LAB 4 – STREPTOCOCCI, ENTEROCOCCI AND VRE

Question 1

You have a plate growing alpha hemolytic colonies, the Gram is G+C, chs, and the
catalase is weak positive.
a. What is likely the organism?
b. Is there a discrepancy? If so, what is the cause of this discrepancy?

Answer 1

a. Enterococci
b. Yes, Streptococcus sp. should be catalase(-)

Question 2

What are the components of the BE hydrolysis test? What are the functions of these
components?

Answer 2

Bile Esculin agar contains:
40% bile = growth differentiates bile resistant bacteria
Esculin = bacteria that hydrolyzes esculin in the presence of bile forms esculetin
Ferric citrate = reacts with formed esculetin = black

Question 3

What organism goes down the middle of the CAMP plate? Why is this organism used for
this test?

Answer 3

• S. aureus
• GBS produces CAMP factor, which enhances Beta hemolysis of S. aureus

GBS - Streptococcus pyogenes

Question 4

What specimens are collected for VRE?

Answer 4

• Anal swabs or stool samples

VRE - vancomycin Resistant Enterococci

Question 5

For the following antibiotics, what zone of inhibition would be considered
susceptible/sensitive?
a. Bacitracin
b. Optochin
c. Novobiocin

Answer 5

a. Any ZOI = S
b. >15 ZOI = S
c. >16 ZOI = S

Question 6

Explain the principle of the PYR test. Include the full names of the reagents and products.

Answer 6

Principle:
- presence of pyrridonyl peptidase hydrolyzes PYR (L-pyrrolidonyl-beta-naphthylamide) substrate
= B-napthylamide product
- addition of cinnanaldehyde reagent = pink after 10 min

Question 7

What method is used for typing pneumococci?
a. Why is this method used instead of Lancefield grouping?

Answer 7

Neufeld Quelling method: typing based on type-specific capsular polysaccharide

a. Pneumococci do not possess group-specific cell wall carbohydrate antigens

Question 8

What conditions must be met for decarboxylation to occur? Why do you need to include a
control tube?

Answer 8

• Mineral oil must be added to provide an anaerobic condition
• A negative control tube shows that decarboxylation activity acts on a specific substrate = negative without amino acid

Question 9

You set up an arginine decarboxylase test, but you forgot to overlay the tubes with mineral
oil. What color would expect the tubes to be?

Answer 9

yellow = negative
- same as the control

Question 10

What is the clinical significance of group B Streptococci in a vaginal smear of a pregnant
woman?

Answer 10

• GBS can cause serious neonatal sepsis and meningitis after vaginal delivery

Question 11

What post-infection sequelae can result from bacterial pharyngitis which demonstrated a
colony morphology of sm hem, Gram stain of g+c chns, cat neg and was bacitracin sensitive?

Answer 11

• acute rheumatic fever
• acute glomerulonephritis

Question 12

S. pneumoniae ID Scheme

Answer 12

colony morph - α flt/wet
% - gpc, prs
Cat - neg
TP (optochin) - S
BileSol - pos

Question 13

Viridans ID Scheme

Answer 13

Colony morph - α
% - gpc
Cat - neg
TP (optochin) - R
BileSol - neg

Question 14

Aerococcus urinae ID Scheme

Answer 14

Colony morph - α
% - gpc tet
Cat - neg
PYR - neg
BEA - neg
6.5% NaCl - pos

Question 15

Enterococcus faecalis ID Scheme

Answer 15

Colony morph - ent, gry
% - gpc, ch
Cat - neg/ wk pos
PYR - pos
BEA - pos
6.5% NaCl - pos

Question 16

Streptococcus bovis ID Scheme

Answer 16

Colony morph - ty gry
% - gpc
Cat - neg
PYR - neg
BEA - pos
6.5% NaCl - neg

Question 17

All Streptococci-like bacteria are catalase __ EXCEPT __ are weakly __.

Answer 17

“All” cocci are catalase NEGATIVE EXCEPT for ENTEROCOCCUS FAECALIS and AEROCOCCUS URINAE are weakly POSITIVE.

Question 18

S. pyogenes ID Scheme

Answer 18

Colony morph - β
% - gpc, ch
Cat - neg
PYR - pos
Latex - A pos
CAMP - neg
TA (bacitracin) - S

Question 19

S. pyogenes agalactiae ID Scheme

Answer 19

Colony morph - β
% - gpc, ch
Cat - neg
PYR - neg
Latex - B pos
CAMP - pos
TA (bacitracin) - R