Antibiotic Resistance

Question 1

What are the two fundamental forms of antibiotic resistance?

Answer 1

innate (or intrinsic) and acquired

Question 2

What is innate resistance?

Answer 2

Innate resistance exists prior to drug exposure, and is the result of structural or growth properties of the microbe.

Question 3

Which are more resistance to B-lactams, gram-positive and gram-negative? Why?

Answer 3

gram negative bacteria tend to be more resistant to beta-lactams than Gram positive bacteria because the outer membrane of Gram negative bacteria acts as a barrier to access to the cell wall found in the periplasmic space.

Similarly, intracellular bacteria are resistant to beta-lactams because beta-lactams are poorly taken up by animal cells.

Question 4

What is acquired resistance?

Answer 4

occurs through the exposure of bacteria to antibiotics, resulting in the selection and accumulation of resistant strains.

Upon exposure to antibiotics in the clinical setting, the resistant strain is selected in favor of antibiotic-sensitive strains, and the resistant strain soon becomes the predominant microbe.

Question 5

Acquired resistance most commonly arises through what processes?

Answer 5

chromosomal mutations, the acquisition of a transposon that imparts antibiotic resistance, or the acquisition of a plasmid (R factor) that may encode resistance to multiple antibiotics and contain multiple transposons.

Because R factors can encode multidrug resistance (MDR), it is possible for a patient to be infected by a microbe that is resistant to an antibiotic never used to treat the patient.

Question 6

T or F. Transduction and transformation can also lead to the horizontal transfer of antibiotic resistance, but this occurs far less frequently than plasmid-mediated horizontal transfer.

Answer 6

T

Question 7

Basic tenets of drug resistance:

Answer 7

 Resistance is likely to emerge, given sufficient time and drug use
 Antibiotic resistance is progressive, evolving from low levels through intermediate to high levels
 Organisms that are resistant to one drug are likely to become resistant to other antibiotics
 Once resistance appears, it is likely to decline slowly, if at all
 The use of antibiotics by one person affects others in the extended as well as the immediate healthcare environment

Question 8

Bactericidal antibiotics are the drugs of choice for what kinds of infections?

Answer 8

potentially lethal or chronic infections, or infections known to involve a biofilm.

In contrast, bacteriostatic antibiotics can be used when the immune response can be relied upon to assist in clearance.

Question 9

What is the main measurement of antibiotic efficacy?

Answer 9

determination of the minimum inhibitory concentration (MIC), the lowest concentration that inhibits growth.

Question 10

How can the MIC be measured?

Answer 10

(MIC), the lowest concentration that inhibits growth. Automated methods determine the MIC for specific bacterial isolates, but the MIC can also be determined by the Kirby-Bauer method used in the Basic Microbiology Laboratory of this course or by the E strip test

Question 11

How does E strip test work?

Answer 11

serial dilutions of the antibiotic are spread across a paper strip and the strip is then placed on an agar plate spread with the bacterial isolate. As the bacterial “lawn” grows during incubation, growth will be inhibited where the concentration of antibiotic is high as it diffuses from the strip into the agar. Consequently, a “halo” of no growth will form in those areas; however, the lawn will intersect with the strip at the MIC because the antibiotic concentration in the agar below that point will no longer inhibit growth.

Question 12

What is the minimum bactericidal concentration (MBC)?

Answer 12

the lowest concentration that kills bacteria, can also be determined. The MHC and MBC are basically the same for bactericidal antibiotics.

Question 13

T or F. The MIC must be reached in tissues or blood for efficacious treatment.

Answer 13

T. Antibiotic resistance raises the MIC.

While antibiotic resistance raises the MIC needed for treatment, antibiotic resistance does not increase the virulence of a microbe. Virulence is determined by the pathogenic potential of a microbe, which remains the same in the presence or absence of antibiotics. Antibiotics simply assist the immune response in clearing the infection.

Question 14

What is the breakpoint?

Answer 14

the relative concentration needed for susceptibility instead of resistance, and is an empirically-determined measure of attained MIC in tissues or blood.

Question 15

The combined use of two or more antibiotics simultaneously is one way to reduce the occurrence of resistance as long as ____.

