Microbiome Practice Questions

Q: Which of the following organisms is typically present in a normal vagina?

Lactobacillus species. ***Lactobacillus species*** - **Lactobacillus** species (especially *L. crispatus*, *L. jensenii*, *L. gasseri*, and *L. acidophilus*) are the **predominant organisms** in the normal healthy vagina. - They constitute **95% or more** of the normal vaginal microbiome. - Lactobacilli produce **lactic acid** from glycogen in vaginal epithelial cells, maintaining a **low pH (3.8-4.5)** that inhibits growth of pathogenic organisms. - They also produce **hydrogen peroxide** and **bacteriocins** that provide protection against pathogens. - This is the **hallmark of normal vaginal flora** and essential for vaginal health. *Candida albicans* - *Candida albicans* is an **opportunistic fungal pathogen**, not a typical member of normal vaginal flora. - While it can be found as a **colonizer** in approximately 20-30% of asymptomatic women, it is not considered a "typical" or beneficial component of normal flora. - When present, it exists in small numbers kept in check by Lactobacillus-dominated flora. - Overgrowth causes **vulvovaginal candidiasis** (yeast infection). *Trichomonas vaginalis* - *Trichomonas vaginalis* is a **flagellated protozoan parasite** that is **always pathogenic**. - Its presence indicates **trichomoniasis**, a **sexually transmitted infection**. - It is **never** part of normal vaginal flora. *Gardnerella vaginalis* - *Gardnerella vaginalis* is associated with **bacterial vaginosis (BV)**, a dysbiotic condition. - While it may be present in small numbers in some women, its overgrowth with depletion of Lactobacillus defines BV. - It is not considered a typical or beneficial member of normal vaginal flora.

Q: Disruption of which of the following oropharyngeal commensals predisposes to candidiasis?

Streptococcus. ***Streptococcus*** - **Streptococcus** species, particularly *S. sanguinis* and *S. mitis*, are major commensals in the oral cavity that **inhibit the growth of *Candida albicans*** through competition for nutrients and production of antimicrobial substances. - Disruption of this normal **streptococcal flora**, often by broad-spectrum antibiotics, creates an environment where *Candida albicans* can proliferate, leading to candidiasis. *Staphylococcus* - **Staphylococcus** species are primarily skin and nasal commensals; while *S. aureus* can be found in the oral cavity, it is not a primary competitor against *Candida* in the same way as streptococci. - Their presence or absence is not typically a direct predisposing factor for oral candidiasis compared to the dominant streptococcal flora. *Lactobacillus* - **Lactobacillus** species are common in the gastrointestinal tract and vagina, where they maintain an acidic environment that inhibits pathogen growth; however, they are less dominant in the oropharynx as a defense against *Candida*. - While beneficial for overall host health, their disruption in the oral cavity does not typically precipitate candidiasis as directly as that of the **streptococcal flora**. *Hemophilus influenzae* - *Haemophilus influenzae* is a common inhabitant of the **upper respiratory tract** and can be an opportunistic pathogen, but it is not known to have a significant role in directly inhibiting *Candida* growth in the oropharynx. - Its presence or absence in the commensal flora in the oral cavity does not typically influence the development of candidiasis.

Q: What is the typical bacterial count in the duodenum?

10^2 per gram. ***10^2 per gram*** - The duodenum has a **relatively low bacterial count** (typically 10^2-10^4 CFU/gram) due to the **acidic environment** from gastric acid and **rapid transit** of contents. - A count of **10^2 CFU/gram** represents the **lower end of the normal range** for the proximal duodenum, where gastric acid effects are strongest. - This sparse bacterial population contrasts sharply with the dense colonization seen in the distal gut. *10^1 per gram* - This represents an **extremely low count** more characteristic of the **stomach**, not the duodenum. - Such minimal bacterial presence is due to the **hostile acidic environment** (pH 1-3) in the stomach. - The duodenum, while having low counts, consistently has higher bacterial densities than this. *10^5 per gram* - This count is characteristic of the **distal small intestine (ileum)**, where bacterial concentrations progressively increase. - A bacterial count of **10^5 per gram in the duodenum** would be considered **abnormal** and suggest **small intestinal bacterial overgrowth (SIBO)**. - SIBO occurs when colonic-type bacteria colonize the small intestine inappropriately. *10^10 per gram* - This bacterial density is typical of the **colon** (which harbors 10^11-10^12 CFU/gram), the most densely colonized part of the human gut. - Such a high count in the duodenum would indicate **severe bacterial overgrowth** or gross contamination. - The colon's anaerobic environment supports this massive bacterial population.

