Disk Diffusion Susceptibility Testing and Microorganism Identification

MEDICAL LABORATORY SCIENCE 6. SUMMATIVE EXAM REVIEW QUESTIONS #21 + ANSWERS & RATIONALIZATIONS 1. 2. 3. 4. 5. Quality control results for disk diffusion susceptibility tests yield the following results: aminoglycoside zones too small and penicillin zones too large. This is most likely due to the: a. Inoculum being too heavy b. Inoculum being too light c. pH of Mueller-Hinton agar being too low d. Calcium and magnesium concentration in the agar being too high → Mueller-Hinton Agar used for disk diffusion is standardized at pH 7.2 to 7.4. ▪ Penicillins function better in an acidic environment, so zone sizes would become larger id the media pH is too low. ▪ Aminoglycosides, on the other hand are less effective in an acidic environment, so zone sizes would become smaller if the pH of the media is too low. ▪ Plates should not be incubated in carbon dioxide atmosphere which lowers the pH of the media. Susceptibility testing performed on quality control organisms using a new media lot number yielded zone sizes that were too large for all antibiotics tested. The testing 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 sizes? a. The antibiotic disks were not stored with the proper desiccant b. The depth of the media was too thick c. The depth of the media was too thin d. The antibiotic disks were not properly applied to the media In disk diffusion susceptibility testing, as an antimicrobial agent diffuses away from the disk, the concentration of antibiotic is: a. Increased b. Decreased c. Unchanged d. Inoculum dependent After satisfactory performance of daily disk diffusion susceptibility quality control is documented, the frequency of quality control can be reduced to: a. Twice a week b. Every week c. Every other week d. Every month Overall profile of antimicrobial susceptibility testing results of a specific microorganism to a battery of antimicrobial agents. a. Antibiotic profile b. Antibiogram c. Histogram d. Scattergram 7. Staphylococcus aureus toxin which disrupts the smooth muscle in blood vessels and is toxic to erythrocytes, leukocytes, hepatocytes, and platelets: a. Alpha toxin b. Beta toxin c. Delta toxin d. Gamma toxin Lysostaphin is used to differentiate Staphylococcus from which other genus? a. Streptococcus b. Stomatococcus c. Micrococcus d. Planococcus → Micrococcus vs. Staphylococcus 8. A light yellow colony from a skin lesion grew aerobically and tested as catalase positive and coagulase negative. The organism gram stained as positive cocci in clusters. The organism was modified oxidase positive, bacitracin (0.04U) susceptible and resistant to lysostaphin. What is the identification of this organism? a. Staphylococcus aureus b. Micrococcus luteus c. Staphylococcus epidermidis d. Peptostreptococcus anaerobius 9. Bacitracin resistance (0.04 unit) is used to differentiate: a. Micrococcus spp. from Staphylococcus spp. b. Staphylococcus spp. from Neisseria spp. c. Planococcus spp. from Micrococcus spp. d. Staphylococcus spp. from Streptococcus spp. 10. A wound (skin lesion) specimen obtained from a newborn grew predominantly β-hemolytic colonies of gram-positive cocci on 5% sheep blood agar. The newborn infant was covered with small skin eruptions that gave the appearance of a “scalding of the skin.” The gram-positive cocci proved to be catalase positive. Which tests should follow for the appropriate identification? a. Optochin, bile solubility, PYR b. Coagulase, glucose fermentation, DNase → Ritter-Lyell (S. aureus) c. Bacitracin, PYR, 6.5% salt broth d. CAMP, bile-esculin, 6.5% salt broth 11. An outbreak of Staphylococcus aureus in the nursery department prompted the Infection Control Committee to proceed with an environmental screening procedure. The best screening media to use for this purpose would be: a. CNA agar b. THIO broth c. Mannitol salt agar → selective for S. aureus d. PEA agar 1 12. Inhibitor of Mannitol Salt Agar: a. Mannitol b. Phenol red c. Neutral red d. Sodium chloride → 7.5-10% NaCl 13. An outbreak of Staphylococcus aureus has occurred in a hospital nursery. In order to establish the epidemiological source of the