Answer 15

the two antibiotics do not have the same mechanism of action

consider that the spontaneous mutation frequency in bacteria is about 1 x 10-7 for a specific mutation that would lead to antibiotic resistance. If an infection is treated simultaneously with two antibiotics, the frequency with which resistance to both antibiotics would occur is 1 x 10-7 multiplied by 1 x 10-7 or 1 x 10-14

Question 16

Antibiotic resistance can be accomplished by what four general mechanisms?

Answer 16

(i) reduced permeability (i.e., exclusion),
(ii) active export of the antibiotic by multidrug-resistance (MDR) pumps so that an efficacious concentration cannot be reached within the bacterium,
(iii) target alteration by mutation or creation of new analogous activity, or
(iv) drug inactivation, by modification or destruction.

Question 17

How can innate resistance through exclusion occur?

Answer 17

by the absence of an appropriate porin or transport system in Gram negative outer membranes or by the peptidoglycan cell wall, both of which can exclude antibiotics on the basis of size. This explains the resistance to beta-lactams exhibited by some Gram negative bacteria.

Question 18

Mechanisms of exclusion can also be acquired. How?

Answer 18

compliant porin and transport systems can be deleted or altered so that they no longer allow an antibiotic to cross into the cell, and membrane lipids can be altered to reduce drug binding or diffusion.

An example is the increased thickening of the Gram positive cell wall in vancomycin- intermediate Staphylococcus aureus strains (VISA) that traps the antibiotic before it can reach it’s D-Ala D-Ala target in peptidoglycan

Question 19

Multidrug-resistance efflux pumps are often induced in the presence of drug, and frequently belong to the class of _____.

Answer 19

ATP-binding cassette (ABC) transporters

Question 20

While these transporters can typically export multiple drugs, this is a particularly important mechanism for resistance to ____ antibiotics.

Answer 20

tetracycline

Question 21

Altered targets are key mechanisms of resistance for antibiotics that inhibit what kinds of cellular components?

Answer 21

ribosome, topoisomerase, and RNA polymerase activity.

Some of the more significant examples of altered targets are the penicillin-binding proteins (transpeptidases) where mutations reduce the affinity of beta-lactams and allow cell wall synthesis to continue in the presence of drug. Because such alterations must still allow the transpeptidases to function to some degree, the resistance conferred by such changes is sometimes incomplete and limited in terms of the classes of antibiotics affected.

Question 22

What is unique about methicillin-resistant Staphylococcus aureus (MRSA)?

Answer 22

the microbe is resistant to all beta-lactams.

Question 23

How is MRSA resistant to all beta-lactams?

Answer 23

The difference is that MRSA strains have acquired a transposon containing the mecA gene, which encodes an altered minor transpeptidase that functions when the principal transpeptidases are inactivated by beta- lactams. Because the principal transpeptidases remain unaltered, they are fully functional in the absence of methicillin, and consequently, MRSA strains are not compromised in the absence of drugs.

Question 24

T or F. Target-site mutations most rapidly arise if the antibiotic binds a single site

Answer 24

T. Therefore, newer drugs are designed to bind multiple sites, reducing the likelihood of a single “knockout” alteration that results in antibiotic resistance. Still, as evidence by MRSA, bacteria can acquire new, analogous activities that produce an altered target.

As additional examples, vancomycin-resistant strains of Enterococci (VRE) and Staphylococcus aureus (VRSA) harbor a plasmid containing the vanA gene, which encodes an enzyme that synthesizes D-Ala D-Lactate instead of D-Ala D-Ala at the end of the peptidoglycan pentapeptide

Principal transpeptidases can recognize the D-Ala D-Lac structure, but vancomycin cannot, and therefore resistance occurs. This is particularly distressing when MRSA strains acquire the vanA gene because vancomycin is the drug of choice in treating MRSA infections

Question 25

What is an example of how some microbes have developed chloramphenicol resistance?

Answer 25

resistant microbes acquire the ability to methylate ribosomes at a key site, reducing the affinity of chloramphenicol for the rRNA of the 50S subunit.

Question 26

T or F. Antibiotic inactivation is the most robust form of resistance. Why or why not?

Answer 26

T. because no sacrifice in activity of the antibiotic’s target is required.

Question 27

Antibiotic Inactivation occurs by what two methods?

Answer 27

modification and destruction

Question 28

What are the most common forms of modification that lead to inactivation?

