Proper media for culture of a urethral discharge from a man include:
D. chocolate and modified Thayer-Martin agars - should be used for the recovery of N. gonorrhoeae. Chocolate provides the nutrients required by N. gonorrhoeae and selective media contains antimicrobial agents that inhibits other organisms and permits recovery of pathogenic Neisseria.
A sheep blood agar plate inoculated with 0.001 mL of urine grows 70 colonies of S. aureus. How many colony forming units per mL of urine should be reported?
A. 70,000 - when 0.001 mL of urine is plated, the growth of one colony is equivalent to 1000 CFU/mL.
The lowest concentration of antibiotic that inhibits growth of a test organism is the:
B. minimum inhibitory concentration - the MIC is a basic lab measurement of an antibiotic against an organism. It is the lowest concentration of antibiotic that is lethal to the organism.
Which of the following clean catch urine culture colony counts indicates the patient likely has a UTI?
D. 103 CFU/mL - Patients with an infection often have at least 100,000 bacteria/mL of urine in the bladder. However one third of young women with symptomatic cystitis have less than 100,000 bacteria/mL of urine.
The steam autoclave method of sterilization:
C. uses 15 lbs of pressure for 15 minutes - the traditional gravity displacement steam sterilization cycle is 121C for 15 minutes at 15 pounds per square inch. Ethylene oxide is an alternative sterilization method.
The expected colony count in a suprapubic urine from a healthy individual is:
D. 0 CFU/mL - urine in the bladder is normally sterile. Suprapubic aspiration removes urine directly from the bladder and yields a specimen free of urethral contamination.
An aspirate of a deep wound was plated on BAP and incubated aerobically and anaerobically. At 24 hours there was growth on both plates. This indicates that the organism is a(n):
B. facultative anaerobe - can grow under both aerobic and anaerobic conditions
The proper blood-to-broth ratio for blood cultures to reduce the antibacterial effect of serum in adults is:
B. 1:10 - human blood contains substances that may inhibit microbial growth. Diluting blood in culture broth reduces the concentration of these substances as well as any antibiotics that may present. The recommended blood broth ratio is 1:5-1:10. Dilutions less than this may cause the blood to clot, trapping organisms in the clot. Greater dilutions may increase the time to detection.
A penicillin-resistant Neisseria gonorrhoeae produces:
C. beta-lactamase - antimicrobial resistance in N. gonorrhoeae is widespread. The production of beta lactamase breaks open the beta lactam ring of the penicillin, destroying its activity.
Which selective medium is used for the isolation of gram positive microorganisms?
A. Columbia CNA with 5% sheep blood - Columbia CNA contains colistin and nalidixic acid, which inhibit most gram negative organisms. Eosin methylene blue is selective and inhibits gram positive organisms and modified Thayer-Martin is selective and inhibits gram positive organisms, gram negative bacilli, and yeast.
The most sensitive substrate for the detection of beta lactamase is:
B. nitrocefin - the chromogenic cephalosporin test using nitrocefin is the most sensitive and specific test for detection of beta lactamase. Acidimetric tests employing penicillin are less expensive, but not as sensitive, as the nitrocefin assay.
A Staph aureus isolate has an MIC of 4 ug/mL to Oxacillin. There is uncertainty as to whether this represents an Oxacillin (heteroresistant) resistant strain or a hyperproducer of beta-lactamase.
Based on the above results for Oxacillin and Amoxicillin-Clavulanic Acid, which strain is heteroresistant?
B. strain D - A Staphylococcus aureus isolate with an MIC of 4 ug/mL is resistant to Oxacillin. As per CLSI recommendations, a heteroresistant strain would be defined as a Methicillin (Oxacillin) resistant S. aureus (MRSA/ORSA). An MRSA/ORSA isolate would be reported as resistant to all beta lactam agents, beta lactam/beta lactamase inhibitor combinations, such as Amoxicillin-Clavulanic Acid, and Carbapenems.
