Microbiology: Bacteriology: Gram-Negative Identification Flashcards

Which enzyme produced by Neisseria gonorrhoeae helps it evade complement-mediated killing?

Answer: IgA protease

N. gonorrhoeae secretes IgA protease, which cleaves secretory IgA and also helps evade host mucosal immunity and complement.

Which gram-negative rod is the most common cause of nosocomial urinary tract infections?

Answer: Escherichia coli

E. coli accounts for approximately 80% of community-acquired and 50%+ of nosocomial urinary tract infections.

A gram-negative, oxidase-positive, non-fermenting rod growing at 42°C with a fruity odor is most likely:

Answer: Pseudomonas aeruginosa

Pseudomonas aeruginosa is oxidase-positive, non-fermenting, grows at 42°C, and produces pyocyanin and a characteristic grape-like odor.

The Voges-Proskauer (VP) test detects which end-product of glucose fermentation?

Answer: Acetoin

The VP test detects acetoin (acetylmethylcarbinol), an intermediate in the butanediol fermentation pathway.

Which gram-negative organism characteristically produces a green metallic sheen on EMB agar?

Answer: Escherichia coli

Strong acid fermenters like E. coli produce a metallic green sheen on Eosin Methylene Blue (EMB) agar due to dye precipitation.

Haemophilus influenzae requires which two growth factors supplied in chocolate agar?

Answer: Factors V and X

H. influenzae requires Factor V (NAD) and Factor X (hemin) for growth, both of which are released from lysed red blood cells in chocolate agar.

Which characteristic distinguishes Shigella from Salmonella on triple sugar iron (TSI) agar?

Answer: Shigella produces no gas; Salmonella typically produces gas

Shigella ferments glucose without gas production, while most Salmonella produce gas (CO2 and H2) from glucose fermentation.