outbreak, the most commonly used typing method is: a. Pulsed-field gel electrophoresis b. Serological typing c. Coagulase testing d. Catalase testing 14. An isolate of Staphylococcus aureus was cultured from an ulcer obtained from the leg of a diabetic 79year-old female patient. The organism showed resistance to methicillin. Additionally, this isolate should be tested for resistance or susceptibility to: a. Erythromycin b. Gentamicin c. Vancomycin → drug of choice for MRSA d. Kanamycin 15. Bacitracin A disks (0.04 unit) are used for the presumptive identification of which group of βhemolytic streptococci? a. Group A → S. pyogenes sensitive b. Group B c. Group C d. Group F 16. Two blood cultures on a newborn grew β-hemolytic streptococci with the following reactions: • CAMP test = + • Hippurate hydrolysis = + • 6.5% NaCl = + • Bile esculin = negative • Bile solubility = negative • Bacitracin = resistant • PYR = negative • Trimethoprim–sulfamethoxazole = resistant Which is the most likely identification? a. Group A streptococci b. Group B streptococci c. Group D streptococci d. Nongroup A, nongroup B, nongroup D streptococci 17. An isolate recovered from a vaginal culture obtained from a 25-year-old female patient who is 8 months pregnant is shown to be a gram-positive cocci, catalase negative, and β-hemolytic on blood agar. Which tests are needed for further identification? a. Optochin, bile solubility, PYR b. Bacitracin, CAMP, PYR → S. agalactiae: Bacitracin R, CAMP (+), PYR (-) c. Methicillin, PYR, trehalose d. Coagulase, glucose, PYR 18. Which method is most useful for confirmation that a culture isolate is Group B streptococcus? a. Southern blotting b. Polymerase chain reaction c. Direct hybridization d. Probe capture assay 19. Positive result for the hippurate hydrolysis test: a. Yellow b. Red c. Purple → Glycine + Ninhydrin reagent d. Black 20. S. pneumoniae and the Viridans streptococci can be differentiated by which test? a. Optochin disk test, 5 μg/mL or less b. Bacitracin disk test, 0.04 unit c. CAMP test d. Bile esculin test 21. Which test is used to differentiate the Viridans streptococci from the group D streptococci and enterococci? a. Bacitracin disk test b. CAMP test c. Hippurate hydrolysis test d. Bile esculin test 22. The bile solubility test causes the lysis of: a. Streptococcus bovis colonies on BAP b. Streptococcus pneumoniae colonies on a blood agar plate c. Group A streptococci in broth culture d. Group B streptococci in broth culture 23. A sputum specimen from an 89-year-old male patient with suspected bacterial pneumonia grew a predominance of gram-positive cocci displaying alpha- hemolysis on 5% sheep blood agar. The colonies appeared donut shaped and mucoidy and tested negative for catalase. The most appropriate tests for a final identification are: a. Coagulase, glucose fermentation, lysostaphin b. Penicillin, bacitracin, CAMP c. Optochin, bile solubility, PYR → S. pneumoniae d. Bile esculin, hippurate hydrolysis 24. The Superoxol test is used as a rapid presumptive test for: a. Neisseria gonorrhoeae b. Neisseria meningitidis c. Neisseria lactamica d. Moraxella (Branhamella) catarrhalis 25. Appropriate transport medium for E.coli, Salmonella, Shigella, and Y. enterocolitica: 1. Cary-Blair 2. Stuart 3. Amies 4. Buffered glycerol-saline a. 1 only b. 2 and 3 c. 1, 2 and 3 d. 1, 2, 3 and 4 26. Xylose lysine deoxycholate (XLD) agar is a highly selective medium used for the recovery of which bacteria? a. Staphylococcus spp. from normal flora b. Yersinia spp. that do not grow on Hektoen agar c. Enterobacteriaceae from gastrointestinal specimens d. Streptococcus spp. from stool cultures 2 27. At which pH does the methyl red (MR) test become positive? a. 7.0 b. 6.5 c. 6.0 d. 4.5 28. An organism gave the following reactions: • TSI: acid slant/acid butt; no H2S gas produced • Indole: positive • VP: negative • Citrate: negative • Urea: negative • Motility: positive • Lysine decarboxylase: positive This organism most likely is: a. Klebsiella pneumoniae b. Shigella dysenteriae c. Escherichia coli d. Enterobacteria cloacae 29. A sputum culture from an alcoholic seen in the ER grows gray, mucoid, stringy