Exam 3 - (CH 21) Nonfermenting and Miscellaneous Gram-Negative Bacilli Textbook Review Questions

Question 1

Which of the following is an exotoxin produced by Pseudomonas aeruginosa?
A. DNase
B. Lipopolysaccharide
C. Phospholipase
D. Alginate

Answer 1

A. DNase

Question 2

Which pigment produced by Pseudomonas aeruginosa is water-soluble and fluoresces under short-wavelength ultraviolet light?
A. Pyocyanin
B. Pyoverdin
C. Pyorubin
D. Pyomelanin

Answer 2

B. Pyoverdin

Question 3

Which of the following sets of reactions distinguishes Pseudomonas stutzeri from most other Pseudomonas spp.?
A. Arginine dihydrolase (ADH) negative and starch hydrolysis positive
B. ADH positive and starch hydrolysis positive
C. Growth at 4°C and gelatin hydrolysis positive
D. Growth at 42°C and production of pyverdin

Answer 3

A. Arginine dihydrolase (ADH) negative and starch hydrolysis positive

Question 4

Which best describes Acinetobacter baumannii?
A. Asaccharolytic, oxidase negative, and catalase positive
B. Nonlactose fermenting, oxidase positive, and catalase negative
C. Lactose fermenting, oxidase negative, and nonmotile
D. Asaccharolytic, oxidase and catalase positive, and motile

Answer 4

C. Lactose fermenting, oxidase negative, and nonmotile

Question 5

Stenotrophomonas maltophilia is positive for this test.
A. Oxidase
B. Lactose fermentation
C. DNase
D. o-Nitrophenyl-β-D-galactopyranoside (ONPG)

Answer 5

C. DNase

Question 6

Studies suggest that this plate is most effective in reducing overgrowth while maintaining good recovery of Burkholderia cepacia.
A. B. cepacia–selective agar (BCSA)
B. Oxidative-fermentative base, polymyxin B, bacitracin, lactose (OFPBL)
C. Thiosulfate citrate bile salt sucrose (TCBS) agar
D. Cefsulodin-irgasan-novobiocin (CIN) agar

Answer 6

A. B. cepacia–selective agar (BCSA)

Question 7

This test result indicates an isolate is most likely Oligella ureolytica rather than an Alcaligenes spp.
A. Nonmotile
B. Abundant growth on MacConkey (MAC) agar
C. Nonoxidative
D. Positive phenylalanine deaminase (PDA)

Answer 7

D. Positive phenylalanine deaminase (PDA)

Question 8

Which is the best test to differentiate between Brevundimonas vesicularis and Brevundimonas diminuta?
A. Oxidase
B. Glucose oxidation
C. Maltose oxidation
D. Esculin hydrolysis

Answer 8

D. Esculin hydrolysis

Question 9

Nonpigmented strains of Chromobacterium violaceum have been confused with
A. Aeromonas.
B. Pseudomonas.
C. Citrobacter.
D. Plesiomonas.

Answer 9

A. Aeromonas

Question 10

Which of the following best describes Ralstonia pickettii?
A. Oxidase and catalase negative
B. Nonmotile and does not grow on MAC agar
C. Slow grower that grows on MAC agar
D. Resistant to all cephalosporins and carbapenem

Answer 10

C. Slow grower that grows on MAC agar

Question 11

Based on carbohydrate utilization, what is the difference between nonfermentative and fermentative organisms?

Answer 11

Fermentative gram-negative bacilli are able to metabolize carbohydrates to derive energy under anaerobic conditions. Phenotypically, these bacteria can produce an acid “butt” in specific media (e.g., triple sugar iron [TSI] agar or Kligler iron agar [KIA]). Nonfermenters cannot ferment sugars and are not able to acidify the butt of TSI or KIA.

Question 12

What is the typical natural habitat of most nonfermenters?

Answer 12

Most nonfermenters exist in the environment, often a moist or aquatic environment. They are not usually part of the normal human biota. Nonfermenters may also be found in soil and on plants, as well as in hospital environments on countertops, on equipment, and occasionally in contaminated liquids used for dispensing medications and disinfectants.

Question 13

What types of infections do nonfermenters cause?

Answer 13

Nonfermenters rarely cause infections outside of the hospital environment, except when traumatically implanted (e.g., into the skin from soil, vegetation, or water sources) or in the case of immunocompromised patients. In the hospital, nonfermenters may be the cause of nosocomial urinary tract infections, postsurgical wound infections, pneumonia (particularly ventilator-associated pneumonia), and/ or bacteremia. The incidence of infection is greater in the immunocompromised patient.

