A cystic fibrosis patient on mechanical ventilation has a chronic productive cough. A sputum sample shows Gram-negative rods. Culture yields large, flat irregular colonies with a metallic sheen at 42°C; growth on cetrimide is green; MAC is nonpigmenting; oxidase is positive; motility is present; nitrate is not reduced after zinc. Which organism do you suspect?

Prepare for the CAMLPR Microbiology Test. Utilize flashcards and engaging multiple-choice questions, each equipped with descriptions and explanations. Ready yourself for an excellent performance!

Multiple Choice

A cystic fibrosis patient on mechanical ventilation has a chronic productive cough. A sputum sample shows Gram-negative rods. Culture yields large, flat irregular colonies with a metallic sheen at 42°C; growth on cetrimide is green; MAC is nonpigmenting; oxidase is positive; motility is present; nitrate is not reduced after zinc. Which organism do you suspect?

The main idea here is recognizing a Pseudomonas aeruginosa infection based on a distinctive combo of colony appearance, pigment production, and biochemical traits that are classic for this organism, especially in a cystic fibrosis patient. Key clues are a Gram-negative rod that is oxidase positive and motile, with growth as large, flat irregular colonies that show a metallic sheen when grown at 42°C. The green pigment on a selective medium like cetrimide agar is highly characteristic, produced by pyocyanin (and often accompanied by a yellow-green pigment from pyoverdine). The organism typically does not pig­ment on MacConkey (MAC colonies are nonpigmented), which fits with it being a nonfermenter. Oxidase positivity and motility further support Pseudomonas over other similar nonfermenters. The nitrate reduction result also aligns with Pseudomonas species, which commonly use nitrate as a terminal electron acceptor, giving a characteristic interpretation on nitrate reduction testing. These features help distinguish it from other Gram-negative rods that might appear in CF patients. Stenotrophomonas maltophilia is oxidase negative and usually lacks the same pigment pattern; Burkholderia cepacia can be a CF pathogen but typically does not present with the same green pigment and high-temperature colony morphology; Acinetobacter baumannii is nonmotile and oxidase negative and does not produce the green pigment on cetrimide. Putting the pigment production, oxidase positivity, motility, cetrimide-grown green colonies, and growth characteristics together makes Pseudomonas aeruginosa the best fit.

The main idea here is recognizing a Pseudomonas aeruginosa infection based on a distinctive combo of colony appearance, pigment production, and biochemical traits that are classic for this organism, especially in a cystic fibrosis patient.

Key clues are a Gram-negative rod that is oxidase positive and motile, with growth as large, flat irregular colonies that show a metallic sheen when grown at 42°C. The green pigment on a selective medium like cetrimide agar is highly characteristic, produced by pyocyanin (and often accompanied by a yellow-green pigment from pyoverdine). The organism typically does not pig­ment on MacConkey (MAC colonies are nonpigmented), which fits with it being a nonfermenter. Oxidase positivity and motility further support Pseudomonas over other similar nonfermenters. The nitrate reduction result also aligns with Pseudomonas species, which commonly use nitrate as a terminal electron acceptor, giving a characteristic interpretation on nitrate reduction testing.

These features help distinguish it from other Gram-negative rods that might appear in CF patients. Stenotrophomonas maltophilia is oxidase negative and usually lacks the same pigment pattern; Burkholderia cepacia can be a CF pathogen but typically does not present with the same green pigment and high-temperature colony morphology; Acinetobacter baumannii is nonmotile and oxidase negative and does not produce the green pigment on cetrimide. Putting the pigment production, oxidase positivity, motility, cetrimide-grown green colonies, and growth characteristics together makes Pseudomonas aeruginosa the best fit.

More practice questions