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infection and defence > making a microbiological diagnosis > Flashcards

making a microbiological diagnosis Flashcards

1
Q

what are 5 main methods used to identify bacteria

A
  • microscopy
  • culture
    -serology ( antigen/antibody)
  • molecular biology
  • MALDI-Tof (mass spectroscopy)
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2
Q

what are the different staining used in microscopy

A
  • gram stain
    -ziehl nielsen (mycobacterium)
  • india ink- crytococcus
  • silver stain
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3
Q

are spiral organims seen under gram stain

A
  • no
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4
Q

what organisms are difficult to see under gram stain

A
  • mycobacterium (ziehl-neilson)
  • mycoplasma
  • legionella
  • chlamydia
    -coxiella
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5
Q

what are the advantages of microscopy

A
  • pathogens have characterstic appearances (viruses, bacteria, parasite)
  • examining sterile fluid (blood, joint fluid, ascites)
  • easy and cheap
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6
Q

what are the disadvantages of microscopy

A
  • cannot distinguish between pathogen and commensal e.g. e coli and salmonella look exactly same under microscope
  • organism identification nearly impossible ( s. aureus, s. epidermidis)
  • relatively insensitive
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7
Q

what are the main postitive cocci

A
  • stapylococci
  • streptococcus
  • enterococci
  • peptostreptococci (anaerobes)
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8
Q

what are advantages of culture

A
  • gold standard
  • more sensitive than microscopy
  • allows identification and antibiotic sensitivity
  • variety of culture media and conditions for different organisms
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9
Q

cons of culture

A
  • cannot grow/ culture certain dangerous microbes
  • slow
    -hampered by antibiotic resistance
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10
Q

what is a selective culture medium

A

a type of culture medium that is designed to allow the growth of a specific microorganism while inhibiting the growth of other microorganisms.
example n gonorrhoea would take forever to grow and other bacteria would overide its growth- antibiotics therefore used to suppress the growth of other bacteria

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11
Q

what is differential media

A
  • allows rapid screening of specimens with commensal flora for pathogens- exploit biochemical differences between pathogens and commensal
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12
Q

what is biochemical identification

A
  • multitude of tests containing substrate used to determine the organism (24hours)
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13
Q

what is the staphlococcus coagulase testing

A
  • specimen placed into test tube containing plasma
  • if coagulates bacterium is s. aureus
  • if it does not clot- s. epidermidis possibly
  • process is repeated 20 times and a positivie or negative result is given a number- produces a code which once looked up identifies the organism
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14
Q

what is maldi-tof (mass spec), how long does it take

A
  • takes 5 min to 72 hours- quicker than biochemical test
  • sperate microorganism by mass/charge ratio
    ions hit microorganism
  • ions released, accelerated and detected
  • computer detects organism
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15
Q

what are serology tests

A
  • non culture
  • antibody detection of antigen detection
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16
Q

how does antibody detection work

A
  • dependant on immune response to produce antibodies agaisnt the bacteria (slow 10-14 days)
  • IgG only indicates there there is an immune response- doesnt indicate when/the time
  • immunocompetant patient required to produce a mountain immune response
  • elisa
  • example toxoplasma, leishmania, chlamydia, mycoplasma
17
Q

what is the antigen test

A
  • serology
  • rapid
  • serotype specific only tests wether bacteria is present not whether its not present
  • s. pneumonia and legionella cryptococcus ( tests for currently)
18
Q

what is urine dipstick test

A
  • excellent negative predictive value
  • poor positive predictive value- cannoy diagnose (50%)
  • allows rapid exclusion s of infection
  • tests for nitrites and leukocytes in urine - most uti infections convert nitrates into nitrites
19
Q

what are skin tests

A
  • indicate past exposure
  • based on type 4 hypersensitivity reaction
  • intact immune response essential (mantoux heaf- detect exposure to tb antigens- bcg vaccinated patients have false positive effect- only tells previous exposure and not current infection
20
Q

what is antibiotic susceptibility test

A
  • gold standard, serial dilutions of antibscterial agent
  • incubate for growth and find minimum inhibitory concentration (mic)- compare to international standard to determine wether bacteria is reistant or not
  • disc diffusion can be also used
  • e test as well- to determine mic

infection and defence (9 decks)

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