12.1 Bacteriological Analysis & Colony Calculations

12.1 Bacteriological Analysis & Colony Calculations

2025 WPI alignment: This section teaches conducting and interpreting bacteriological analyses in Laboratory Analysis, the 15-question area containing 3 recall, 12 application, and 4 calculation items.

Why this laboratory task matters

Bacteriological methods estimate indicator organisms associated with fecal contamination or disinfection performance. Sterile technique, dilution, membrane or defined-substrate procedure, incubation controls, countability, and reporting units determine validity.

Analytical foundation

Laboratory workflow

1. Confirm organism, method, sample preservation, sample time, residual neutralization, dilution plan, and required report basis.
2. Prepare sterile workspace, media, filtration apparatus, dilution water, controls, and verified incubation equipment.
3. Mix the sample as directed without contaminating it and filter/inoculate selected volumes or dilutions.
4. Incubate under the exact method conditions and protect plate identity and orientation.
5. Read only colonies/wells meeting the method definition and countable-range rules.
6. Calculate, qualify, review controls, report on the correct basis, and retain required records.

Quality and diagnostic evidence

Calculation and interpretation

WPI supplies CFU/100 mL = colonies counted × 100 / sample volume in mL for the stated simple case. If dilution is used, apply the method’s dilution factor correctly. Do not average plates that the method declares uncountable, and do not report a numeric zero when the method requires “less than” a detection value. Geometric means, where required, are calculated from the defined data set and permit rules.

Worked laboratory scenario

A 10 mL filtered volume yields 18 valid colonies and all controls pass. Using WPI’s simple relationship, the result is 18 × 100 / 10 = 180 CFU/100 mL. Reporting 18 CFU/100 mL would ignore the volume normalization. If the sample was additionally diluted, the specified dilution factor would also be applied.

Common exam traps

• Indicator organisms and methods are not interchangeable simply because both report per 100 mL.
• A failed blank or positive control affects sample validity.
• Use the actual analyzed volume and any dilution factor.
• Do not invent a precise count from a plate outside the method’s countable range.

Field-to-exam checklist

• Maintain sterile collection and analysis through the entire bacteriological chain.
• Controls and countability rules determine whether a result is valid.
• Normalize colonies to the required sample-volume basis.
• Interpret organism results with disinfectant and hydraulic evidence.

Handling nonnumeric outcomes

Microbiological methods can produce results outside the countable range, confluent growth, atypical colonies, or no detected colonies in the volume examined. Report these according to the method rather than forcing every plate into an exact number. The qualifier carries operational meaning: “greater than” indicates the true concentration may be higher, while “less than” reflects a detection boundary, not proof of absolute zero organisms. Preserve plate or instrument observations that support the qualifier.

When multiple dilutions or replicate results are available, apply the approved method’s countability and calculation rules consistently; selecting only the most favorable plate destroys the sampling inference.

Countable ranges, dilutions, and geometric means

Countable ranges are method-specific and are not suggestions. For membrane filtration, fecal coliform plates are generally read in the range of about 20 to 60 colonies, and total coliform plates in the range of about 20 to 80. Below the range, the count is statistically unreliable; above it, colonies crowd and suppress one another. A plate outside the range is reported with the method's estimation rule or with a greater-than or less-than qualifier, never with invented precision.

Different methods produce different units. Membrane filtration yields colony-forming units (CFU) from a direct count. Multiple-tube fermentation and defined-substrate tray methods yield a most probable number (MPN) derived from a statistical table of positive and negative results. Both are reported per 100 mL, and both may satisfy a permit, but they are not the same measurement and should not be averaged together in a single trend.

Worked calculation with dilution. One millilitre of a 1:10 dilution is filtered and yields 34 countable colonies. The effective volume of original sample analyzed is 1.0 mL x 0.1 = 0.1 mL. The result is 34 x 100 / 0.1 = 34,000 CFU/100 mL. Forgetting the dilution factor would report 3,400 — a tenfold understatement, and exactly the error that turns a violation into an apparent pass.

Bacteriological limits are usually geometric means, because the data are log-normally distributed. The geometric mean of n values is the nth root of their product. That structure makes the statistic far less sensitive to a single high result than an arithmetic mean would be — which is why one bad day rarely fails a monthly bacteriological limit by itself, and why a persistent moderate elevation can fail it even when no single value looks alarming.