An Enterococcus isolated from multiple blood cultures in a patient endocarditis should be:
A. screened for high level aminoglycoside resistance - systemic enterococcal infections, such as endocarditis, are commonly treated with a cell-wall-active agent and an aminoglycoside. These agents act synergistically to kill the organism. If the organism is resistant to one or both, there is no synergy, and the combination will fail. It is important to detect aminoglycoside and beta-lactam resistance in these cases. Enterococci have intrinsic moderate level resistance to aminoglycosides. Acquired resistance corresponds to very high MICs (>500 ug/mL) for gentamicin and is termed high level resistance.
In the Kirby-Bauer disc diffusion susceptibility test, which variable is critical when testing Pseudomonas spp. for antibiotic susceptibility to aminoglycosides?
A. cation content of media - variations in the concentrations of divalent cations primarily calcium and magnesium affect the results of aminoglycoside, tetracycline, and colistin tests with P. aeruginosa isolates. A cation concentration that is too high results in smaller zone sites, and a concentration that is too low increases zone sizes.
The procedure that assures the most accurate detection of mecA-mediated Oxacillin resistance to routine broth microdilution susceptibility testing against S. aureus is:
C. use of Cefoxitin for testing - Cefoxitin is used as a surrogate for mecA-mediated oxacillin resistance in S aureus. S aureus with cefoxitin MICs >4 ug/mL are considered oxacillin resistant. CLSI recommends the addition of 2% NaCl, incubation at 35 +/- 2C, and incubation for 24 hours when performing susceptibility testing of S aureus against oxacillin.
When performing a stool culture, a colony type typical of an enteric pathogen is subcultures on a BAP. The resulting pure culture is screened with several tests to obtain the following results:
TSI: K/A, no gas, no H2S
Phenylalanine deaminase: Negative
Serological typing: Shigella flexneri (Shigella subgroup B)
The serological typing is verified with new kit and controls. The best course of action would be to:
A. verify reactivity of motility medium with positive and negative controls - all Shigella are nonmotile
Susceptibility testing performed on QC organisms using a new media lot number yielded zone sizes that were too large for all antibiotics tested. The tested was repeated using media from a previously used lot number, and all zone sizes were acceptable. Which of the following best explains the unacceptable zone sites?
B. the depth of the media was too thin - all Mueller Hinton agar used for disk diffusion should be poured to a depth of 4mm.
Three sets of blood cultures were obtained form an adult patient with fever and suspected endocarditis. The aerobic bottle of one set had growth of Staph epidermidis at 5 days of incubation. This indicates that:
C. the organism is most likley a contaminant
In order to isolate Campylobacter coli/jejuni, the fecal specimen should be:
A. inoculated onto selective plating media and incubated in reduced O2 with added CO2 at 42C
An expectorated sputum is sent to the lab for culture from a patient with respiratory distress. The gram stain of the specimen shows many squamous epithelial cells (>25/lpf) and rare neutrophils. The microscopic appearance of the organisms present include:
moderate GPC in chains and diplococci
moderate palisading GPR all in moderate amounts
The gram stain is most indicative of:
B. oropharyngeal flora
Which type of microscope would be most useful in examining viruses and the structure of microbial cells?
After satisfactory performance of daily disk diffusion susceptibility quality control is documented, the frequency of QC can be reduced to:
D. every week - daily disk diffusion QC can be converted to weekly testing when 30 days of consecutive testing demonstrates no more than 3 antibiotic/organism combinations outside of the acceptable limits.
In disk diffusion susceptibility testing, as an antimicrobial agent diffuses away from the disk, the concentration of the antibiotic is:
The most common cause for failure of a GasPak anaerobic jar to establish an adequate environment for anaerobic incubation is:
D. catalysts that have become inactivated after repeated use
Which of the following is the most appropriate method for collecting a urine specimen from a patient with an indwelling catheter?
B. aspirate urine aseptically from the catheter tubing
Which one of the following groups of specimens would be acceptable for anaerobic culture?
