Which Bacterium Is Not Part of the Human Microbiome? 

Propionibacterium acnes, Lactobacillus casei, and Bacteroides fragilis reside in human skin, gut, and other sites as normal microbiota, while Streptococcus suis primarily affects animals like pigs and is not a typical human microbiome member. The correct answer is (C) Streptococcus suis.​

Correct Answer

C. Streptococcus suis stands out as the option not belonging to the human microbiome, as it mainly colonizes pigs and causes zoonotic infections in humans rather than persisting as a commensal.​

Option Breakdown

• A. Propionibacterium acnes: This bacterium dominates the skin microbiome, especially in sebaceous areas like hair follicles, aiding skin health but linked to acne.​

• B. Lactobacillus casei: A key gut and vaginal Lactobacillus species, it supports probiotic functions, acidifies environments, and prevents pathogen overgrowth in human microbiota.​

• C. Streptococcus suis: Primarily a porcine pathogen causing meningitis and septicemia in pigs, it rarely colonizes healthy humans and does not form part of the core human microbiome.​

• D. Bacteroides fragilis: Abundant in the human colon as an anaerobic gut commensal, it aids fermentation, vitamin synthesis, and immune modulation.​

Human Microbiome Overview

The human microbiome encompasses trillions of microbes across body sites like skin, gut, mouth, and vagina, influencing digestion, immunity, and pathogen resistance. Commensals like P. acnes and B. fragilis maintain balance, while opportunists or transients like S. suis do not qualify as residents.​