Question 1

What principle does the TREK ARIS 2X System use to determine antimicrobial susceptibility?

Accepted Answer

The Sensititre Automated Incubator Reader (ARIS 2X, TREK Diagnostics, Cleveland, OH) was the first system to be marketed in the United States that used a fluorometric detection system for detecting growth endpoints of common, rapidly growing bacteria using minimal inhibitory concentration (MIC) or breakpoint formats with either a 5-hour or 18-hour incubation period.

Question 2

A man goes to his doctor because he has a cloudy substance dripping from his penis.A Gram stain is performed and gram-negative intracellular diplococci are observed.What is the drug of choice to treat this infection?

Accepted Answer

Although for many years penicillin was the drug of choice for treating uncomplicated gonorrhea, the increased incidence of penicillin-resistant isolates has led to the use of ceftriaxone or a fluoroquinolone as first-line therapy.

Question 3

When testing Escherichia coli, Klebsiella spp., and Proteus mirabilis, which drug is mostly likely to indicate the presence of an extended-spectrum b-lactamases (ESBL)?

Accepted Answer

Strategies for laboratory detection of ESBL-producing E.coli, Klebsiella spp., and P.mirabilis include testing of those drugs that are most likely to indicate the presence of an ESBL (the "indicator" drugs).These include, in decreasing order of sensitivity for detection, cefpodoxime, ceftazidime, cefotaxime, ceftriaxone, aztreonam, and use of special screening zone or minimal inhibitory concentration (MIC) breakpoints to facilitate recognition of ESBL production.

Question 4

The Clinical and Laboratory Standards Institute (CLSI) has published methods for susceptibility testing of several agents of potential bioterrorism, including Brucella anthracis, Yersinia pestis, Burkholderia mallei, B.pseudomallei, Francisella tularensis, and Brucella spp.How does CLSI suggest clinical laboratories perform antimicrobial susceptibility testing on these isolates?

Accepted Answer

Primarily for use by public health laboratories capable of safely working with select agents, the CLSI has published methods for susceptibility testing of several agents of potential bioterrorism, including B.anthracis, Y.pestis, B.mallei, B.pseudomallei, F.tularensis, and Brucella spp.Clinical laboratories should not attempt to perform susceptibility tests on these species, but should refer any suspected isolates to appropriate public health laboratories.

Question 5

On what media is Kirby-Bauer testing performed for Neisseria gonorrhoeae?

Accepted Answer

GC agar base is supplemented with various nutrients for testing N.gonorrhoeae.Disk diffusion tests are performed on the same agar (GC base), and all tests are incubated in an atmosphere containing 5% to 7% CO₂.Clinical and Laboratory Standards Institute (CLSI) has specified interpretive criteria unique for this species.

Question 6

Which enterococcus is/are the most common species demonstrating vancomycin resistance among clinical isolates?

Accepted Answer

The incidence of vancomycin resistance in enterococci increased sharply in the 1990s and the early part of the decade of 2000s.E.faecium is the most common species demonstrating vancomycin resistance among clinical isolates, followed by E.faecalis.

Question 7

What principle does the MicroScan Walkaway SI use to determine antimicrobial susceptibility?

Accepted Answer

Most current instruments use the principle of turbidimetric detection of bacterial growth in a broth medium by use of a photometer to examine the test wells.

Question 8

Oxacillin-resistant Staphylococcus aureus is called _____-resistant S.aureus.

Accepted Answer

Isolates of oxacillin-resistant S.aureus are commonly referred to as methicillin-resistant S.aureus (MRSA) for historical reasons.

Question 9

Spontaneous mutations occur in the b-lactamase genes that result in extended-spectrum b-lactamases (ESBL).These ESBL inactivate:

Accepted Answer

Spontaneous mutations occur that may result in novel b-lactamases that can inactivate extended-spectrum cephalosporins, penicillins, and aztreonam.These b-lactamases are known as extended-spectrum b-lactamases.

