Which of the following is a laboratory method used to observe living bacteria motility and study their size, shape and arrangement?

Appeared in: PGICH Noida NO-2025

Explanation

The hanging drop slide method is a type of wet mount preparation specifically used to observe living organisms.
It allows for the direct visualization of true bacterial motility (purposeful movement) versus Brownian motion (random jiggling).
Since the bacteria are not heat-fixed or stained, their natural size, shape, and arrangement can be observed without distortion.

Option A: The agglutination test is a serological method that detects antigen-antibody reactions, seen as clumping. It does not allow for the observation of individual bacterial motility or morphology.
Option B: Acid-fast staining is a differential staining procedure that requires heat-fixing the bacteria, which kills them. It is used to visualize specific types of bacteria, not to observe motility in living cells.
Option D: Gram staining is a primary method for bacterial classification, but it involves heat-fixing the smear, which kills the bacteria. Therefore, it cannot be used to observe the motility of living organisms.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Microbiological techniques for observing bacteria as background academic context rather than a clinical decision trigger.
Identifying bacterial motility is clinically important. For example, the motility of Proteus species is a key factor in their ability to cause ascending urinary tract infections (UTIs).
In gastroenterology, distinguishing between motile pathogens like Vibrio cholerae (causes cholera) and non-motile ones is a critical diagnostic step.
What if? If a urine sample shows bacteria in a wet mount but they are not moving, it could suggest a non-motile pathogen or that the bacteria are dead. However, if they show true, directional movement, it strongly suggests an infection with a motile organism like E. coli or Proteus, guiding initial antibiotic choice.

How to Approach the Question

First, identify the key requirements in the question: the method must be for observing 1) 'living' bacteria and 2) their 'motility', 'size', 'shape', and 'arrangement'.
The word 'living' is the most critical clue. This immediately suggests that any method involving heat-fixing or harsh chemicals is likely incorrect.
Evaluate each option: 'Acid fast stain' and 'Gram staining' are staining procedures that famously use heat-fixation, which kills the bacteria. These can be eliminated.
Consider the 'Agglutination test'. Recall that this is an immunological test for clumping, not for observing individual cell characteristics like motility.
This leaves the 'Hanging drop slide method'. Even if you are unsure, by elimination, it is the most plausible answer. Recalling that it's a type of wet mount confirms it is used for living organisms.

Concept Tested & Keywords

Concept Tested: Microbiological techniques for observing bacteria
Stem keywords: laboratory method, living bacteria, motility, size, shape, arrangement
Lead-in keywords: Which of the following

Qtfal72tPtulGl_BJl_llB

E6 Microbiology Applied AnjuDhir 3e Part 1 pp. 102-104, 103-105

Practise the full PGICH Noida NO-2025

Attempt every question from this paper in a timed mock, then review the full solution for each one.