Final answer:
Pseudomonas aeruginosa (option b) is a nonfermenter; it is an aerobic, gram-negative bacterium and does not ferment lactose, unlike the other bacteria listed which belong to the Enterobacteriaceae family and are capable of carbohydrate fermentation.
Step-by-step explanation:
The question asks which of the following is a nonfermenter among the provided list of bacteria:
- Escherichia coli
- Pseudomonas aeruginosa
- Yersinia enterocolitica
- Serratia marcescens
Pseudomonas aeruginosa is the correct answer. It is a gram-negative enteric bacillus that is strictly aerobic and does not ferment lactose, which classifies it as a nonfermenter. In contrast, bacteria from the Enterobacteriaceae family, like Escherichia coli, Serratia marcescens, and Yersinia enterocolitica, are facultative anaerobes and are capable of fermenting carbohydrates.
Although some Enterobacteriaceae like Yersinia pestis are noncoliforms because they either cannot ferment lactose or can only ferment it incompletely, Pseudomonas aeruginosa is not part of the Enterobacteriaceae family and stands out as a nonfermenter among the options listed.