Microbiology lab

Over the past few weeks, we have been spending some of our lecture hours in lab playing with bacteria. There are mixed opinions about the lab sessions among my classmates, but I enjoyed the labs overall.

Their purpose, really, was to have us perform experiments and observe the results to learn key concepts of bacterial morphology and development of resistance to antibiotics. That works out great for people like me who need a few modes of delivery for the material to sink in properly. Another up-side was that it cut us all some slack by decreasing the "information concentration" (volume of information per unit time).

I particularly enjoyed learning how to do a Gram stain, the archetypal protocol for distinguishing between Gram-positive bacteria (which have a thick cell wall and stain purple) and Gram-negative bacteria (which have a thin cell wall and stain red). Whether a bacterial is Gram-positive or Gram-negative is usually instructive as to which types of drugs it will likely be responsive to.


My favorite part of the lab was the experiment demonstrating how bacteria have developed resistance to antibiotics. The three plates above were all plated with Staphylococcus aureus, a particularly nasty bug that's responsible for skin (staph) infections, pneumonia, and a whole range of other problems. The plate on the left is a strain of Staph isolated from the community in 1945, the middle plate is a strain from 1965, and the plate on the right is a strain isolated in 1999. The little white disks each contain a different type of antibiotic, and you know that the bacteria are susceptible to a given antibiotic if there's a zone of clearance around a disk: the larger the zone of clearance, the more effective the antibiotic is at preventing bacterial growth or killing the bacteria.

Notice how every antibiotic was effective at clearing the 1945 version of Staph. aureus, but that effectiveness was markedly decreased against the 1965 strain. By the time you get to 1999, five of those eight antibiotics are completely useless against Staph. aureus. The doomsayers are worried about what happens when we don't have any antibiotics that are effective against highly resilient bacteria. As future doctors, we must be careful when prescribing antibiotics in order to delay the generation of new resistant strains.