SUCCESS! In Clinical Laboratory Science Bacteriology - Antimicrobial Agents and Antimicrobial Susceptibility Testing

Which of the following antimicrobial agents acts by inhibiting protein synthesis? A. Gentamicin B. Methicillin C. Rifampin D. Ampicillin

A. Gentamicin is a member of the aminoglycoside group of antibiotics. These drugs act on the 308 ribosomal subunit to inhibit protein synthesis. Gentamicin is particularly effective against a wide variety of gram-negative bacilli.

Which of the following is not one of the standard control organisms used for the weekly testing of antimicrobial disks? A. Staphylococcus epidermidis (ATCC 25833) B. Pseudomonas aeruginosa (ATCC 27853) C. Escherichia coli (ATCC 25922) D. Enterococcus faecalis (ATCC 29212)

A. Standard quality control strains maintained by the American Type Culture Collection (ATCC) should be tested routinely as recommended by the Clinical and Laboratory Standards Institute. Guidelines developed for the quality assurance of the disk-diffusion antimicrobial susceptibility test procedure recommended that the following organisms be used for this purpose: Pseudomonas aemginosa (ATCC 27853), Staphylococcus aureus (ATCC 25923), Escherichia coli (ATCC 25922), and Enterococcus fecalis (ATCC 29212). Cultures of these organisms should be frozen or lyophilized to maintain their antimicrobial susceptibility pattern. Testing should not be done from stored cultures but rather from freshly grown 18- to 24-hour cultures.

A suspension of the test organism for use in broth dilution and disk diffusion testing is adjusted to match the turbidity of a A. #0.5 McFarland standard B. #1.0 McFarland standard C. #2.0 McFarland standard D. #3.0 McFarland standard

A. Standardization of the susceptibility testing procedure is essential for determining the susceptibility of an organism to antimicrobial agents. A #0.5 McFarland standard is used when adjusting the turbidity of the suspension of test organism. A #0.5 McFarland standard has a turbidity consistent with approximately 1.5 X 108organisms/mL of broth or saline.

Chloramphenicol is an important antimicrobial agent for the treatment of meningitis as well as several other serious infections. Unfortunately, chloramphenicol exhibits significant complications that limit its clinical usefulness. These effects include A. Allergic reactions and anaphylaxis B. Bone marrow suppression and aplastic anemia C. Significant gastrointestinal manifestations D. Photosensitivity

B. Bone marrow toxicity is the major complication of chloramphenicol. Reversible bone marrow suppression with anemia, leukopenia, and thrombocytopenia occurs as a direct result of the agent on hematopoiesis. The second form of bone marrow toxicity is a rare but usually fatal aplastic anemia. The mechanism of this response is not known.

Clavulanic acid is classified as a A. Beta-lactam B. Beta-lactamase inhibitor C. Macrolide D. Aminoglycoside

B. Clavulanic acid is a beta-lactamase inhibitor. It can be administered with amoxicillin or ticarcillin and is effective in treating infections caused by beta-lactamase-producing bacteria such as staphylococci, Klebsiella, and Haemophilus influenzae. Sulbactam and tazobactam are also beta-lactamase inhibitors.

Resistance to clindamycin can be induced in vitro by A. Ampicillin B. Erythromycin C. Gentamicin D. Penicillin

B. Even though clindamycin and erythromycin are in different classes, the mechanisms of resistance are similar. The presence of erythromycin can induce clindamycin resistance. The D-zone test is used to detect the presence of this inducible resistance.

Rapid testing for beta-lactamase production is recommended, before initiation of antimicrobial therapy, for isolates of A. Serratia marcescens B. Haemophilus influenzas C. Staphylococcus epidermidis D. Streptococcus pyogenes

B. Haemophilus influenzae should be tested for beta-lactamase production. The test can be performed directly, and the methods are rapid and reliable for the detection of penicillin and ampicillin resistance. Rapid test methods, in general, rely on a color change to detect the presence of this enzyme. A pH indicator may be used to detect the penicilloic acid produced when the beta-lactam ring of penicillin is cleaved, or a color change can be observed when the beta-lactam ring of a chromogenic cephalosporin is hydrolyzed by the enzyme.

Which drug known to be active against parasitic infections has importance as a therapeutic agent in cases of disease caused by anaerobic bacteria? A. Isoniazid B. Metronidazole C. Rifampin D. Trimethoprim

B. Metronidazole, a drug recommended for the treatment of amebic dysentery and trichomoniasis, is a synthetic compound that acts by inhibiting DNA synthesis. The use of this drug for treating anaerobic infections has gained emphasis in light of resistance patterns of many of the commonly recovered anaerobes. Metronidazole is consistently active against all gram-negative, anaerobic bacilli; is able to cross the blood-brain barrier; and is the only agent consistently bactericidal against susceptible isolates.

