Mastering Microbiology Ch 20

5) In the table, as illustrated by the data shown, the minimal bactericidal concentration of antibiotic X is

15 μg/ml.

Consider the different mechanisms through which antibiotics inhibit microbial growth, and consider what changes in the microbe might enable it to resist the inhibitory effects of antibiotics. Select all of the statements that describe a mechanism that would enable a microorganism to resist the action of an antibiotic.

Altered porins in the cell wall block passage of antibiotic through the cell wall. A microbe develops a transport mechanism in the plasma membrane that rapidly pumps antibiotic out of the bacterial cell. An enzyme that destroys the antibiotic is produced. Target site is modified, so that an antibiotic is unable to bind to its target.

Identify the statements below that accurately describe the mechanisms through which organisms acquire antibiotic resistance. Select all of the statements that accurately describe the origins and spread of antibiotic resistance.

Antibiotic-resistance genes can be passed horizontally via transduction. Antibiotic resistance is readily transmitted to the next generation during binary fission. Mutations are the ultimate source of antibiotic-resistance genes. Antibiotic-resistance genes can be passed horizontally via bacterial conjugation. Antibiotics select for those microbes that have developed resistance, increasing their frequency in the bacterial population. Antibiotic-resistance genes can be passed from one bacterium to another by bacterial transformation.

What is meant by selective toxicity?

Chemotherapeutic agents should act against the pathogen and not the host.

Quinolones and fluoroquinolones act against what bacterial target?

DNA gyrase

There are numerous examples of microorganisms displaying each of the four major resistance mechanisms. In this activity, you are asked to identify the mechanism that each example best illustrates. Drag each item into the appropriate bin.

Enzymatic inactivation of the antibiotic o Many strains of Neisseria gonorrhoeae are resistant to penicillin because of the production of beta-lactamases. · Prevention of penetration to the target site within the microbe o Resistance to tetracycline may result from modified pore proteins in the outer membrane that form a porin too small for the tetracycline to pass through · Alteration of the drug's target site o Resistance to clindamycin develops when mutations in bacteria alter the ribosomal binding site to which clindamycin would normally bind o MRSA (methicillin-resistant Staphylococcus aureus) is resistant to all beta-lactam drugs because of a mutation in its penicillin-binding protein (PBP) · Rapid efflux of the antibiotic o Pseudomonas aruginosa has membrane pumps that export a number of different antibiotics from its cells

Why are chemotherapeutic agents that work on the peptidoglycan cell wall of bacteria a good choice of drug?

Humans and other animal hosts lack peptidoglycan cell walls.

Why is polymyxin only used on the skin?

It can also damage living human cell membranes, but the drug is safely used on the skin, where the outer layers of cells are dead.

There are a large number of antibiotics that inhibit protein synthesis at 70S ribosomes found in bacterial cells but do not interfere with protein synthesis at the 80S ribosomes found in eukaryotic cells. Some of these antibiotics bind to the smaller ribosomal subunit and interfere with the reading of the mRNA code, whereas others bind to the larger ribosomal subunit and inhibit the formation of peptide bonds. Unfortunately, some of the antibiotics that inhibit protein synthesis in bacteria exhibit some toxicity to the eukaryotic host cells as well. What is the most likely reason for this toxicity to the host cell? Select the correct answer.

These antibiotics interfere with protein synthesis within eukaryotic mitochondria.

Why is it difficult to find good chemotherapeutic agents against viruses?

Viruses depend on the host cell's machinery, so it is hard to find a viral target that would leave the host cell unaffected.

Which of the following antimicrobial agents is recommended for use against fungal infections?

amphotericin B

Niclosamide prevents ATP generation in mitochondria. You would expect this drug to be effective against

helminths.

In this activity, you will identify the general mechanism of inhibition for the antimicrobial actions listed. Drag each item into the appropriate bin.

inhibiting cell wall synthesis o inhibiting synthesis of peptide cross-links o inhibiting bonding of N-acetyl glucosamine to N-acetylmuramic acid o inhibiting lipopolysaccharide synthesis o inhibiting mycolic acid synthesis · injuring the plasma membrane o inhibiting fatty acid synthesis · interfering with DNA replication o interfering with DNA polymerase o inhibiting DNA gyrase · interfering with RNA synthesis (transcription) o interfering with RNA polymerase · interfering with protein synthesis o interfering with activity of 30s ribosomal subunit o interfering with activity of 50s ribosomal subunit o interfering with attachment of tRNA to mRNA o interfering with peptide bond formation, catalyzed by the ribosome

Which of the following methods of action would be bacteriostatic?

inhibition of protein synthesis

The antibiotic tetracycline binds to the 30S subunit of the ribosome, as shown by the dark black bar on the right portion of the diagram in the figure. The effect is to

interfere with the attachment of the tRNA to mRNA-ribosome complex.

In the presence of penicillin, a cell dies because

it undergoes lysis.

Which of the following is an antiprotozoan drug that interferes with anaerobic metabolism?

metronidazole

Which of the following antibiotics is recommended for use against gram-negative bacteria?

polymyxin