Principles of Laboratory Tests Flashcards

Question

What is the principle behind the egg yok agar - Naglar reaction? (4)

Answer 1

Used to detect lecithinase and lipase production and proteolytic activity of anaerobes -> recommended in the Naglar Test for the ID of C. perfringens Egg yolk agar is used which acts as a supply of lecithin and free fats which allows detection of lecithinase and lipase production and proteolytic activity Lecithin break down results in a white opaque zone of precipitation that spreads beyond the edge of the colony Lipase activity forms an iridescent sheen on the surface of the agar

Answer 2

A non-selective medium Supplemented with a suspension of egg yolk and enriched with haemin and vitamin K Haemin and vitamin K enhance the growth of anaerobic microorganisms

Answer 3

A test used to differentiate C. perfringens from other perfringens -> detects the alpha toxin (lecithinase) produced by it Works by neutralising lecithinase C produced through a specific antitoxin It works by exploiting the lecithinase ability of the organism already seen on egg yolk agar -> antitoxin is added only to one half of the plate

Answer 4

We need to prove that the bacilli are in fact anaerobic Need to put up an aerobic and anaerobic plate -> the organism must grow on the anaerobic plate Anaerobic GNBs are susceptible to metronidazole so a disc should be put up on the anaerobic plate to prove its an anaerobic GNB

Answer 5

Anaerobic Identification Discs A commercial system for the identification of gram negative anaerobic bacteria based on the susceptibility to six antimicrobials Erythomycin, Eifampicin, Colistin sulphate, Penicillin, Kanamycin, Vancomycin

Answer 6

Haemophilus species are all dependant on one or both of the growth factors, X and V X = haemin V = nicotinamide adenine dinucleotide (NAD)

Answer 7

Horse blood agar contains X factor normally Horse blood agar contains a NADase enzyme which inactivates factor V(NAD) However when the blood is heated to 80 degrees (chocolate agar) the NADase activity is destroyed and thus makes the V factor available

Answer 8

Contains magnesium chloride and potassium sulphate which enhances pigment profuction Selective supplement = antimicrobial cocktail comprising centrimide (antiseptic) and nalidixic acid (antibiotic) P. aeruginosa is resistant to both these selective agents

Answer 9

Contains specific chromogenic substrates (Red Gal) which are cleaved by the B-galactosidase enzymes prouced by E. Coli and coliforms, resulting in pink colonies Medium also contains phenylalanine and tryptophan, as an indication of tryptophan deaminase activity, indicating the presence of Proteus species, morganella and providencia species which appear brown

Answer 10

Test for an organisms ability to grow with citrate as its sole carbon source Utilisation of citrate releases alkaline bicarbonate ions that cause the medium pH to increase above 7.4 Incorporated bromothymol blue indicator turns green to blue under alkaline conditions

Answer 11

Test for the hydrolysis of aa tryptophan, which leads to formation of indole If indole is produced then the addition of 8 drops kovaks reagent a cherry red/pink colour change will be seen

Answer 12

Test for production of acids from the metabolism of glucose to produce a pH of 4.4 in the medium These acids are not further metabolised and are said to be stable acids At a pH of 4.4. or less the methyl red indicator will stay red

Answer 13

Detection of products of glucose metabolism -> further breakdown of acids found in MR test -> this produces neutral end products e.g. acetoin and 2,3-butanediol These neutral products raise the pH from 4.4 to create a negative MR test acetoin and 2,3-butanediol react with a-naphthol to produce a mahogany red colour

Answer 14

Test for the production of decarboxylase enzyme which removes the carboxyl group from an amino acid Decarboxylase broth contains nutrients, dextrose, pyridoxal, bromocresol purple and cresol red indicators, plus a single aa e.g. lysine or arginine Bromocresol purple turns purple at an alkaline pH and turns yellow at an acidic pH No colour change if organism cannot ferment dextrose If dextrose fermenter -> acidic byproducts are formed -> low pH -> medium turns yellow -> decarboxylation begins -> alkaline byproducts produced -> pH raised = purple

