making a microbiological diagnosis Flashcards
what are 5 main methods used to identify bacteria
- microscopy
- culture
-serology ( antigen/antibody) - molecular biology
- MALDI-Tof (mass spectroscopy)
what are the different staining used in microscopy
- gram stain
-ziehl nielsen (mycobacterium) - india ink- crytococcus
- silver stain
are spiral organims seen under gram stain
- no
what organisms are difficult to see under gram stain
- mycobacterium (ziehl-neilson)
- mycoplasma
- legionella
- chlamydia
-coxiella
what are the advantages of microscopy
- pathogens have characterstic appearances (viruses, bacteria, parasite)
- examining sterile fluid (blood, joint fluid, ascites)
- easy and cheap
what are the disadvantages of microscopy
- 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
what are the main postitive cocci
- stapylococci
- streptococcus
- enterococci
- peptostreptococci (anaerobes)
what are advantages of culture
- gold standard
- more sensitive than microscopy
- allows identification and antibiotic sensitivity
- variety of culture media and conditions for different organisms
cons of culture
- cannot grow/ culture certain dangerous microbes
- slow
-hampered by antibiotic resistance
what is a selective culture medium
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
what is differential media
- allows rapid screening of specimens with commensal flora for pathogens- exploit biochemical differences between pathogens and commensal
what is biochemical identification
- multitude of tests containing substrate used to determine the organism (24hours)
what is the staphlococcus coagulase testing
- 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
what is maldi-tof (mass spec), how long does it take
- 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
what are serology tests
- non culture
- antibody detection of antigen detection
how does antibody detection work
- 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
what is the antigen test
- serology
- rapid
- serotype specific only tests wether bacteria is present not whether its not present
- s. pneumonia and legionella cryptococcus ( tests for currently)
what is urine dipstick test
- 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
what are skin tests
- 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
what is antibiotic susceptibility test
- 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
