillus cereus Listeria Norovirus Giardia lamblia
What are the main pathogens that cause inflammatory diarrhoea?
Salmonella Shigella Campylobacter C.Diff STEC
What does a presentation of diarrhoea with vomiting suggest?
S.aureus food poisoning or viral gastroenteritis
What should be suspected if symptoms come on within 6 hours of ingesting food?
S. aureus of B. cereus (preformed exotoxin) infection
What is the main cause of travellers diarrhoea?
Enterotoxigenic E. coli (ETEC)
What is the most common cause of Osteomyelitis?
Staph. aureus
What is the most common cause of septic arthritis?
S. aureus
Young sexually active adults: Neisseria gonorrhoea
What is the most common bacterial STI?
Chlamydia
What are the main symptoms of chlamydia?
70% asymptomatic
Men: Urethritis, Testicular pain
Women: Vaginal discharge, lower abdominal/ pelvic pain
What is the first line investigation for STI?
NAAT (Nucleic acid amplification test)
How is chlamydia treated?
Doxycycline/ Azithromycin antibiotics
What is the second most common STI in the UK?
Gonorrhea (Niesseria gonorrhoea)
How does gonorrhea present?
Usually asymptomatic
Men: Urethritis with purulent discharge and dysuria
Women: Pelvic pain with vaginal discharge, itch and dysuria
What is the most common cause of viral meningitis?
Enteroviruses
Herpes simplex
What pathogen causes TB?
Mycobacterium tuberculosis
What kind of antibiotic is usually used to treat gram positive bacteria and why?
Those that inhibit cell wall synthesis as gram positive bacteria have thick cell walls
What are the different types of antibiotic that inhibit cell wall synthesis?
Penicillins
Glyopeptides
Cephalosporins
Carbapenems
What are the main types of glyopeptides/
Vancomycin
Teicoplanin
What are the main types of penicillins?
Benzylpenicillin
Amoxicillin
Flucloxacillin
What are the main types of cephalosporins?
Cephalexin
Cefotaxime
Ceftriaxone
What are the main types of carbapenems?
Imipenem
Estapenem
When are Carbapenems usually used?
For the treatment of severe infections or infection caused by multiple resistant bacteria
What types of antibiotic inhibit protein synthesis?
Cholaramphenicol
Macrolides
Tetracyclines
Aminoglycosides
What would be used instead of penicillin in those with a penicillin allergy?
Macrolides (e.g. Clarithromycin, Erythromycin)
What are the different ways antibiotics can inhibit nucleic acid synthesis?
Inhibit folate synthesis
Inhibit DNA Gyrase
Bind to DNA polymerase
Break DNA strands
What are the main types of antibiotic that inhibit nucleic acid synthesis?
Rifamipcin Fluroquinolones Ciprofloxacin Trimethoprim Sulphonamids Sulphamethoxazole Metronidazole
Why should you never give the antibiotic Trimethroprim to pregnant women?
It inhibit folate synthesis and folate is very important during pregnancy to prevent spina bifida
What antibiotic is the first line in staph infections?
Penicillins e.g. Flucloxacillin
What is the main antibiotic used for UTI’s?
Nitrofurantoin or Trimethoprim
What antibiotic is used to treat MRSA?
Vancomycin
What are mycobacteria?
Aerobic, non-spore forming bacilli
What are the two main types of mycobacteria?
TB + Leprosy
How are Mycobacteria investigated?
Zeihl-Neelsen stain instead of gram staining
What are the two outcomes of Zeihl-Neelsen staining?
Acid-fast bacteria stain red
Non acid-fast bacteria stain blue
How can viruses be identified>
PCR
Nucleic acid amplification tests (NAAT)
Serology: look for antibodies in response
What can be given to treat severe viral infections?
Antivirals: End in ‘vir’ e.g. Acyclovir
What are protozoa?
Microscopic unicellular eukaryotes
How are protozoa classified?
Based on movement: ameoiboids, ciliates, sporozoan, flagellates
What are the most common types of protozoa?
Malaria
Giardia Lamblia
Toxoplasmiosis
Trichomonas vaginalis