Here's a special treat for all my visual learners! Check out these infographics I created to DDx between all the Gram positive Cocci...
Laboratory workup of Gram POS Cocci
1. Determine the morphology/grouping of the gram-pos organism
GRAM POSITIVE COCCI IN...
2. Perform a Catalase Test
Catalase: an enzyme that catalyzes the release of oxygen from hydrogen peroxide (H2O2)
No bubbles are formed
Bubbles are formed
Staphylococcus Species Workup
If catalase is POSITIVE, then perform a Coagulase Test
No clot formed on slide (left) or tube (right) tests
Clots/clumps formed on slide (left) or tube (right) tests
If Coagulase Test is NEGATIVE, determine Novobiocin sensitivity
Perform a Clindamycin Induction Test (D Test)
What you need to know:
Since clindamycin and streptogramin are among the few drugs of choice in the treatment of methicillin resistant S. aureus (MRSA) infections, knowing the resistance to these antibiotics is imperative.
The D test is used to study the macrolide lincosamide streptogramin resistance (MLSB), both constitutive and inducible as well as macrolide streptogramin resistance (MSB) in Staphylococcus aureus.
Basically, the D test is performed to determine if S. aureus is susceptible to Clindamycin.
How testing is performed:
Erythromycin (macrolide) and clindamycin (lincosamide derivative) discs are placed adjacent to each other over the Mueller Hinton agar medium inoculated with the test organism.
The test is performed in the same MHA plate in which the antibiotic sensitivity test is being done, taking into consideration that the discs are placed adjacent to each other maintaining the distance.
Clindamycin resistance can be induced by Erythromycin which causes the Kirby Bauer zone around the Clindamycin disc to be blunted and form a "D" if clindamycin can be induced by erythromycin to be resistant. So this is called INDUCABLE RESISTANCE.
The organism susceptible to clindamycin without any flattening of the zone (D test negative) near clindamycin disc (resistant) = Macrolide streptogramin resistance (MSB)
Susceptible to both antibiotics = No resistance
Flattening of the clindamycin zone (D test positive) near the erythromycin disc (resistant) = Inducible MLSB (IMLSB)
The growth of the organism up to the edges of the disc = Constitutive MLSB (CMLSB) Resistance
Streptococcus Species Workup
S. pneumonoiae (encapsulated)
Viridans streptococci (S. mutans)
S. pyogenes (Group A Strep)
S. agalactiae (Group B Strep)
Enterococcus faecalis (Group D Strep)
If catalase is NEGATIVE, then determine the pattern of hemolysis on sheep blood agar, then continue the appropriate testing algorithm to determine the bacterial organism.
Some bacteria produce extoxins that cause the lysis of RBCs which can be seen on a blood agar plate
Degrades hemoglobin into biliverdin
Negative for hemolysins
Aerobe vs. Anaerobe
Tests for presence of a capsule
Swelling around capsule
Clearing around disk
Clearing of broth in presence of bile
No lysis present
Bile Solubility testing
Some bugs make a self-lysing enzyme to depress the growth of old colonies. The presence of bile salt will accelerate this .
Clearing around disk
Growth near top of tube
(highest O2 concentration)
Grows in 6.5% NaCl
Growth at base of tube
(lowest O2 concentration)
No growth in 6.5% NaCl
See below for how to differentiate between the various Streptococci bacteria
based on hemolytic patterns and other laboratory tests.
MICRO & IMMUNO
Gram pos vs. Gram neg classification
Gram positive RODS