D. pleural fluid, brain abscess
Which one of the folowing specimen requests is acceptable?
A. urine for culture of AFB
A liquid fecal specimen from a three-month-old infant is submitted for micro examination. In additoin to culture on routine media for Salmonella and Shigella, this specimen should be routinely:
B. examined for the presence of Campylobacter spp.
CSF from a febrile 25 year old man with possible meningitis is rushed to the lab for a stat gram stain and culture. While performing the gram sstian, the technologist accidentally spills most of the specimen. The smear shows many neutrophils and no microorganisms. Since there is only enough CSF to inoculate one plate, the technologist should use a:
A. CHOC - has no inhibitory effects on bacterial growth and contains additional nutrients that support the growth of fastidious organisms such as H. influenzae and Neisseria.
A diabetic foot swab from an 82 year old womann with recurrent infections is submitted for culture. The gram stain reveals:
Many neutrophils, no squamous epithelial cells
Many gram negative bacilli
Many gram positive cocci in chains
The physician requests that all pathogens be worked up. In addition to the sheep blood and MAC routinely used for wound cultures, the technologist might also process a(n):
A. CNA - is a selective medium commonly used in the isolation of gram positive aerobic and anaerobic organisms.
Which of the following is the most appropriate specimen source and primary media battery?
A. endocervical - CHOC, Martin-Lewis
Sputum and urine specimens are routinely processed using a general purpose agar (BAP) and a selective agar (EMB or MAC). CSF is routinely processed using BAP and CHOC.
Which of the following is the most appropriate organism and media combination?
B. Yersinia enterocolitica - cefsulodin-irgasan-novobiocin (CIN)
A gram stain from a swab of a hand wound reveals:
No squamous epithelial cells
Moderate GPC in chains
Moderate large GNR
Select the appropriate media that will selectively isolate each organism:
D. Columbia CNA, MAC
Columbia CNA is a selective medium used for the isolation of gram positive organisms. The medium contains colistin and nalidixic acid, which inhibits gram negative organisms.
Upon review of a sputum gram stain, the technician notes that the nuclei of all of the neutrophils present in the smear are staining dark blue. The best explanation of this finding is:
D. the slide was inadequately decolorized with acetone/alcohol
When performing a Kovac indole test, the substrate must contain:
The indole test is used for determining an organisms abilits to produce indole from deamination of tryptophan by tryptophanase.
The ONPG test allows organisms to be classified as a lactose fermenter by testing for which of the following:
Through the action of the enzyme beta-galactosidase, ONPG cleaves into galactose and o-nitrophenol (a yellow compound).
Sodium bicarbonate and sodium citrate are components of which of the following?
D. JEMBEC system
The bicarbonate-sodium citrate pellet absorbs moisture to generate a carbon dioxide-rich environment.
Chocolate agar base containing vancomycin, colistin, anisomycin, and trimethoprim is also known as:
D. Martin Lewis agar
ML is a modification of modified Thayer-Martin formulation, and contains a higher concentration of vancomycin and anisomycin instead of nystatin. These modifications provide better inhibition of gram positive organisms and Candida.
A medium that aids in the presumptive ID of organisms based on their appearance on the medium is called:
Differential media contain compounds, often carbohydrates, that provide a presumptive ID based on colony color or a precipitate around the colony.
SPS is used as an anticoagulant for blood cultures because it:
C. inactivated neutrophils and components of serum complement
When evaluating a new susceptibility testing system, if the new characterizes a susceptible isolate as resistant, this is termed a:
D. major error
A very major error occurs when the system characterizes a resistant isolate as susceptible; a major error occurs when the system characterizes a susceptible isolate as resistant; and a minor error occurs when the system characterizes a susceptible or resistant isolate as intermediate, or an intermediate isolates as susceptible or resistant.
Which of the following methods is inadequate for the detection of vancomycin-intermediate S aureus?
