Counting colonies inside the countable window

Decide whether a plate can be counted at all, and say why a crowded plate and an almost bare plate both produce an untrustworthy number.

Builds on (2 levels back)inferred · high confidence

One cell grows into one visible colony: Counting colonies is only counting cells if each dot started from one cell, so that link has to be in place before any window matters.
Diluting spreads the same cells thinner: Landing inside a countable window is done by choosing a dilution, so the effect of diluting has to be understood first.

Prerequisites are inferred: pending teacher review.

Re-learn the skill with worked practice and clear examples.

Apply a published range to a real series, and keep the range attached to its method.

Step 1: The standard plate count range is 30 to 300

Standard Methods for the Examination of Water and Wastewater sets the trusted range for a spread or pour plate at 30 to 300 colonies. Below 30 the reading is too easily moved by chance; above 300 colonies interfere with one another.

Step 2: Different methods publish different ranges

A membrane filter coliform count uses a narrower range than the spread plate does, because the colonies sit crowded together on a small filter disc rather than spread across a whole dish. The range belongs to the method, and quoting the wrong one produces a confidently wrong answer.

Step 3: The dilution series exists to hit the range

You cannot know in advance how loaded a sample is, so several strengths go on at once. One of them lands inside the range and gets counted; the rest are recorded as outside it and set aside with a reason.

Practice

Three plates from one sample hold, in order of decreasing strength, an uncountable sheet, 168 colonies, and 7 colonies. Using the 30 to 300 standard plate count range, which plate is counted?

Reviewed

A.The sheet, once a grid is drawn on the base and one square is scaled up
B.All three, with the three figures averaged into one final answer
C.The plate holding 7, because the dots on it are cleanly separated
D.The plate holding 168, because that count sits inside the trusted range

Show the worked solution ▾

Answer: D. The plate holding 168, because that count sits inside the trusted range

Step 1: Test each figure against the range: The sheet is unresolvable, 168 sits inside 30 to 300, and 7 is below the bottom.
Step 2: Count the one that qualifies: Exactly one plate is inside the range, and that is the plate the method uses.

Why it's right: Only 168 falls inside the trusted range, so it is the plate the method counts and the other two are recorded as outside the range with the reason given.

Why the others miss:

A: Scaling one square of an unresolvable sheet assumes an even spread the plate gives no way to check, and merged colonies still hide from the count.
B: Averaging a valid count with two invalid ones drags a trustworthy figure toward two untrustworthy ones.
C: Clean separation is real and does not rescue a count below the bottom of the range, where chance dominates.

Aligned to Culturing · reading level ~grade 10

A student quotes the spread plate range of 30 to 300 while reading a membrane filter coliform count. What is the risk?

Reviewed

A.The filter will hold exactly ten times as many colonies as a dish would
B.The range belongs to a different method, so plates may be misjudged
C.Membrane filters cannot be counted by eye and need an instrument
D.None, because a colony is a colony however the plate was prepared

Show the worked solution ▾

Answer: B. The range belongs to a different method, so plates may be misjudged

Step 1: Ask where a published range comes from: From the geometry and crowding behavior of one particular method, worked out for that method.
Step 2: Ask what happens when it is moved: Plates get accepted or rejected against limits that were never established for them.

Why it's right: A published range is derived from one method's geometry and crowding, so carrying it to another method accepts and rejects plates against limits never established for it.

Why the others miss:

A: No fixed multiplier connects the two. The relationship is not a conversion factor.
C: Colonies on a filter disc are read by eye routinely, often with a low power lens for comfort.
D: Colonies behave the same way and the area they are competing in does not, which is exactly what sets the range.

Aligned to Culturing · reading level ~grade 10

Where you'd see this

A laboratory prints the method beside every count, because the acceptance range a reader should apply depends on it.

Careers that use this

Quality Control Analyst: Rejects counts outside the range published for the method used, rather than the range they remember.

Guided notes

Fill these in as you work through the lesson.

Big idea: A counting method publishes a range of colony numbers it can be trusted inside. Above that range colonies crowd each other and you undercount; below it, chance alone moves the number around too much to mean anything. Landing inside the range is the point of diluting.

Key terms: write the meaning

Countable window (the range a method can be trusted in):
Too numerous to count (the report line for a crowded plate):
Colony forming unit (one dot, from one starting cell):
Dilution series (several strengths, so one lands right):

The rule

A standard plate count is trusted between and colonies. Above the top, colonies each other and the count runs low. Below the bottom, alone moves the number too much to rely on.

Check yourself

Why is a plate holding 4 colonies a weaker measurement than one holding 80?
What is actually happening on a plate that comes back as a solid sheet of growth?
Why plate three different dilutions of the same sample instead of guessing one?

Work one example

You plate three dilutions of one sample. The first is a solid lawn, the second holds 168 colonies, and the third holds 7. Say which plate you count, and write the one-line reason you would give for setting the other two aside.