Question 14

What risk factors are associated with infections caused by nonfermentative, gram-negative bacilli?

Answer 14

Risk factors for infection by nonfermenters include immunocompromised states from cancer or cancer chemotherapy, transplantation, and steroid use. In the immunocompetent individual, infections are associated with burns, catheters, prior use of broad-spectrum antimicrobial agents, metabolic disorders (e.g., diabetes mellitus), and foreign body implantation, traumatically or via transplanted organs.

Question 15

What are the four most common nonfermentative, gram-negative bacilli isolated in the clinical laboratory?

Answer 15

Pseudomonas aeruginosa is the most common nonfermenter associated with clinical infections, especially nosocomial infections. Acinetobacter baumannii complex, Burkholderia spp., and Stenotrophomonas maltophilia are often isolated from hospitalized patients, especially from respiratory specimens, but they are more often colonizers and are not always clinically significant. The isolation of a nonfermenter from a single blood culture, or as part of a polymicrobial infection, often indicates that the organism is acting as a colonizer or a laboratory contaminant rather than a relevant pathogen. However, if one of the nonfermenters is seen on a Gram stain from a sterile site, is the only organism isolated, and is present in high numbers, its clinical significance needs to be considered.

Question 16

What are the typical susceptibility patterns of the most commonly isolated nonfermenters?

Answer 16

Many of the nonfermenters, especially P. aeruginosa, A. baumannii complex, and S. maltophilia, can be resistant to agents used to treat infections caused by fermentative gram-negative bacilli. They are resistant to penicillin, ampicillin, most third-generation cephalosporins (except ceftazidime), macrolides, lincosamides, and agents active against gram-positive bacteria. There is variability of their in vitro and in vivo responses to aminoglycosides; quinolones; aminopenicillins, such as piperacillin and ticarcillin; and SXT. Specific susceptibility tests need to be performed if the nonfermenter is considered clinically relevant.

Question 17

What initial clues indicate that an isolate is a nonfermenter?

Answer 17

Most nonfermenters will grow on media selective for gram-negative bacilli, such as MacConkey agar, but will remain lactose-negative. In addition, many of the nonfermenters will be oxidase-positive. The Gram stain of many nonfermenters is that of thin bacilli, often a little longer than the rods of the fermenters, such as E. coli and other Enterobacteriaceae. When a nonfermenter is placed on media to determine carbohydrate utilization, such as TSI or KIA, there will be no acidification of the butt. This result will increase the likelihood of a nonfermenter.

Question 18

How would you differentiate P. aeruginosa from other fluorescent group pseudomonads?

Answer 18

The ability to grow at 42 ° C and the presence of a blue-green pigment diffusing through the medium are characteristics of P. aeruginosa that can be used to differentiate this organism from other members of the pseudomonad fluorescent group. In addition, the distinct grapelike odor of P. aeruginosa may be a clue.

Question 19

What are the identifying characteristics of Acinetobacter spp.?

Answer 19

Acinetobacter spp. are gram-negative coccobacillary organisms rather than true bacilli. They are oxidase-negative, which is uncharacteristic of most other nonfermenters. In addition, they have a bluish-purple appearance on MacConkey agar and are often susceptible to the antimicrobial combination of ampicillin and sulbactam, which is unusual for many other nonfermenters.

Question 20

Which nonfermenters are most commonly isolated from patients with cystic fibrosis?

Answer 20

Pseudomonas aeruginosa is the species most often isolated; occasionally Burkholderia cenocepacia and Burkholderia multivorans, both members of the B. cepacia complex, are recovered from lower respiratory tract specimens from patients with cystic fibrosis.

Question 21

Points to Remember

Answer 21

■ Nonfermenters will not acidify the butt of TSIA or KIA.
■ Most nonfermenters are oxidase positive, a key test in differentiation from most members of the family Enterobacteriaceae.
■ Nonfermenters are environmental isolates, which rarely cause disease in healthy humans.
■ Nonfermenters can be more resistant to antimicrobial agents than members of the family Enterobacteriaceae. There are specific methods and breakpoints available for many of the nonfermenters in the CLSI standards.
■ The most common nonfermenters isolated in clinical microbiology laboratories are P. aeruginosa, A. baumannii complex, S. maltophilia, and B. cepacia complex.
■ Acinetobacter spp. may retain the crystal violet stain resembling gram-positive bacteria.
■ Phenotypic identification will not identify all nonfermenters; MALDI-TOF or 16S rRNA DNA sequencing will be needed for confirmation, if needed.
■ Specimens from patients with CF often demonstrate a wide variety of nonfermentative bacilli and may require that more definitive identification methods be used. The most common, however, is P. aeruginosa.