A. disk diffusion
The disk diffusion procedure will not differentiate S aureus strains with reduced susceptibility to vancomycin (MICs 4-8 ug/mL) from susceptible strains even when incubated for 24 hours
In a disk diffusion susceptibility test, which of the following can result if disks are placed on the inoculated media and left at room temperature for an hour before incubation?
A. zones of larger diameter would result
A delay of more than 15 minutes between placing the disk on an inoculated plate and incubation permits excess prediffusion of the antimicrobial agent from the disk.
Which of the following factors would make an organism appear to be more resistant on a disk diffusion susceptibility test?
B. too many organisms in the inoculum
First generation cephalosporins can be adequately represented by:
Cefotetan and cefoxitin are second generation cephalosporins, and ceftriaxone is a third generation cephalosporin.
An antibiotic that inhibits cell wall synthesis is:
Inhibits penicillin binding proteins that are essential to peptidoglycan (cell wall) synthesis. Chloramphenicol inhibits protein synthesis, colistin increases cell membran permeability, and sulfamethoxazole inhibits folate metabolism.
Which one of the following organisms does not require susceptibility testing to the antimicrobial indicated when isolated from a clinically significant source?
B. Strep pyogenes - penicillin
Which of the following must be incubated in a microaerophilic environment for optimal recovery of the organism?
A. Campylobacter jejuni
Which of the following antibiotics would routinely be tested and reported for isolates of Pseudomonas aeruginosa?
Aminoglycoside antibiotics such as gentamicin are active against Pseudomonas and routinely tested and reported on these isolates. Penicillin, erythromycin, and clindamycin are not active against Pseudomonas.
Diagnosis of typhoid fever can be confirmed best by culture of:
A. bone marrow
The sensitivity of blood culture is only 50-70%. Stool cultures are positive in <50% of cass and urine cultures are positive even less frequently. Bone marrow has a sensitivity of up to 90%. Higher colony counts are present in bone marrow and counts are not decreased by up to 5 days of antimicrobial therapy prior to specimen collection.
Vibrio parahaemolyticus can be isolated best from feces on:
B. thiosulfate citrate bile salts (TCBS) agar
TCBS is a highly selective and differential medium for the recovery of most Vibrio spp. HE and SS are selective and differential for the isolation and differentiation of enteric pathogens such as Salmonella and Shigella. EMB is selective and differential for gram negative enteric bacilli.
Which of the following media can be used to culture Campylobacter jejuni?
B. Skirrow medium
Skirrow medium is an enriched selective blood agar medium used for the isolation for Campylobacter from specimens with mixed flora. CIN and bismuth sulfate agars are selective and differential for Yersinia enterocolitica and Salmonella, respectively. CNA is selective for gram positive organisms.
Tests for beta lactamase production in Haemophilus influenzae:
A. should be performed on all blood and CSF isolates
As many as 20-40% of H. influenzae produce beta lactamase.
Media used to support growth of Legionella pneumophilia should contain whcih of the following additives?
B. charcoal and yeast extract
Buffered charcoal yeast extract medium is a specialized enrichment medium for the isolation of Legionella. The nutritive base includes yeast extract. Charcoal is added to the medium as a detoxifying agent.
The best medium for culture of Bordetella pertussis is:
C. Regan-Lowe agar
Cephalexin is added to inhibit nasopharyngeal flora. It provides better isolation of B pertussis that Bordet-Gengou medium.
The best medium for culture of Francisella tularensis is:
C. cystine glucose blood agar
The most rapid method for detection of Francisella tularensis is:
A. polymerase chain reaction
Serological diagnosis may take weeks to confirm. Fluorescent Ab staining and antigen detection in urine using specific monoclonal antibodies are not as widespread as PCR.
When processing throat swabs for GAS culture, the medium of choice is:
D. sheep blood agar
The ability to detect oxacillin-resistant S aureus may be enhanced by:
D. using Mueller-Hinton brother with 2% NaCl
The optimal wound specimen for culture of anaerobic organisms should be:
D. a syringe filled with pus, obtained before administration of antibiotics