Get Contaminated

It was the final practical exam in Microbiology. But the real drama happened after the exam was over.

Each student was supplied with a question paper with one question on it: "You are provided with a sealed plastic ampoule. Aseptically transfer 1 ml to each of the two provided nutrient media & incubate for 24 hours. Microscopically identify your contaminant, if present."

Curious as all students are, we started checking around for friends in the laboratory who have received similar ampoules with codes that matched our own. The ampoules' labels were packed with 'em codes! Some students cared to venture a guess as to which of the many codes were actually meaningful (you'd be surprised what students do during exam time) although this remained a mostly futile exercise. I thought I was lucky enough when one of my friends, Raghda, came up to me beaming and showing me the ampoule she received. It had similar inscriptions to mine — hence, supposedly the same contaminant we were asked to identify. On the following day, after the incubation had been performed, she could apparently identify Staphylococcus aureus, but I protested: it was definitely Candida albicans! Such disagreements put the thrill in undergraduate microbiology.

Later on that day, as our friends gathered around discussing how they did in the exam, Mahmoud came up with a rather intriguing question: "How do the examiners contaminate the ampoules?" Honestly, I had no idea, although this question prompted a hearty discussion. Mahmoud himself suggested that the ampoules might simply have been filled with a solution containing the contaminants. Others pointed out that if this was really how it was done, the solution should have initially looked quite turbid (it definitely was not — even Raghda and I agreed on that one). See, whenever bacteria grow in a clear liquid medium, they tend to rapidly multiply and eventually turn the "once clear" medium into a turbid one. Mahmoud, however, was not impressed by the explanation and defended his case by pointing out that the number of microbial cells inoculated into the ampoules could have been very little, also adding that the solution itself may have been sufficiently diluted — could be, as these two conditions would have contributed to a much less turbid medium.

In an attempt to end the debate, a smartie (for my own well-being, I shall not mention the name) started off by announcing that all the discussion was pointless. His view — the supreme truth in his opinion — was that the examiners simply ordered different batches of expired ampoules and tested each batch for whichever type of contaminants the ampoules fell victim to. Then, these ampoules are provided to the students, say, for exams. Surely, smartie was wrong that time.

Recently, because the mystery was too intoxicating, I went and asked an assistant lecturer in the Microbiology Department about this whole ampoule contamination thing. How do they actually contaminate those ampoules then?!? As it would turn out, Mahmoud was on the right track. (But most importantly, smartie was definitely wrong on this one.) The department is provided with different batches of ampoules containing minute amounts of the desired microorganisms, delivered by a supplier shortly before exams. The solutions inside the ampoules are free from any nutrients, though, so the bacteria do not replicate — hence, the lack of  turbidity in the observed solutions. The bacteria will obviously not survive for long under these conditions, which is why the batches are delivered on a day-to-day basis up to the time those glorious exams are over.

Turns out that exams not only mean work for the students but also for the lecturers and lab attendants. And did I mention that smartie got it all wrong wrong wrong?