Question 10

What antibiotics are active against extended-spectrum b-lactamases (ESBL)-producing strains of bacteria?

Accepted Answer

Consequently, when an ESBL-producing isolate is identified, it should be reported as clinically resistant to all cephalosporins, penicillins, and aztreonam, despite the standard in vitro test results.The carbapenems (imipenem, meropenem, ertapenem) are active against ESBL-producing strains, as are the cephamycins (cefoxitin and cefotetan).

Question 11

How many hours of incubation are necessary for the antimicrobial susceptibility test run on the BD Phoenix?

Accepted Answer

The Phoenix reads the susceptibility determinations following approximately 6 to 8 hours of incubation.

Question 12

Therapy of disseminated meningococcal infections and various types of gonococcal infections are:

Accepted Answer

Neisseria gonorrhoeae and N.meningitidis are organisms of public health significance that may be isolated by clinical laboratories with varying frequencies.Therapy of disseminated meningococcal infections and various types of gonococcal infections are generally empiric based on recommendations from CDC and various professional groups.Most of the time, clinical microbiology laboratories are not required to perform antimicrobial susceptibility testing of these two species.

Question 13

What principle do most current automated antimicrobial susceptibility instruments use to read growth?

Accepted Answer

Most current instruments use the principle of turbidimetric detection of bacterial growth in a broth medium by use of a photometer to examine the test wells.

Question 14

How long are minimal inhibitory concentration (MIC) panels for anaerobes incubated?

Accepted Answer

Additionally, the number of organisms in the test inoculum is 0.5 log₁₀ higher (10⁶ colony-forming units [CFU]/mL) than that for testing aerobes, and panels are incubated anaerobically at 35° C for 48 hours.

Question 15

To obtain a bactericidal effect in enterococci, ampicillin or penicillin must be given in combination with:

Accepted Answer

To obtain a bactericidal effect, ampicillin or penicillin (or vancomycin in the penicillin-allergic patient or with Enterococcus faecium) must be given in combination with an aminoglycoside-usually gentamicin or sometimes streptomycin.

Question 16

What automated system was originally designed for use in outer space?

Accepted Answer

The VITEK System (bioMérieux Vitek, Durham, NC) was originally designed for use in the United States space exploration efforts of the 1970s as an onboard test system for spacecraft exploring other planets for life.

Question 17

Oxacillin-resistant staphylococci can appear susceptible in vitro to other b-lactam agents such as:

Accepted Answer

Sometimes, oxacillin-resistant staphylococci can appear susceptible in vitro to other b-lactam agents, such as cephalosporins; however, these are clinically ineffective.Consequently, all oxacillin-resistant staphylococci must be reported as resistant to all b-lactam agents, including cephalosporins, b-lactam/b-lactamase inhibitor combinations, and carbapenems, if those agents are tested, regardless of the in vitro test results.

Question 18

What type of antimicrobial susceptibility testing method does Clinical and Laboratory Standards Institute (CLSI) recommend for anaerobes?

Accepted Answer

The reference method described by the CLSI for testing anaerobic bacteria is an agar dilution method, and the recommended medium is supplemented Brucella blood agar.As previously mentioned, however, agar dilution is not practical for use in the routine clinical laboratory, and a broth microdilution method is more often used.

Question 19

In vitro testing conditions that can be modified to enhance the expression of oxacillin resistance include all the following except:

Accepted Answer

In vitro testing conditions can be modified to enhance the expression of oxacillin resistance; they are as follows: preparation of inocula using the direct inoculum suspension procedure, incubation of tests at temperatures no greater than 35° C, and making final test readings after a full 24 hours of incubation.

Question 20

The BD Phoenix system uses what principle to measure bacterial growth in the susceptibility test wells?

Accepted Answer

The Phoenix uses a redox indicator system (similar to resazurin) to measure bacterial growth in the susceptibility test well.The indicator is added to the broth at the time of organism inoculation.

Quiz 13: Antimicrobial Susceptibility Testing