BM110 Microbiology Laboratory: Statistical Support

Preface to the 2026-27 presentation

Welcome to the 2026-27 edition of the statistics supporting information for the BM110 microbiology laboratory.

We would be very grateful to hear feedback about this workshop material by email or through the GitHub repository Issues page.

Overview

These pages provide support and background to the statistical calculations you will be using in the BM110 microbiology laboratories.

The practical work aims to determine whether antimicrobial use in factory farming might contribute to spillover of antimicrobial resistance genes in the environment.

To test this hypothesis, that antimicrobial use on a farm results in greater occurrence of antimicrobial-resistant bacteria, a number of soil samples have been collected from two different farms, A and B (Figure 1). These two farms have differing practices, most specifically concerning their prophylactic use of antimicrobials.

Figure 1: OS-style map showing the locations of the fictional farms providing the simulated data. (Figure generated using ChatGPT)

In the laboratory, you plated out (collectively) two serial dilutions (\(10^{-1}\) and \(10^{-2}\)) of bacteria from the two farm soil samples, and observed the number of colonies on each plate to obtain (again collectively) colony-forming unit (CFU) measurements for each of four conditions:

1. Farm A, nutrient agar (NA)
2. Farm A, nutrient agar + Ampicillin (Amp)
3. Farm B, nutrient agar (NA)
4. Farm B, nutrient agar + Ampicillin (Amp)

This allows us to test statistically whether there is quantitative support for the following hypotheses:

1. Do we recover different counts of bacteria from farms A and B?
2. Do we recover different counts of bacteria on nutrient agar + Ampicillin than on nutrient agar alone?
3. Does the difference or proportion in count between nutrient agar + Ampicillin, and nutrient agar differ between the two farms?

These pages will walk you through the relevant statistical analyses.