Q: What is the predominant bacterial genus found in the human colon?

Bacteroides. ***Bacteroides*** - **Bacteroides** is the most abundant bacterial genus in the human colon, representing a significant proportion of the gut microbiota. - The colon is a predominantly **anaerobic environment**, which is ideal for Bacteroides growth. - This genus belongs to the phylum Bacteroidetes and plays crucial roles in **polysaccharide fermentation** and vitamin synthesis. *Escherichia* - While **Escherichia coli** is well-known and commonly studied, it represents only a small fraction (typically <1%) of the total colonic bacteria. - E. coli is a facultative anaerobe but is vastly outnumbered by obligate anaerobes like Bacteroides. *Clostridium* - **Clostridium** is an important genus within the phylum Firmicutes and is abundant in the colon. - However, as a single genus, it does not reach the numerical dominance of Bacteroides. - Multiple Clostridium species contribute to butyrate production and gut health. *Staphylococcus* - **Staphylococcus** species are typically found in higher concentrations on the skin and in nasal passages. - In the colon, they are present in very small numbers and are not among the predominant genera.

Q: What is the estimated range of bacteria present on human skin?

10^5 - 10^10. ***10^5 - 10^10*** - The human skin harbors a vast and diverse community of microorganisms, with bacterial counts often ranging between **10^5 and 10^10 colonies per square centimeter**, depending on the body site (e.g., sebaceous, moist, or dry areas). - This broad range reflects the varying conditions (e.g., moisture, pH, sebum production) across different skin regions, which support different microbial populations. - This is the **correct estimated range** that accounts for the variability across different skin environments. *10^1 - 10^2* - This range is **too low** to accurately represent the typical bacterial density on human skin. - Even relatively sterile or dry areas of skin would have significantly higher bacterial counts than this. *10^2 - 10^5* - While some limited or highly specialized areas might approach the higher end of this range, it generally **underestimates** the total bacterial population on most human skin surfaces. - Many common skin environments support much higher concentrations of bacteria. *10^10* - While **10^10 represents the upper end of the estimated range**, this single value does not represent a range as asked in the question. - The actual bacterial count varies across skin surfaces from approximately 10^5 to 10^10 CFU/cm², not a single fixed number. - Stating only the upper limit ignores the variation across different body sites and skin conditions.

Question 1

Which of the following organisms is typically present in a normal vagina?

Accepted Answer

Lactobacillus species

Answer

Gardnerella vaginalis

Answer

Trichomonas vaginalis

Answer

Candida albicans

Question 2

Disruption of which of the following oropharyngeal commensals predisposes to candidiasis?

Accepted Answer

Streptococcus

Answer

Staphylococcus

Answer

Lactobacillus

Answer

Haemophilus influenzae

Question 3

What is the typical bacterial count in the duodenum?

Accepted Answer

10^2 per gram

Answer

10^1 per gram

Answer

10^10 per gram

Answer

10^5 per gram

Question 4

What is the predominant bacterial genus found in the human colon?

Accepted Answer

Bacteroides

Answer

Escherichia

Answer

Clostridium

Answer

Staphylococcus

Question 5

What is the estimated range of bacteria present on human skin?

Accepted Answer

10^5 - 10^10

Answer

10^10

Answer

10^1 - 10^2

Answer

10^2 - 10^5

Microbiome — MCQs

Microbiome — MCQs

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