There is an interesting Editorial in the March issue of Nature Reviews Microbiology (7, 174; 2009; subscription or site licence required) about the development of the field in the past 300-or so years since the time of van Leeuwenhoek. From the Editorial:

Advances in microbiology are largely driven by improvements in technology. The awe-inspiring size of the experiments performed today — such as studies of entire microbial communities — is the result of a centuries-long path of discovery, one advance built on the next. Current technologies have provided us with new insight into entire communities and biomes, and promise to unravel many secrets of the microbial world…..In 300 years, we have gone from observing mixed species in a drop of pond water to isolating and studying individual bacteria to sequencing all species in a sample of seawater. In the process, we have still only discovered perhaps 1% of all bacteria, and possibly an even lower percentage of phages and viruses. Microbiology has come a long way, and has a longer way to go. It will be fun.