Closing the Brief Case: Staphylococcus intermedius Group—Look What the Dog Dragged In

ANSWERS TO SELF-ASSESSMENT QUESTIONS

1. Which biochemical test can be helpful as a screen to differentiate SIG species from Staphylococcus aureus?

   1. Catalase
   2. Pyrrolidonyl arylamidase (PYR)
   3. Coagulase
   4. Ornithine decarboxylase

   Answer: B. Whereas both SIG members and S. aureus are catalase and coagulase positive, only SIG species are PYR positive. Both are negative for ornithine decarboxylase. However, an ornithine-positive Staphylococcus species that also gives a positive reaction in latex agglutination and PYR testing is Staphylococcus lugdunensis.

2. Disk diffusion testing with which antimicrobial agent should be tested as a surrogate for methicillin resistance testing with members of the Staphylococcus intermedius group?

   1. Cefazolin
   2. Methicillin
   3. Oxacillin
   4. Cefoxitin

   Answer: C. Oxacillin has been shown to be a better surrogate marker to predict the presence of mecA and methicillin resistance in SIG species. This is in contrast to S. aureus, where cefoxitin is the surrogate for prediction of mecA-mediated methicillin resistance. This was added to the CLSI M100 guidelines in 2016.

3. Which staphylococcal species is not a member of the Staphylococcus intermedius group?

   1. Staphylococcus pseudintermedius
   2. Staphylococcus delphini
   3. Staphylococcus lugdunensis
   4. Staphylococcus intermedius

   Answer: C. The SIG consists of three species: Staphylococcus intermedius, Staphylococcus pseudintermedius, and Staphylococcus delphini. Like the members of the SIG, Staphylococcus lugdunensis can test positive for both coagulase (latex agglutination testing) and PYR but is also ornithine decarboxylase positive.

TAKE-HOME POINTS

• SIG species can easily be misidentified as S. aureus in clinical laboratories, but they can be differentiated using MALDI-TOF MS.

• Biochemically, PYR can be used to differentiate SIG members (PYR positive) from S. aureus (PYR negative).

• Whereas the prevalence of methicillin resistance in SIG veterinary isolates was low historically (approximately 5% until the early 2000s), it has been increasing rapidly, with estimates as high as 30% in 2007.

• Methicillin resistance in SIG isolates can be detected using oxacillin disk diffusion as a surrogate marker, unlike in S. aureus and coagulase-negative staphylococci, for which cefoxitin disk diffusion is used.

• Immunochromatographic testing for the presence of PBP2a, which confers methicillin resistance, can be performed for SIG isolates, though induced PBP2a testing (using cefoxitin) is needed for some SIG isolates.

See https://doi.org/10.1128/JCM.00839-17 in this issue for case presentation and discussion.