Lesson 3 Flashcards
Describe the commonly used types
and forms of culture media used in
veterinary practice laboratories
The types of media available for culturing bacteria are: transport media general purpose media enriched media selective media differential media enrichment media.
Transport media
Designed to keep microbes alive while not encouraging growth and reproduction. The Culturette used for specimen collection contains prepared transport media.
general purpose media
sometimes referred to as nutrient media, are not commonly used in veterinary practice.
enriched media
formulated to meet the requirements of the most fastidious pathogens. They are basic nutrient media with extra nutrients added, such as blood, serum, or egg. Examples include blood agar and chocolate agar.
selective media
contain antibacterial substances, such as bile salts or antimicrobials, that inhibit or kill all but a few types of bacteria. They facilitate the isolation of a particular genus from a mixed inoculum. Ex. MacConkey agar
differential media
allow bacteria to be differentiated into groups based on their biochemical reactions on the medium. Simmons citrate is a differential medium.
enrichment media
liquid media that favor the growth of a particular group of organisms. They contain nutrients that encourage the growth of the desired organisms or that contain inhibitory substances that suppress competitors. Examples include tetrathionate broth and selenite broth.
Describe the tests used to identify
bacteria
Swabbing, aspiration, and imprints.
Describe the procedures for inoculation
of culture media
lant tubes can be inoculated on the slant surface, *** area, or in both areas. If agar slants are used, only the surface of the slant may be inoculated, or the butt and the surface may be inoculated. To inoculate only the surface of the slant, a straight flamed wire is used to obtain a colony of bacteria from the primary isolation plate. The surface of the slant tube is streaked in an S shape. To inoculate both the butt and slant, the butt of the slant is stabbed with the tip of the inoculating wire and then carefully withdrawn up the same insertion path. The surface of the slant is then streaked in an S shape.
List biochemical tests that might be
needed to confirm the identity of bacteria in samples
The catalase test, the coagulase test and Oxidase Activity test.
Catalase test
performed on gram-positive cocci and small gram-positive bacilli. It tests for the enzyme catalase, which acts on hydrogen peroxide to produce water and oxygen. A small amount of a colony from a blood agar plate is placed on a microscope slide, and a drop of catalase reagent (3% hydrogen peroxide) is added. If the colony is catalase positive, gas bubbles are produced No bubble production indicates a negative result.
Coagulase test
performed on catalase-positive, gram-positive cocci. Staphylococcus aureus produces coagulase, which is an enzyme that coagulates plasma. Two versions of the test are available: the slide coagulase test and the tube coagulase test. The coagulase test is used to differentiate among coagulase-positive S. aureus, Staphylococcus intermedius, and coagulase-negative Staphylococcus (e.g., Staphylococcus epidermidis or Staphylococcus saprophyticus).
Oxidase Activity Test
oxidase test depends on the presence of cytochrome C oxidase in bacteria. A drop of 1% tetramethyl-p-phenylenediamine is added to a piece of filter paper in a Petri dish. The filter paper must be damp but not saturated. Prepared dry reagent slides are also available for performing the oxidase test and eliminate the need for reagent handling. A short streak of the sample is made on the filter paper or prepared slide using a glass rod or the end of a Pasteur pipette that has been bent into a hook. The sample should be applied with a gentle rubbing action. A nickel-chromium wire bacteriologic loop should not be used, because any traces of iron may give a false-positive result.
Explain the principle of antibiotic sensitivity testing
An antimicrobial sensitivity test may be performed to determine the bacterium’s susceptibility or resistance to specific antimicrobial drugs. The sensitivity test results allow the veterinarian to choose the most appropriate antimicrobial treatment for the patient.
List the steps needed to perform antibiotic sensitivity testing
It uses paper disks that have been impregnated with antimicrobials. This is a quantitative test that requires the measurement of inhibitory zone sizes to give an estimate of antimicrobial susceptibility.
