Which of the following is an obligate intracellular organism?

INI-CET EXAM -2025

Applied Microbiology & Infection control

Easy

Appeared in: INI-CET EXAM -2025

Explanation

An obligate intracellular organism is a microorganism that cannot reproduce outside of a living host cell.
Ehrlichia chaffeensis is a small, gram-negative bacterium that is a well-established obligate intracellular pathogen.
It is transmitted by ticks and causes human monocytic ehrlichiosis (HME) by infecting and replicating within mononuclear phagocytes (monocytes and macrophages).
Inside the host cell, these bacteria form characteristic clusters called morulae, which are visible under a microscope.

Option B: Staphylococcus aureus is a facultative intracellular pathogen, not an obligate one. It can grow and reproduce both inside and outside of host cells.
Option C: Tropheryma whipplei is an intracellular pathogen, but Ehrlichia chaffeensis is the more classic and definitive example of an obligate intracellular organism among the choices.
Option D: Streptococcus pyogenes is a classic extracellular pathogen. It replicates in the extracellular spaces of the host and does not require entry into a host cell to reproduce.

Visual 1: Micrograph - A peripheral blood smear showing a monocyte containing a morula (a cluster of Ehrlichia chaffeensis bacteria), which is a pathognomonic sign of ehrlichiosis.
Visual 2: Diagram - A flowchart illustrating the difference between obligate intracellular, facultative intracellular, and extracellular bacteria, with examples for each category.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Bacterial classification based on host cell dependency as background academic context rather than a clinical decision trigger.
Understanding that a pathogen is an obligate intracellular organism is critical for clinical practice. It influences the choice of antibiotics, as the drug must be able to penetrate the host cell to be effective. For example, doxycycline is a first-line treatment for ehrlichiosis because it has good intracellular penetration.
Diagnostic methods are also affected. Culture is often difficult or impossible for obligate intracellular organisms, so diagnosis relies on methods like PCR to detect bacterial DNA or serology to detect host antibodies.
What if? If a patient with suspected ehrlichiosis were treated with an antibiotic that has poor intracellular penetration, like a beta-lactam, the treatment would likely fail, leading to worsening of the illness and potential for severe complications like renal failure, meningoencephalitis, or septic shock.

How to Approach the Question

First, understand the key term in the question: 'obligate intracellular organism'. This means the organism MUST live inside a host cell to replicate.
Next, review the options provided. These are all names of bacteria.
Recall or deduce the characteristics of each bacterium. Try to classify them as intracellular or extracellular.
Eliminate the known extracellular pathogens. Streptococcus pyogenes and Staphylococcus aureus are classic examples of bacteria that primarily live outside cells.
Compare the remaining options. Both Ehrlichia chaffeensis and Tropheryma whipplei are intracellular. However, Ehrlichia (along with Rickettsia and Chlamydia) is a textbook example of an OBLIGATE intracellular parasite.
Select Ehrlichia chaffeensis as the best fit for the definition of an obligate intracellular organism.

Concept Tested & Keywords

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