Microbiology, 2021

3.3 • Unique Characteristics of Prokaryotic Cells 99
polysaccharides, glycoproteins, or glycolipids.
Glycocalyces allows cells to adhere to surfaces, aiding in the formation of biofilms (colonies of microbes that
form in layers on surfaces). In nature, most microbes live in mixed communities within biofilms, partly
because the biofilm affords them some level of protection. Biofilms generally hold water like a sponge,
preventing desiccation. They also protect cells from predation and hinder the action of antibiotics and
disinfectants. All of these properties are advantageous to the microbes living in a biofilm, but they present
challenges in a clinical setting, where the goal is often to eliminate microbes.
Figure 3.29
(a) Capsules are a type of glycocalyx composed of an organized layer of polysaccharides. (b) A capsule stain of Pseudomonas
aeruginosa, a bacterial pathogen capable of causing many different types of infections in humans. (credit b: modification of work by
American Society for Microbiology)
The ability to produce a capsule can contribute to a microbe’s pathogenicity (ability to cause disease) because
the capsule can make it more difficult for phagocytic cells (such as white blood cells) to engulf and kill the
microorganism. Streptococcus pneumoniae, for example, produces a capsule that is well known to aid in this
bacterium’s pathogenicity. As explained in Staining Microscopic specimens, capsules are difficult to stain for
microscopy; negative staining techniques are typically used.
An S-layer is another type of cell envelope structure; it is composed of a mixture of structural proteins and
glycoproteins. In bacteria, S-layers are found outside the cell wall, but in some archaea, the S-layer serves as
the cell wall. The exact function of S-layers is not entirely understood, and they are difficult to study; but
available evidence suggests that they may play a variety of functions in different prokaryotic cells, such as
helping the cell withstand osmotic pressure and, for certain pathogens, interacting with the host immune
system.
Clinical Focus
Part 3
After diagnosing Barbara with pneumonia, the PA writes her a prescription for amoxicillin, a commonlyprescribed
type of penicillin derivative. More than a week later, despite taking the full course as directed,
Barbara still feels weak and is not fully recovered, although she is still able to get through her daily
activities. She returns to the health center for a follow-up visit.
Many types of bacteria, fungi, and viruses can cause pneumonia. Amoxicillin targets the peptidoglycan of
bacterial cell walls. Since the amoxicillin has not resolved Barbara’s symptoms, the PA concludes that the
causative agent probably lacks peptidoglycan, meaning that the pathogen could be a virus, a fungus, or a
bacterium that lacks peptidoglycan. Another possibility is that the pathogen is a bacterium containing
peptidoglycan but has developed resistance to amoxicillin.