The pH of the agar used for the KirbyBauer test should be A. 7.0-7.2 B. 7.2-7.4 C. 7.4-7.6 D. 7.6-7.8

B. The Kirby-Bauer or disk-agar diffusion susceptibility test requires that the pH of the agar be tested at room temperature to ensure an optimal range of 1.2-1.4 before use in the procedure. A sample of the Mueller-Hinton medium can be tested by macerating it in distilled water and testing with a pH meter electrode; a surface electrode is acceptable for direct testing. Another acceptable method is to allow the agar to solidify around the electrode of a pH meter and then obtain a reading.

The extended-spectrum beta-lactamases confer resistance to A. Amoxicillin B. Ceftriaxone C. Erythromycin D. Rif ampin

B. The extended spectrum beta-lactamases (ESBLs) confer resistance to the extended spectrum cephalosporins such as ceftriaxone and cefotaxime. ESBLs cleave the antibiotic, inactivating it. So far, ESBLs have only been found ingram-negative bacteria.

Beta-lactamase-producing strains of Haemophilus influenzas are resistant to A. Chloramphenicol B. Erythromycin C. Penicillin D. Trimethoprim sulfamethoxazole

C. Beta-lactamase production by strains of Haemophilus influenzae renders them resistant to the antibacterial effect of penicillin and ampicillin. It is recommended that rapid beta-lactamase testing be performed on isolates in life-threatening clinical infections such as meningitis. The rapid tests all rely on this enzyme's ability to act on a beta-lactamase ring and in turn produce a color change, which denotes a positive result due to the production of penicilloic acid.

Penicillin is active against bacteria by A. Inhibition of protein synthesis at the 30S ribosomal subunit B. Reduction of dihydrofolic acid C. Inhibition or peptidoglycan synthesis D. Inhibition of nucleic acid function

C. Inhibitors of peptidoglycan synthesis such as penicillin act to inhibit cell wall development. Bacteria unable to produce peptidoglycan for their cell walls are subject to the effects of varying osmotic pressures. The peptidoglycan component of the cell wall protects the bacterium from lysis.

Metronidazole is most commonly recommended for treatment of infections caused by A. Aerobic microorganisms B. Microaerophilic microorganisms C. Obligate anaerobic microorganisms D. Obligate intracellular microorganisms

C. Metronidazole, a nitroimidazole derivative, is active against most of the clinically significantanaerobes. Only some of the non-spore forming, gram-positive anaerobic bacilli and gram-positive anaerobic cocci are resistant to this agent. This drug acts to disrupt bacterial DNA through the production of cytotoxic intermediates.

Which of the following media should be used for in vitro susceptibility testing of S. pneumoniae? A. Chocolate agar B. Charcoal yeast extract agar C. Mueller-Hinton base supplemented with 5% lysed horse blood D. Mueller-Hinton base supplemented with 1% hemoglobin and 1% IsoVitaleX

C. Most fastidious bacteria do not grow satisfactorily in standard in vitro susceptibility test systems that use unsupplemented media. For certain species, such as Haemophilus influenzae, Neisseria gonorrheae, Streptococcus pneumoniae, and other Streptococcus species, modifications have been made to the standard Clinical and Laboratory Standards Institute (CLSI) methods. In the case of S. pneumoniae, current CLSI broth dilution test conditions include cation-supplemented Mueller-Hinton broth with 5% lysed horse blood.

When using the rapid chromogenic cephalosporin method for the detection of beta-lactamase production by an organism, a positive test is indicated by the color A. Yellow B. Green C. Red D. Blue

C. The chromogenic cephalosporin method is the most sensitive test for detecting the production of beta-lactamase enzymes. This yellow compound will become red if the organism produces the enzyme that breaks the beta-lactam ring. Nitrocefin, the commonly used compound, has a high affinity for most bacterial beta-lactamases.

The agar recommended by the Clinical and Laboratory Standards Institute for routine susceptibility testing of nonfastidious bacteria is A. MacConkey agar B. MiddlebrookVHIOagar C. Mueller-Hinton agar D. Trypticase soy agar

C. The recommended plating medium for use in both the disk diffusion and tube dilution susceptibility test procedures is Mueller Hinton. Low in tetracycline and sulfonamide inhibitors, this medium has been found to show only slight batch-to-batch variability. For the susceptibility testing of fastidious organisms (e.g., Streptococcus pneumoniae), 5% lysed sheep blood may be added.

The minimum bactericidal concentration (MBC) of an antimicrobial agent is defined as the lowest concentration of that antimicrobial agent that kills at least of the original inoculum. A. 95.5% B. 97% C. 99.9% D. 100%

C. The requirement of 99.9% killing defines the minimum bactericidal concentration (MBC) of an antimicrobial agent. The MBC test is an additional quantitative assessment of the killing effect of a drug on a specific patient isolate. This test, done to evaluate a drug's activity, is sometimes requested in cases of life-threatening infections.

