Early learning: basic GPC

Question 1

If you get a dark purple cocci, what test would you do next and why?

Answer 1

A catalase: To diff Staphylococcus - positive- from Streptococci- neg

Question 2

If you get a catalase positive GPC, what test is most helpful next?

Answer 2

Coagulase (a latex agglutination test): To diff btw coagulase POSITIVE S. Aureus from coagulase negative Staphs

Question 3

What are 3 notable CoNS? Give differentiating tests between them

Answer 3

S. saprophyticus- is PYR negative and Novobiocin resistant S. epidermidis- is Novobiocin sensitive and PYR neg S. lugdunensis- is PYR positive and ornithine decarboxylase positive

Question 4

What important CoNS is PYR positive?

Answer 4

S. lugdunensis

Question 5

What test(s) help diff btw Micrococcus and Coagulase neg Staphs?

Answer 5

oxidase(microdase) is positive in MIcrococcus; Micrococcus is susceptible to bacitracin (lysostaphin)

Question 6

You have catalase positive, coagulase negative GPC; what are you thinking it could be and what tests would you try next?

Answer 6

One of the coag-neg Staphylococci due to the catalase positivity; to diff out S. saprophyticus, do a novobiocin sensitivity–> if resistant, it is sapro; if susceptible, must determine whether Staph lugdunensis or epidermidis—> do PYR and ornithine decarboxylase, S. lugdunensis is POS for both

Question 7

what will a PBP2 test accomplish on you Staph aureus culture?

Answer 7

determine if it is methicillin resistant (MRSA)—- has penicillin binding proteins

Question 8

What is the first step in ID of a CATALASE NEGATIVE GPC?

Answer 8

determine hemolysis pattern–> if beta, alpha,or gamma

Question 9

You have an alpha-hemolytic lancet-shaped diplococci GPC. What test is most useful right off and why?

Answer 9

The optochin disk; if it is sensitive, it is Strep pneumoniae. Then what?

Question 10

Your alpha-hemolytic gram positive coccus is optochin resistant and bile INsoluble, eliminating Strep pneumo……what are you thinking, what test is next?

Answer 10

Do a bile esculin —> if NEG, probably S. viridans; If POS, could be group D Strep or Enterococcus, need to do PYR and 6.5% NaCl plate to see. Enterococcus is PYR POS, and salt tolerant

Question 11

what is the diff btw a bile solubility test and bile esculin agar/ test? what is “bile”?

Answer 11

a bile solubility uses sodium desoxycholate (a bile salt) to enhance autolysis inherent in Strep pneumoniae–> clears colonies. Bile esculin contains the bile as well as esculin which both enterococcus and group D can hydrolyze, causing blackening of the medium; S. viridans is bile esculin negative

Question 12

What steps follow discovery of a beta-hemolytic gram positive cocci, catalase negative?

Answer 12

PYR and bacitracin: S. pyogenes is PYR positive and bacitracin (lysostaphin) sensitive; if negative/resistant do a CAMP and hippurate to see if GBS (S. agalactiae)

Question 13

You have a beta-hemolytic catalase negative, PYR negative /bacitracin resistant GPC. What has been eliminated and what remains a possibility? how will testing proceed?

Answer 13

Staph is eliminated by catalase negativity; Strep pyogenes is eliminated by the PYR and bacitracin results; Other beta-hemolytic streps now remain: do CAMP and hippurate hydrolysis to rule in or out GBS

Question 14

Your beta-hemolytic catalase neg, PYR neg, bacitracin resistant cocci is also CAMP and Hippurate negative, now what? what could it be and how would you narrow it down?

Answer 14

It could be a group D strep which is also PYR negative: DO bile esculin and 6.5% NaCl growth test—> if positive bile esculin and negative 6.5% it is Grp D; if negative bile esculin it could be a non group A, B or D beta-hemolytic strep

Question 15

what diffs enterococcus from group D strep?

Answer 15

PYR and 6.5% NaCl growth. (enterococcus positive in both)