Skip to content

Add your earlier progress to your account?

From before you signed in, this browser has:

Which plan do you want to keep?

You answered the plan questions before you signed in, and those answers don't match the plan saved on your account.

SettingSaved planNew answers

Study progress is waiting to be saved

Vancomycin, enterococci and pneumococci

17 min

Skills you'll build4 skills
  • Select a validated vancomycin MIC method for S. aureus
  • Investigate unexpected reduced vancomycin susceptibility in S. aureus
  • Read enterococcal high-level gentamicin and streptomycin screens for synergy
  • Choose meningitis or nonmeningitis breakpoints for a pneumococcal MIC

At the bench: Review susceptibility results for method validity and expected resistance

Try first

Try first

A Staphylococcus aureus from a blood culture was set up by disk diffusion, and the vancomycin disk shows a clear zone. What does the laboratory do for vancomycin?

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Vancomycin and S. aureus

Vancomycin-intermediate S. aureus (VISA) has an MIC of 4 to 8 µg/mL and is caused by a thickened cell wall. Vancomycin-resistant S. aureus (VRSA) has an MIC of 16 µg/mL or more and has acquired the enterococcal vanA genes.1 Disk diffusion detects neither, so vancomycin is tested by an MIC method read at a full 24 hours.1,2

When an MIC comes back higher than expected:

  1. Check purity. A mixed culture can produce an apparent high MIC.
  2. Repeat the identification and the MIC.
  3. Save any S. aureus with an MIC of 8 µg/mL or more, and send it to the state public health laboratory for confirmation.1

An MIC of 4 µg/mL is repeated and confirmed as well.1

Enterococci and aminoglycosides

Aminoglycosides are not active by themselves against enterococci. They are used for synergy with a cell-wall agent, so they are reported only through high-level resistance screens:2

  • Gentamicin 500 µg/mL in broth, or a 120-µg disk.
  • Streptomycin 1,000 µg/mL in broth or 2,000 µg/mL on agar, or a 300-µg disk.
  • Growth in the screen, or a resistant zone, indicates no synergy with that drug.
  • A disk zone of 7 to 9 mm is inconclusive and goes to a dilution test.
  • A streptomycin broth screen that shows no growth at 24 hours incubates to a full 48 hours.

Gentamicin and streptomycin are screened separately, because one does not predict the other. A routine aminoglycoside MIC does not predict synergy.2

Pneumococci and the infection site

Pneumococci resist beta-lactams through altered penicillin-binding proteins and make no beta-lactamase.2 Their breakpoints are in separate sets for meningitis and nonmeningitis:

  • A 1-µg oxacillin disk screens nonmeningitis isolates. A zone of 20 mm or more predicts penicillin susceptibility.
  • A zone of 19 mm or less needs a penicillin MIC.
  • Isolates from cerebrospinal fluid (CSF) are not screened with the disk. Penicillin and third-generation cephalosporin MICs are interpreted with the meningitis breakpoints.2

Follow the bench demonstration

Sources for this step
References
  1. Centers for Disease Control and Prevention. Laboratory testing for vancomycin-resistant Staphylococcus aureus. Reviewed April 15, 2024. Accessed September 27, 2026. https://www.cdc.gov/staphylococcus-aureus/php/laboratories/index.html
  2. Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing. 36th ed. CLSI supplement M100. CLSI; 2026. Accessed September 27, 2026. https://clsi.org/shop/standards/m100/

Watch one

Read each decision and its explanation.

The automated panel reports a vancomycin MIC of 8 µg/mL on an S. aureus from a blood culture. Three earlier isolates from this patient had MICs of 1 µg/mL.

What does the laboratory do before any vancomycin result is final?

  1. Note the change. An MIC three doubling dilutions above the earlier isolates is unexpected.

    An unexpected result is compared with the patient's history first.

  2. Check the purity plate. It shows a second, smaller colony type, so the culture was mixed.

    A second organism in the well can grow through the drug and raise the MIC.

  3. Reisolate the S. aureus and repeat the identification.

    Only a pure culture gives an MIC for the S. aureus itself.

  4. Repeat the MIC on the pure isolate. It is again 8 µg/mL.

    The repeat uses a validated MIC method with its full 24-hour read.

  5. Save the isolate and refer it to the state public health laboratory. Notify infection prevention by the laboratory's procedure.

    An MIC of 8 µg/mL or more needs confirmation outside the hospital.

Reisolate from the mixed culture, repeat the identification and a validated MIC, and when the pure isolate still reads 8 µg/mL, save it and refer it to the state public health laboratory.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

Which approach is appropriate for assessing vancomycin susceptibility in S. aureus?

A validated MIC method can detect reduced vancomycin susceptibility. Review purity and confirm an unexpected result.

Disk diffusion is unreliable for this organism-drug combination and is not recommended.

Cefoxitin evaluates methicillin resistance. It cannot provide a vancomycin susceptibility result.

