The Royal Society announced that it has put 60 of its most memorable papers online. One of them concerns the discovery of bacteria by Antonie van Leeuwenhoek. The original paper as published in the Society’s Philosophical Transactions of 1677–1678 was entitled:
Observations, communicated to the Publisher by Mr. Antony van Leewoenhoeck, in a Dutch Letter of the 9th of Octob. 1676, here English’d: Concerning little Animals by him observed in Rain-Well-Sea- and Snow water; and also in water wherein Pepper had lain infused.
Here are two excerpts that can be expected to tickle the present day microbiologist's mind. (Note that in the style of the time, the letter "s" looks like an "f" without the crossbar: ſ)
The eye of a body louse is approximately 100 µm in diameter or 104 µm3. The organisms van Leeuwenhoek saw were 1/10 that length, or 10 µm, and 1/1000 that volume, around 10 µm3. Many bacteria reach such dimensions. The movement described matches the runs and tumbles of chemotactic bacteria.
Let's assume he was talking about lengths. A honey bee is about 1.2 cm in length, a horse, about 250 cm. Thus, the ratio of their lengths is approximately 200×. Cheese mites are around 0.5 mm in length, thus making the organisms he observed approximately 2.5 µm long. I haven't found out how thick the hair of a cheese-mite is.