colonies on sheep blood agar. The isolate grows readily on MacConkey agar and forms mucoid, dark pink colonies. The colonies yield the following test results: • ONPG: + • Indole: • Glucose: + • Oxidase: • Citrate: + • VP: + The organism is most likely: a. Edwardsiella tarda b. Klebsiella pneumoniae c. Escherichia coli d. Proteus vulgaris 30. Which of the following sets of tests best differentiates Salmonella and Citrobacter species? a. KCN, malonate, beta-galactosidase, lysine decarboxylase b. Dulcitol, citrate, indole, H2S production c. Lactose, adonitol, KCN, motility d. Lysine decarboxylase, lactose, sucrose, malonate, indole → Salmonella are positive for lysine decarboxylase and most are negative for KCN, malonate and ONPG. → Citrobacter are negative for lysine decarboxylase and positive for growth in KCN. 31. Which biochemical tests should be performed in order to identify colorless colonies growing on MacConkey agar (swarming colonies on blood agar) from a catheterized urine specimen? a. Indole, phenylalanine deaminase, and urease b. Glucose, oxidase, and lactose utilization c. Phenylalanine deaminase and bile solubility d. H2S and catalase 32. Greater than 100,000 CFU/mL of a gram-negative bacilli were isolated on MacConkey from a urine specimen. Biochemical results are as follows: • Glucose: acid, gas produced • Indole: negative • Urea: positive • TDA: positive • H2S: positive The organism is most likely: a. Morganella morganii b. Proteus mirabilis c. Proteus vulgaris → Indole (+) d. Providencia stuartii 33. Three blood cultures taken from a 30-year-old cancer patient receiving chemotherapy and admitted with a urinary tract infection grew lactose-negative, motile, gram-negative rods prior to antibiotic therapy. Given the following biochemical reactions, which is the most likely organism? • H2S (TSI): positive • VP: negative • DNase: positive • Gelatin hydrolysis: positive • Ornithine decarboxylase: negative • Indole: positive • Citrate: negative • Phenylalanine deaminase: positive • MR: positive • Urease: positive a. Proteus vulgaris b. Proteus mirabilis → Indole (-) c. Serratia marcescens d. Klebsiella pneumoniae 34. A 64-year-old male with lymphoma has a positive blood culture at 18 hours incubation. The organism is a non-lactose fermenting gram-negative bacillus on MacConkey agar. Further testing gives the following reactions: • Oxidase: negative • TSI: alkaline/acid, no hydrogen sulfide • Motility: positive • Indole: positive • Citrate: positive • Ornithine decarboxylase: negative • Urea: positive • Phenylalanine deaminase: positive • VP: positive The genus is: a. Morganella b. Proteus c. Providencia d. Serratia 35. A favored energy source for many enteric pathogens, but it is not utilized by many other nonpathogenic enteric organisms. a. Lysine b. Mannitol c. Phenol red d. Bile salts 3 36. Fever, abdominal cramping, watery stools, and fluid and electrolyte loss preceded by bloody stools 2–3 days before is characteristic of shigellosis but may also result from infection with: a. Campylobacter spp. b. Salmonella spp. c. Proteus spp. d. Yersinia spp. 37. Which group of tests best differentiates Helicobacter pylori from C. jejuni? a. Catalase, oxidase, and Gram stain b. Catalase, oxidase, and nalidixic acid sensitivity c. Catalase, oxidase, and cephalothin sensitivity d. Urease, nitrate, and hippurate hydrolysis 38. The following results were obtained from a pure culture of gram-negative rods recovered from the pulmonary secretions of a 10-year-old cystic fibrosis patient with pneumonia: • Oxidase: positive • Glucose OF (open): positive • Pigment: red (nonfluorescent) • Growth at 42°C: positive • Motility: positive • Gelatin hydrolysis: positive • Arginine dihydrolase: positive • Flagella: polar, monotrichous • Which is the most likely organism? 