ennumeration of microorganism Flashcards
(13 cards)
What is the purpose of microbial enumeration?
To determine the number of microorganisms present in a sample for purposes such as quality control in pharmaceuticals, evaluating antimicrobial effects, research, and diagnostics.
What is the difference between viable and total counts?
Viable counts measure living cells capable of division, while total counts include both living and dead cells.
Why might total counts be important even if the cells are dead?
Because dead cells may still contain endotoxins or indicate prior contamination, relevant in pharmaceutical quality control.
What are colony-forming units (CFU)?
A colony-forming unit is a single cell or clump of cells that gives rise to a visible colony on solid media.
How does the spread plate technique work?
A diluted sample is spread on an agar plate, incubated, and colonies growing on the surface are counted. Only surface colonies are considered.
What is the principle behind the pour plate technique?
A diluted sample is mixed with molten agar, poured into a plate, and incubated. Colonies form throughout the medium.
What is the standard equation for calculating CFU/mL?
CFU/mL = average colony count × (1/volume plated in mL) × dilution factor
Why are dilutions necessary in enumeration techniques?
To reduce colony numbers to a countable range (25-250 colonies), enabling accurate enumeration.
What is membrane filtration used for?
Used for low microbial loads or antimicrobial-containing samples. It filters microorganisms onto a membrane which is then incubated on agar.
What is the typical pore size for membranes in filtration enumeration?
0.45 microns – sufficient to trap most bacteria for enumeration.
What are the benefits of pour plates over spread plates?
Larger sample volume allows detection of low levels of contamination, but colony visualization and oxygen availability may be limited.
When is turbidity measurement used in microbial enumeration?
When rapid estimation of total cell count is needed, using spectrophotometry to measure light scattering from suspended cells.
How does turbidity correlate with cell concentration?
More cells cause higher turbidity and light scattering, up to a linear limit.