The supervisor of a microbiology laboratory has been asked to begin performing in vitro antimicrobial susceptibility testing of Mycobacterium tuberculosis because of an increase in the reported resistance in his community. Which of the following methods would be appropriate for this testing? A. Broth microdilution method using Mueller-Hinton broth B. Kirby-Bauer method C. BACTEC method D. Schlichter method

C. The rise in antimicrobial-resistant isolates of Mycobacteria tuberculosis has been an important public health crisis. The accepted methods for determining the in vitro antimicrobial susceptibility of mycobacteria are based on the growth of the microorganisms on solid or in liquid media containing a specified concentration of a single drug. Two such methods that have been described and are in common use in the U.S. are the agar proportion method and the BACTEC 460TB radiometric method.

Which of the following organisms would not routinely undergo antimicrobial susceptibiity testing? A. Escherichia coli B. Proteus mirabilis C. Staphylococcus aureus D. Streptococcus pyogenes

D. Antimicrobial susceptibility testing is not routinely performed on all bacteria. Certain organisms are predictably susceptible to a variety of antimicrobial agents. Therefore, testing is not usually performed even when these organisms are the etiologic agents of infection. Bacteria for which susceptibility tests are usually not performed include Streptococcus pyogenes (group A Streptococcus), Streptococcus agalactiae (group B Streptococcus), and Neisseria meningitidis.

When testing the antimicrobial susceptibility of Haemophilus influenzae strains by disk-agar diffusion, the recommended medium is A. Chocolate agar B. Charcoal yeast extract agar C. Mueller-Hinton base supplemented with 5% sheep blood D. Mueller-Hinton base supplemented with hematin, NAD, and yeast extract

D. Hemophilias Test Medium is recommended for use in the disk-agar diffusion susceptibility testing procedure of Haemophilus. The testing of Haemophilus spp. requires supplemented media to support the growth of these fastidious organisms. In vitro growth of H. influenzae requires the presence of accessory growth factors: X factor (hemin) and V factor (NAD).

The chemotherapeutic agents structurally similar to the vitamin p-aminobenzoic acid that act by inhibiting bacteria via inhibition of folic acid synthesis are A. Aminoglycosides B. Penicillins C. Macrolides D. Sulfonamides

D. Sulfonamides act to interfere with the ability of bacteria to use p-aminobenzoic acid, which is a part of the folic acid molecule, by competitive inhibition. These chemotherapeutic agents are bacteriostatic and not bactericidal. The drug sulfisoxazole is a member of this group and is used in the treatment of urinary tract infections, especially those caused by Escherichia coli, which must synthesize folic acid for growth

The term that denotes a situation in which the effect of two antimicrobial agents together is greater than the sum of the effects of either drug alone is A. Additivism B. Antagonism C. Sensitivity D. Synergism

D. The therapeutic effect of antimicrobial therapy is often increased by the use of a combination of drugs. A combination of antimicrobials is said to be synergistic when the sum of their effects is greater than that derived from either drug when tested independently. A tenfold decrease in the number of viable cells from that obtained by the most effective drug in the combination is the definition of synergism. Synergistic combinations of antimicrobials are used primarily in the treatment of tuberculosis, enterococcal endocarditis, and certain gram-negative bacillus infections.

An example of a bactericidal antibiotic is A. Chloramphenicol B. Erythromycin C. Tetracycline D. Tobramycin

D. Tobramycin, an aminoglycoside, is the only antibiotic, of those listed, that is bactericidal. Bactericidal antibiotics actually destroy the bacteria, whereas bacteriostatic drugs only arrest the growth of the microorganism. All aminoglycosides, with the exception of spectinomycin, are bactericidal in their activity.

The phenomenon of bacterial resistance to the bactericidal activity of penicillins and cephalosporins, with only inhibition of the organism's growth, is known as A. High-level resistance B. Intrinsic resistance C. Inducible resistance D. Tolerance

D. Tolerance is described as the ability of certain strains of organisms to resist lethal concentrations of antimicrobial agents like penicillin. The growth of these organisms is only inhibited by these cidal drugs. This mechanism of bacterial resistance is attributed to a deficiency of cell wall autolysins.

Which of the following antimicrobial agents acts by inhibiting cell wall synthesis? A. Clindamycin B. Gentamicin C. Naladixic acid D. Vancomycin

D. Vancomycin, which acts to inhibit cell wall synthesis of susceptible bacteria, is produced by an actinomycete. The main activity of this drug is to inhibit peptidoglycan synthesis, but it also has an effect on other aspects of bacterial metabolism. Vancomycin is a bactericidal antibiotic