Hint
  1. Consider which method detects an MIC of 4 to 8 µg/mL.
  2. Zone diameters and MICs correlate poorly for this organism and drug.

Review Vancomycin-intermediate and -resistant S. aureus

Problem 2 of 3 · Hints available

An Enterococcus faecalis from a blood culture gives a 120-µg gentamicin disk zone of 8 mm. What does the laboratory do?

A zone of 7 to 9 mm is inconclusive. It does not show resistance.

The zone is too small to interpret as susceptible. An inconclusive disk does not support synergy.

Zones of 7 to 9 mm are inconclusive and need a dilution test before gentamicin synergy is reported.

A routine aminoglycoside MIC does not predict synergy for enterococci. Only the high-level screen does.

Read a routine aminoglycoside result as synergy

Aminoglycosides alone are clinically inactive against enterococci and are reported only through high-level resistance screens for synergy with a cell-wall agent. A routine aminoglycoside MIC does not predict synergy, and gentamicin and streptomycin are screened separately because one does not predict the other.

Hint
  1. The high-level disks have a zone range that cannot be interpreted.
  2. Consider which method resolves an inconclusive disk result.

Review Enterococcus

Problem 3 of 3 · On your own

A Streptococcus pneumoniae from CSF has a penicillin MIC of 0.12 µg/mL. The laboratory's penicillin breakpoints are, for meningitis, susceptible 0.06 µg/mL or less and resistant 0.12 µg/mL or more, and for nonmeningitis, susceptible 2 µg/mL or less, intermediate 4 µg/mL and resistant 8 µg/mL or more. What is reported?

That interpretation uses the nonmeningitis breakpoints. A CSF isolate is interpreted with the meningitis set.

Applied a nonmeningitis interpretation to a CSF isolate

Pneumococcal penicillin and cephalosporin results have separate meningitis and nonmeningitis breakpoints, and CSF isolates go straight to MIC testing without the oxacillin screen. Reading a CSF isolate against nonmeningitis breakpoints can report a drug as susceptible at an MIC that fails in meningitis.

The meningitis set has no intermediate category for penicillin. An MIC of 0.12 µg/mL falls in its resistant range.

CSF isolates are not screened with the oxacillin disk. They go directly to MIC testing.

A CSF isolate is interpreted with the meningitis breakpoints, where an MIC of 0.12 µg/mL is resistant.

Review Pneumococcal and streptococcal resistance

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • An E. faecalis grows from three blood culture sets in a patient being worked up for endocarditis.
  • Ampicillin tests susceptible.
  • The gentamicin high-level broth screen, 500 µg/mL, shows growth.
  • The streptomycin high-level broth screen, 1,000 µg/mL, shows no growth at 24 hours.
  • The automated panel also prints a routine gentamicin MIC of 4 µg/mL.
Decision 1 of 3

What does the gentamicin screen mean?

Growth in the 500-µg/mL screen is high-level resistance. Gentamicin gives no synergy with a cell-wall agent against this isolate.

The routine MIC does not predict synergy. The high-level screen shows resistance.

Read a routine aminoglycoside result as synergy

Aminoglycosides alone are clinically inactive against enterococci and are reported only through high-level resistance screens for synergy with a cell-wall agent. A routine aminoglycoside MIC does not predict synergy, and gentamicin and streptomycin are screened separately because one does not predict the other.

Growth at 24 hours is a resistant result. The 48-hour rule applies to a streptomycin broth screen with no growth.

Review Enterococcus

Decision 2 of 3

What happens to the streptomycin screen?

A streptomycin broth screen with no growth at 24 hours must incubate the full 48 hours before it is read as susceptible.

Gentamicin and streptomycin are screened separately. One does not predict the other.

Read a routine aminoglycoside result as synergy

Aminoglycosides alone are clinically inactive against enterococci and are reported only through high-level resistance screens for synergy with a cell-wall agent. A routine aminoglycoside MIC does not predict synergy, and gentamicin and streptomycin are screened separately because one does not predict the other.

The streptomycin broth screen needs a full 48 hours. A susceptible reading at 24 hours is read again after the full incubation.

Review Enterococcus

Decision 3 of 3

What happens to the routine gentamicin MIC of 4 µg/mL?

Aminoglycosides are not active by themselves against enterococci. The report gives only the synergy screen result.

A routine aminoglycoside result implies activity by itself, and it does not predict synergy. Enterococcal gentamicin is reported only from the high-level screen.

Read a routine aminoglycoside result as synergy

Aminoglycosides alone are clinically inactive against enterococci and are reported only through high-level resistance screens for synergy with a cell-wall agent. A routine aminoglycoside MIC does not predict synergy, and gentamicin and streptomycin are screened separately because one does not predict the other.

The routine MIC does not predict synergy, and a comment does not change that.

Review Enterococcus

Each result depends on which test was used. For enterococci, the high-level screens are the only reportable aminoglycoside results. Gentamicin shows no synergy, streptomycin is read again at 48 hours, and the routine MIC is not reported.

For reference

Sources checked