Module 11: Streptococcus (Group D; Non-Hemolytic Strep)

Question 1

Group D Streptococci characteristics

Answer 1

Cellular morphology: similar to other streptococci but some cells may be oval

Growth requirements: Same as other streptococci but growth on non-enriched media is better than for Groups A and B

Colonies: 1-2mm, gray, convex, buttery, most are non hemolytic
Group D will grow on some types of MacConkey agar “MacConkey agar without crystal violet” ; may have deep pink color due to lactose fermentations

Question 2

Genus ID of Enterococci and Nonenterococci

Answer 2

same a S. pyogenes

Some strains of Enterococci give a weak reaction int he catalase test - “pseudocatalase” production

Question 3

Species ID of Group D strep

Answer 3

Group D antigen
Both latex and co-agglutination reagent available for group D antigen
Group D antigen contains teichoic acids (located beneath cell wall) - causes some problems in detection of group antigen
Rapid acid extraction method used with latex reagents may not detect group D antigen while the longer enzyme extraction generally will

Other genera may show false pos results

Question 4

BEA test for group D

Answer 4

bile tolerance and esculin hydrolysis
Group D are only strep that should grow int he presence of 40% bile salts in media
Group D also hydrolyse esculin while only the occasional other strain of Strep will give pos result (low chance)
Must use pure culture of strep

*Bile esculin pos Streptococci may be confidently reported as Group D

Question 5

PYR test for Group D

Answer 5

may be used to differentiate group D enterococci from noneneterococci
Enterococci = POS
Nonenterococci = NEG

Group A strep are also PYR pos
Beta hem Strep that is PYR pos should only be ID’d as Enterococcus when bile esculin test is also POS

Question 6

6.5% NaCl test for Group D

Answer 6

Enterococci will grow

Nonenterococci will not

Question 7

Differentiation of Group D Enterococci and Nonenterococci

Answer 7

If isolate is from blood culture, species ID is desirable

Isolation of Streptococcus bovis from blood culture indicates GI tract cancer

Question 8

Group D strep Antimicrobial susceptibility

Answer 8

testing should be done on all clinically significant isolates
Group D Nonenterococcus are MORE susceptible than Enterococci
Most strains of S. bovis are susceptible to penicillin but combination antimicrobial therapy is occasionally needed for eradication

Group D enterococci known to be resistant to low concentration of penicillin and ampicillin (concentrated in urine)
Endocarditis often treated with combo of aminoglycoside and penicillin

Question 9

Clinical significance of Group D strep

Answer 9

Group D streptococci and Enterococci are normal fecal flora (intestines)
Enterococcus faecalis = most common isolate (UTI, wound)

Of Nonenterococci, S. bovis is most common isolate (in blood cultures, associated with carcinoma of GI tract)

Question 10

Vancomycin Resistant Enterococcus (VRE)

Answer 10

Vanc used for treatment when high level aminoglycoside resistance is present
VRE emerged in recent years
Strict isolation procedures

Screening usually done with solid agar plate with vancomycin (agar dilution test) at concentration of 6mg/L
Selective agents slike sodium azide may be included in medium
72hrs incubation with colonies being identified as Enterococci by usual methods
New Chrom agars for VRE now available
Most VRE strains are identified as Enterococcus faecium
Pulse field gel electrophoresis and testing for genetic markers: Van A, B and C