Bacterial Cell Envelopes

Welcome to this week’s #micromeded on the bacterial cell envelope. Understanding the diversity of the bacterial cell envelope, is central to the study of microbiology. If you know what the composition of the bacterial cell envelope, you can predict a lot about the bacteria’s environmental stability, pathogenicity, growth requirements, antibiotic susceptibility, and evolutionary relatedness to other bacteria.

Bacteria can be stained with Gram stain to determine if they are Gram negative (pink/red), Gram positive (purple), or Gram variable. If a bacterial strain does not hold onto dye well, you can try to stain it with an acid-fast stain which would help determine if the bacteria are mycobacterial or possibly a member of the Nocardia genus (which would appear pick on a blue background). Wall-less bacteria will be Gram negative on Gram stain, but because of the lack of a rigid cell wall, they will also be pleomorphic (no uniform shape across individual bacteria). As you can see from this figure that wall-less bacteria acquire cholesterols from the environment/host, so in order to grow this type of bacteria in the lab, you’ll need to include cholesterols.

A Gram stain from a specific source can tell you a lot about what a patient might be infected with. For instance, if there is an intracellular bacterium that is Gram positive and rod-shaped (also called bacillus) in a CSF sample in a newborn/neonate, you can almost assume it is Listeria monocytogenes, but analyzing a Gram stain well is more challenging than you might think. If you do not spend time looking at real patient samples under the microscope, you won’t be able to make a reliable call when a patient needs it—that’s why medical laboratory scientists are so amazing (learn more about that career and others here).

In coming #micromeded posts, we will be exploring some of the bacteria in these categories, so tune in each Wednesday at 5pm CST for more mini-lesson on medical microbiology.