Antimicrobials

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Chapter 29 Antimicrobials

Chapter contents

Bacterial structure 217

Antimicrobial medication 218

Bacterial resistance 221

Antiviral drugs 221

Bacterial Structure

The cell wall of a bacterium is unique. Although not present in every bacterial species, it is usually a very important part of maintaining the integrity of the bacterium.

Animal and plant cells exist in a relatively constant chemical environment, but bacterial cells mostly live in an unfavourable environment. The cell wall is therefore a very important part of maintaining the integrity of the bacterium. Bacteria do not have a nucleus, but instead the genetic material exists in the form of a single chromosome that holds all the genetic information of the cell lying free in the cells. Bacteria have no mitchondria.

The function of the cell wall of a bacterium is to:

• Maintain the cell’s characteristic shape: to prevent the phosphoplipid membrane of the cell wall from adopting a spherical shape.

• Withstand considerable changes in osmolarity.

• Provide a rigid platform for structures such as flagellae.

The structure of the bacterial cell wall varies considerably between species, and classification is largely on the reaction of the bacterial cell wall to the Gram stain (named after the Danish bacteriologist Hans Christian Gram). Gram-positive bacteria retain more of the stain than Gram-negative bacteria, due to the different constituents of the cell walls.

Most of the work on wall structure has been done with Gram-positive cocci and bacilli, and with enteric bacteria (found in the intestine) and other Gram-negative rods.

Gram-Negative Bacteria

• Exhibit some flexibility in their nutritional needs, using inorganic nitrogen compounds, mineral salts and a simple carbon source for synthesis of their whole structure.

• Have a low internal osmotic pressure (see Figure 16.1, p. 124).

Gram-Positive Bacteria

• Gram-positive cocci or bacilli have more specific nutritional requirements. Several bacteria lack some synthetic abilities and require various amino acids, vitamins and accessory factors for growth

• These have a high internal osmotic pressure.

However, these characteristics do not always hold true:

• Some cocci, rickettsias, the Chlamydias and spirochaetes are Gram-negative and have exacting growth requirements.

• Mycoplasmas lack a rigid wall structure; although technically Gram-negative, they are a separate group.

Finding medication that is effective against these microorganisms is anything but straightforward.

The Bacterial Cell Wall

• Gram-Positive Bacteria

• Relatively simple wall.

• 90% of the Gram-positive cell wall comprises peptidoglycans (a polymer of sugars and amino acids) (Figure 29.1).

Figure 29.1 The structure of the bacterial cell wall.

• Gram-Negative Bacteria

• The cell wall is much more complex, consisting of layers of membranes.

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