41. A patient with a nosocomial pneumonia has a sputum Gram stain that shows many neutrophils and numerous small gram-negative coccobacilli. The organism grew in 24 hours as a mucoid, hemolytic colony on blood agar and a colorless colony on a MacConkey agar. The organism had the following characteristics: • Oxidase: negative • Catalase: positive • Nitrate: negative • ONPG: negative • Ornithine decarboxylase: negative • Lysine decarboxylase: negative The organism is: a. Burkholderia pseudomallei b. Pseudomonas stutzeri c. Burkholderia cepacia d. Pseudomonas aeruginosa 39. A 17-year-old female with cystic fibrosis is diagnosed with pneumonia. A sputum sample grew gramnegative bacilli with yellow, smooth colonies that have the following biochemical reactions: • Oxidase: positive • TSI: alk/alk • Glucose: oxidized • Fluorescence: negative • Lysine decarboxylase: positive The most likely organism is: 43. a. Burkholderia cepacia b. Klebsiella pneumoniae c. Shewanella putrefaciens d. Stenotrophomonas maltophilia 40. While swimming in a lake near his home, a young boy cut his foot, and an infection developed. The culture grew a nonfastidious gram-negative, oxidase positive, beta hemolytic, motile bacilli that produced deoxyribonuclease. The most likely identification is: a. Enterobacter cloacae b. Serratia marcescens c. Aeromonas hydrophila d. Escherichia coli 42. 44. 45. 46. a. Stenotrophomonas maltophilia → LDC (+) b. Alcaligenes faecalis → oxidase (+) c. Moraxella lacunata → oxidase (+) d. Acinetobacter baumannii A community hospital microbiology laboratory is processing significant numbers of stool cultures because of an outbreak of diarrhea following heavy rains and flooding in the country. A media that should be incorporated in the plating protocol is: a. Colistin nalidixic acid for Listeria b. MacConkey agar with sorbitol for Campylobacter c. Mannitol salt agar for Enterococcus species d. Thiosulfate citrate bile salts sucrose for Vibrio species Culture medium for Vibrio spp., EXCEPT: a. Buffered glycerol saline b. Cary-Blair c. Alkaline peptone water d. Thiosulfate citrate bile salt (TCBS) agar Type of blood used in blood agar plates for better production of beta hemolysis of Haemophilus: a. Sheep b. Rabbit c. Horse d. Human A 4-year-old boy is admitted to the hospital with suspected meningitis. He has not had most of the childhood vaccines. The suspected pathogen is: a. Listeria monocytogenes b. Haemophilus influenzae c. Streptococcus agalactiae d. Neisseria meningitidis An aerobic, gram-negative coccobacillus was isolated on Bordet- Gengou agar from a nasopharyngeal swab 48 hours after culture from a 6-month-old infant with suspected pertussis. The organism exhibited the following characteristics: • MacConkey agar: no growth • Oxidase: negative • Urea: negative at 4 hours, positive at 24 hours → slow urease • Catalase: positive The most probable identification of this isolate is: a. b. c. d. Moraxella lacunata Pasteurella ureae Bordetella pertussis → oxidase (+), urease (-) Bordetella parapertussis → slow urease producer 4 47. A culture from an infected dog bite on a small boy’s finger yielded a small, gram-negative coccobacillus that was smooth, raised and beta-hemolytic on blood agar. The isolate grew on MacConkey agar, forming colorless colonies. The organism was motile, catalase positive, oxidase positive, reduced nitrate, and was urease positive within 4 hours. No carbohydrates were fermented. The most likely identification of this isolate is: a. Brucella canis b. Yersinia pestis c. Francisella tularensis d. Bordetella bronchiseptica 48. Which of the following characteristics best differentiates Bordetella bronchiseptica from Alcaligenes species? a. Flagellar pattern b. Growth at 24ºC c. Oxidase activity d. Rapid hydrolysis of urea 49. A branching gram-positive, partially acid-fast organism is isolated from a bronchial washing on a 63-year-old woman receiving chemotherapy. The organism does not hydrolyze casein, tyrosine or xanthine. The most likely identification is: a. Actinomadura madurae b. Nocardia caviae c. Streptomyces somaliensis d. Nocardia asteroides 50. A sputum specimen is received for culture and Gram stain. The specimen contains 15 epithelial cells/LPF. The technologist’s best course of action would be to: a. Inoculate appropriate media and incubate anaerobically b. Inoculate appropriate media and incubate aerobically c. Call the physician and notify him of this lifethreatening situation d. Call the patient care area and request a new specimen 5