ID: Streptococci and Enterococci Flashcards
What are the three classifications of streptococci
Alpha haemolytic
Beta haemolytic
Gamma haemolytic
What are the basic characteristics for streptococci
Could be alpha, beta or gamma haemolyisis
KOH negative
Catalase negative*
Oxidase negative
What tests would you carry out for streptococci?
(6)
Lancefield grouping (b) (g)
Bacitracin (b)
Optochin (a)
MacConkey (g)
Bile Aesculin (g)
Vancomycin (g)
What are the basic characteristic test results for enterococci?
(4)
Cocci*
Gram positive*
Aerobe*
Catalase negative *
What’s the most important step when identifying streptococci/enterococcus?
If you have a gram positive, aerobe, catalase negative cocci, you need to be able to identify haemolysis
Only certain tests carried out for each type of haemolysis
Gamma haemolysis usually means an enterococcus
What are the results of the preliminary ID tests for streptococci?
Cocci
Gram positive
Catalase negative
What are the five clinically significant strains of streptococci?
Strep pneumonia
Strep pyogenes
Strep viridans
Strep mutans
Strep agalactiae
How can you tell a strep is strep pneumoniae from how is appears on a blood agar plate and under a microscope
Alpha haemolysis
Draughtsman colonies or mucoid colonies
Lanceolate -> flame shaped cocci
What strep strains have alpha haemolysis
S pneumoniae (most likely)
S viridians
What strep strains have beta haemolysis
S pyogenes (most likely)
S agalactiae
What strep strains have gamma haemolysis
S. bovis however any gram positive cocci which are catalase negative and don’t have haemolysis are more than likely enterococcus than strep
What four tests are used to speciate streptococci?
Optochin susceptibility
Bacitracin susceptibility
MacConkey agar -> bile susceptibility
Lancefield grouping (classifies B-haemolytic strep)
What is the only test needed to speciate alpha-haemolytic strep; S. pneumoniae and S viridans
Optochin susceptibility test
What is the principle behind the optochin susceptibility test
(2)
This test detects an organism’s susceptibility to the chemical optochin
Optochin tests the fragility of the bacterial cell membrane and causes S. pneumoniae to lyse due to changes in surface tension, as shown by a zone of inhibition around the optochin disc
How do you carry out an optochin susceptibility test
(7)
Ask for blood agar, optochin disk x3, bijuo jar of sterile water and a swab x3
Divide a blood agar into three section, half for your test and two quarters for positive and negative control
Prepare a suspension of test organism and controls using a reduced volume of sterile water
Lawn inoculate your test and controls
Place one optochin disc in centre of test inoculum and the controls
Incubate overnight at 37 degrees
Examine for a zone of inhibitions
What is your positive control for optochin susceptibility, i.e which strep species is susceptible to optochin?
Streptococcus pneumoniae
What is your negative control for optochin susceptibility, i.e. which strep species is resistant to optochin
Streptococcus viridans
What three tests must be carried out for B-haemolytic strep
Lancefield grouping
Bacitracin susceptibility test
Ability to grow on MacConkey agar
What is the principle behind the lancefield grouping test
(4)
The majority of pathogenic strep possess specific carbohydrate antigens, which permit the classification of strep into groups
Strep group antigens can be extracted from bacterial cells and their presence demonstrated with latex particles previously coated with group-specific antibodies
These latex particles will agglutinate in the presence of homologous antigen, but will remain in smooth suspension if absent
The use of enzymatic extraction procedure considerably shortens the time required for antigen extraction and much improves the antigen yield
What is the Lancefield grouping test kit called
Oxoid Streptococcal Grouping Kit
How do you prepare your bacteria for the Lancefield agglutination test?
(4)
Dispense 0.4ml of Oxoid Streptococcus Extraction Enzyme into a labelled test tube
Select 2-5 test colonies and emulsify in the enzyme preparation
Incubate for 10 minutes at 37 degrees Celsius in a water bath -> make sure to shake vigorously at 5 mins in
Remove and allow to cool to room temp
How do you carry out the Lancefield grouping test
(6)
Ask for a reaction card, sticks, latex reagent A, B, C, D, F and G,
Ask for S. pyogenes or Enterococcus control?
Ask for (pre-mixed?) enzyme (and test) extract??? -> don’t know if this will be done for you previously or you will have to do it yourself
Add 1 drop of latex reagent A, B, C, D, F, G to a circle on your reaction card
Look for agglutination
Carry out this test using the positive control first so you know the reagents are working
What does a Lancefield group A indicate?
Streptococcus pyogenes
What does a Lancefield group B indicate?
Streptococcus agalactiae