Answer 28

include acetylation, phosphorylation, and adenylation of hydroxyl or amino groups on antibiotics, especially aminoglycosides. These modifications reduce the affinity of the antibiotic for its target in the microbe.

Question 29

What do Beta-lactamases do?

Answer 29

cleave the beta-lactam ring of penicillins and cephalosporins, leading to their destruction and inactivation. Bacteria expressing beta- lactams typically have MICs well above achievable ranges.

The first described beta-lactamase was a plasmid-encoded penicillinase from Staphylococcus aureus. Because S. aureus is a Gram positive bacterium, the enzyme is secreted into the immediate environment, and this requires a relatively high density of bacteria to effectively inactivate penicillin. The original S. aureus beta-lactamase cleaves penicillin and ampicillin, but not later developed “antistaphylococcal” beta-lactams such as methicillin, oxacillin or cephalosporins. They are bound by clavulanic acid.

Question 30

Gram negative beta-lactamases reside in the periplasmic space and may cleave cephalosporins and/or penicillins (some penicillin-resistant strains can be treated with cephalosporins). Some Gram negative bacteria express a cephalosporinase at low levels from the chromosome, but many penicillinases and cephalosporinases are plasmid encoded

Answer 30

Gram negative beta-lactamases reside in the periplasmic space and may cleave cephalosporins and/or penicillins (some penicillin-resistant strains can be treated with cephalosporins). Some Gram negative bacteria express a cephalosporinase at low levels from the chromosome, but many penicillinases and cephalosporinases are plasmid encoded

Question 31

Why do beta-lactamases play a much more significant role in resistance for Gram negative rather than Gram positive bacteria?

Answer 31

Because of the high potential for horizontal transfer of plasmids among Gram negative bacteria

Question 32

What do extended-spectrum beta-lactamases (ESBLs) do?

Answer 32

cleave penicillins and broad- spectrum (later generation) cephalosporins.

Question 33

ESBLs are most commonly associated with what types of bacteria?

Answer 33

Escherichia coli, Klebsiella spp., and Pseudomonas spp., but other Gram negative rods can harbor the plasmids encoding them.

Question 34

More than 200 types of ESBLs are known; a complex system is used to name them. Three common types are TEMs (over 140 known), SHVs (over 60 known), and CTX-M (over 40 known). The use of any generation of penicillins or cephalosporins on ESBL strains can likely lead to treatment failure. Moreover, some ESBL isolates are multidrug resistant (MDR)

Answer 34

More than 200 types of ESBLs are known; a complex system is used to name them. Three common types are TEMs (over 140 known), SHVs (over 60 known), and CTX-M (over 40 known). The use of any generation of penicillins or cephalosporins on ESBL strains can likely lead to treatment failure. Moreover, some ESBL isolates are multidrug resistant (MDR)

Question 35

ESBL strains usually remain sensitive to what type of antibiotic class?

Answer 35

carbapenems. However, carbapenemases, which generally also cleave penicillins and cephalosporins, are on the rise and are spreading among enteric Gram negative rods (i.e., Enterobacteriaceae)

Question 36

Klebsiella pneumoniae carbapenemase (KPC) is increasing in incidence in nosocomial infections. KPC strains are multidrug resistant and extremely hard to “purge” from hospitals, seemingly present on all surfaces of a ward. Another prominent example of a carbapenemase is the New Delhi metallo-betalactamase (NDM-1). Carbapenemase-resistant Enterobacteriaceae (CRE) are one of the most important antibiotic resistance challenges of today.

Answer 36

Klebsiella pneumoniae carbapenemase (KPC) is increasing in incidence in nosocomial infections. KPC strains are multidrug resistant and extremely hard to “purge” from hospitals, seemingly present on all surfaces of a ward. Another prominent example of a carbapenemase is the New Delhi metallo-betalactamase (NDM-1). Carbapenemase-resistant Enterobacteriaceae (CRE) are one of the most important antibiotic resistance challenges of today.

Question 37

T or F. those resistances that are associated with plasmid transmission often result in MDR strains.

Answer 37

T. For example, plasmid-mediated resistance to fluoroquinolones among the Enterobacteriaceae is often associated with resistance to beta-lactams and aminoglycosides. Therefore, fluoroquinolone use should be limited in treating Gram negative enteric infections.