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 ofPseudomonas aeruginosa, a bacterial pathogen capable of causing many different types of infections in
humans.
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 inStaining Microscopic specimens, capsules are difficult to stain for
microscopy, and are visualized by negative staining.
AnS-layeris 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 servesasthe 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.
Part 3
After diagnosing Barbara with pneumonia, the PA writes her a prescription for amoxicillin, a commonly-
prescribed 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.- How can the PA definitively identify the cause of Barbara’s pneumonia?
- What form of treatment should the PA prescribe, given that the amoxicillin was ineffective?
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Clinical Focus
108 Chapter 3 | The Cell
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