Answer 15

A transparent, highly selective medium for the isolation of Neisseria gonorrhoeae from clinical specimens. It contains peptone, corn starch agar base buffered with phosphates and supplemented with horse plasma, horse haemoglobin, dextrose, yeast autolysate and antibiotics. Phosphate buffers the medium and sodium chloride provides electrolytes Starch neutralises the toxic metabolites produced by Neisseria Yeast fulfils the CO2 requirements needed to enhance CO2 growth The cocktail of antibiotics prevents the growth of organisms other than Neissera

Answer 16

Hoyle's agar is a selective medium that uses tellurite to differentially select Corynebacterium diphtheriae from other upper respiratory tract flora. The medium appears cream to yellow colored, and takes the form of a free-floating powder. It is a modification of Neill's medium.[1] Hoyle's tellurite agar contains beef extract, peptones, sodium chloride, blood, tellurite and agar C. diptheria grows with grey colonies

Answer 17

Fraser broth can also be used for the selective enrichment of Listeria. It contains acriflavine and nalidixic acid and ferric ammonium citrate which makes it selective

Answer 18

A selective agar for the growth of Listeria. Contains peptones, salts, chrogenic substrate lithium chloride and antibiotics. B-glucosidase is common to all Listeria species and gives the colonies their blue colour. L. monocytogenes has lipase which causes an opaque white halo around blue colonies. Incubate at 37 degrees for 24 hours and incubate negative strains for another 24 hours.

Answer 19

Polymyxin-lysozyme-EDTA-thallous acetate agar for the selective growth of Bacillus anthracis Contains: beef heart infusion from solids and tryptose provide the carbonaceous and nitrogenous compounds necessary for growth whereas sodium chloride (NaCl) provides the osmotic equilibrium. Thallous acetate, polymyxin, lysozyme (FD185) are inhibitory agents allowing growth of B.anthracis while inhibiting contaminants. Lysozyme specifically suppresses the growth of gram-negative contaminants. Colonies of B.anthracis appear in 36-40 hours after incubation at 37°C. Roughly circular, creamy-white colonies with a ground-glass texture.

Answer 20

Selective and differential medium for non-lactose fermenting enterobacterales. Selective agents are sodium deoxycholate and sodium citrate. Differential agents are lactose and sodium ferric chloride. Shigella appears as a pale coloured colony (NLF), lactose fermenters/commensals are pink, non-lactose fermenters are colourless/pale, Salmonella often have a dark centre (H2S positive)

Answer 21

Xylose lysine decarboxylase Selective and differential media for shigella flexneri Sodium deoxycholate is the selective agent Xylose, Lysine and H2S are the differential agents Lactose Fermenters/Commensals yellow Shigella = red (xylose Lysine) Salmonella red with black centres

Answer 22

Chromagar specific for shiga-toxin E.Coli serotype, these appear as a mauve colour while other enterobacteraciae appear colourless, blue or are inhibited.

Answer 23

Proteus mirabilis are pale pink colonies with tan centres, salmonella enteric is teal coloured colonies with black centres, E. Coli are pink to violet colonies, Shigella sonnei are teal coloured colonies (non H2S producing salmonellas look the same)

Answer 24

Broth which enhances culture yield by reducing growth of faecal coliforms and faecal streptococci. Used for the selective enrichment for the cultivation of Salmonella species. The broth is used by taking a sub-culture of broth from the upper third of the broth column at least 5cm in depth. Its used for the better enrichment of Salmonella Typhi. Incubate for 37 hours for not longer than 18 hours.

Answer 25

Selective Media for Salmonella Typhi Bismuth sulfite indicator along with brilliant green inhibits the intestinal gram-positive and gram-negative bacteria. S. Typhi grow as black colonies with a surrounding metallic sheen resulting from hydrogen sulfide production and reduction of sulfite to black ferric sulfide. Salmonella Paratyphi A grows as light green colonies

Answer 26

Used for the selective isolation of Staphylococci species. It contains lithium chloride and tellurite to inhibit the growth of alternative microbial flora, while the included pyruvate and glycine promote the growth of Staphylococci. Staphylococcus colonies show up black in colour with clear zones produced around them.

Answer 27

Charcoal cefoperazone deoxycholate (CCD) agar-modified An improved version of the original CCD agar. Modified CCD agar is developed for the direct isolation of campylobacters from human and animal feces and its major constituents are charcoal, cefoperazone, and sodium deoxycholate.