Which bacterium is a gram-negative aerobic coccobacillus commonly found in the throat as part of normal microbiota?

• A. Streptococcus pyogenes
• B. Escherichia coli
• C. Haemophilus influenzae
• D. Moraxella catarrhalis

Answer: (C)

The main idea here is recognizing a gram-negative, small, coccobacillus that commonly lives in the upper respiratory tract as part of normal microbiota. Haemophilus influenzae fits that description well: it is a gram-negative coccobacillus often found in the throat and nasopharynx of healthy people. It is fastidious and commonly described as needing X (hemin) and V (NAD) factors to grow, so it’s typically cultured on chocolate agar or with media supplying these factors. In contrast, the other organisms listed don’t match the shape and typical habitat: Streptococcus pyogenes is gram-positive cocci; Escherichia coli is a gram-negative rod from the gut; Moraxella catarrhalis is a gram-negative diplococcus. So the bacterium that best fits a gram-negative aerobic coccobacillus living in the throat as normal microbiota is Haemophilus influenzae.