Rising Challenges in Antibacterial Drug Resistance: Navigating the Advanced Panorama of Microbial Adaptation and Therapeutic Implications”

Rising Challenges in Antibacterial Drug Resistance: Navigating the Advanced Panorama of Microbial Adaptation and Therapeutic Implications

 

PRINCIPLES OF ANTIBIOTIC

RESISTANCE

There are 4 main mechanisms that mediate bacterial resis-

tance to medication (Desk 11–1). (1) Micro organism produce enzymes that

inactivate the drug (e.g., β-lactamases can inactivate penicil-

lins and cephalosporins by cleaving the β-lactam ring of the

drug). (2) Micro organism synthesize modified targets in opposition to which

the drug has a decreased impact (e.g., a mutant protein within the 30S

ribosomal subunit can lead to resistance to streptomycin, and

a methylated 23S rRNA can lead to resistance to erythromy-

cin). (3) Micro organism scale back permeability to the drug such that an

efficient intracellular focus of the drug shouldn’t be achieved

(e.g., adjustments in porins can scale back the quantity of penicillin

coming into the bacterium). (4) Micro organism actively export medication

utilizing a “multidrug-resistance pump” (MDR pump, or “efflux”

pump). The MDR pump imports protons and, in an exchange-

sort response, exports a wide range of international molecules together with

sure antibiotics, similar to tetracyclines.

Most drug resistance is because of a genetic change within the organ-

ism, both a chromosomal mutation or the acquisition of a

plasmid or transposon. Nongenetic adjustments, similar to micro organism

inside an abscess being tougher to succeed in with the antibi-

otic, are mentioned later on this chapter.

The time period high-level resistance refers to resistance that

can’t be overcome by growing the dose of the antibiotic.

A special antibiotic, normally from one other class of medication, is

used. Resistance mediated by enzymes similar to β-lactamases

usually ends in high-level resistance, as all of the drug is destroyed.

Low-level resistance refers to resistance that may be overcome

by growing the dose of the antibiotic. Resistance mediated by

mutations within the gene encoding a drug goal is commonly low degree,

because the altered goal can nonetheless bind among the drug however with

decreased power.

As an example using these phrases, strains of Neisseria gonor-

rhoeae that produce penicillinase can’t be handled efficiently

with penicillin G. They exhibit high-level resistance, and a dif-

ferent drug similar to ceftriaxone have to be used. Nonetheless, strains

of N. gonorrhoeae that synthesize altered penicillin-binding pro-

teins exhibit low-level resistance and might be handled efficiently

with high-dose penicillin G.

Hospital-acquired infections are considerably extra probably

to be brought on by antibiotic-resistant organisms than are

community-acquired infections. That is very true for

hospital infections brought on by Staphylococcus aureus and enteric

gram-negative rods similar to Escherichia coli and Pseudomonas

aeruginosa. Antibiotic-resistant organisms are frequent within the

hospital setting as a result of widespread antibiotic use in hospitals

selects for these organisms. Moreover, hospital strains are

usually proof against a number of antibiotics. This resistance is normally

as a result of acquisition of plasmids carrying a number of genes that

encode the enzymes that mediate resistance.

Desk 11–2 describes sure medically necessary bacte-

ria and the primary medication to which they’re resistant. Observe that

though these micro organism are proof against different medication as properly, for

simplicity, solely essentially the most attribute medication are listed. Some

strains of the micro organism listed in Desk 11–2 are extremely resistant

to a number of antibiotics, particularly methicillin-resistant S. aureus

(MRSA; see Chapter 15), vancomycin-resistant Enterococcus

faecium (VRE; see Chapter 15), multidrug-resistant Streptococcus

pneumoniae (MDR-SP; see Chapter 15), P. aeruginosa (see

Chapter 18), and multidrug-resistant Mycobacterium tuberculosis

(MDR-MTB; see Chapter 21).

GENETIC BASIS OF RESISTANCE

Chromosome-Mediated Resistance

Chromosomal resistance is because of a mutation within the gene that

codes for both the goal of the drug or the transport system

within the membrane that controls the uptake of the drug. The fre-

quency of spontaneous mutations normally ranges from 10–7 to

10–9, which is way decrease than the frequency of acquisition of

resistance plasmids. Due to this fact, chromosomal resistance is much less

of a medical downside than is plasmid-mediated resistance.

The therapy of sure infections with two or extra medication

is predicated on the next precept. If the frequency {that a} bac-

terium mutates to turn out to be proof against antibiotic A is 10–7 (1 in

10 million) and the frequency that the identical bacterium mutates

to turn out to be proof against antibiotic B is 10–8 (1 in 100 million),

then the possibility that the bacterium will turn out to be proof against

each antibiotics (assuming that the antibiotics act by completely different

mechanisms) is the product of the 2 chances, or 10–15.

It’s subsequently extremely unlikely that the bacterium will turn out to be

proof against each antibiotics. Acknowledged one other method, though an

organism might turn out to be resistant to at least one antibiotic, it’s probably that

it will likely be successfully handled by the opposite antibiotic.

Plasmid-Mediated Resistance

Plasmid-mediated resistance is essential from a medical

viewpoint for 3 causes:

(1) It happens in many various species, particularly gram-

unfavourable rods.

(2) Plasmids ceaselessly mediate resistance to a number of medication.

(3) Plasmids have a excessive price of switch from one cell to

one other, normally by conjugation.

Resistance plasmids (resistance elements, R elements) are

extrachromosomal, round, double-stranded DNA molecules

that carry the genes for a wide range of enzymes that may degrade

antibiotics and modify membrane transport methods (Determine 11–1).

Desk 11–3 describes a very powerful mechanisms of resis-

tance for a number of necessary medication.

R elements might carry one antibiotic resistance gene or might

carry two or extra of those genes. The medical implication of a

plasmid carrying a couple of resistance gene is twofold: first

and most evident is {that a} bacterium containing that plasmid

FIGURE 11–1 Resistance plasmid (R plasmid, R issue). Most

resistance plasmids have two units of genes: (1) resistance switch

genes that encode the intercourse pilus and different proteins that mediate

switch of the plasmid DNA throughout conjugation and (2) drug resis-

tance genes that encode the proteins that mediate drug resistance.

The underside half of the determine depicts (from left to proper) the genes

that encode resistance to tetracycline, streptomycin, penicillin

(β-lactamase), chloramphenicol, erythromycin, and gentamicin.

might be proof against a couple of class of antibiotics (e.g.,

penicillins and aminoglycosides), and second, using an

antibiotic that selects for an organism resistant to at least one antibiotic

will choose for an organism that’s proof against all of the antibiotics

whose resistance genes are carried by the plasmid. For instance,

if an organism has the R plasmid depicted in Determine 11–1, then

using penicillin will choose for an organism resistant not

solely to penicillin, but in addition to tetracyclines, aminoglycosides

(e.g., streptomycin and gentamicin), chloramphenicol, and

erythromycin.

Along with producing drug resistance, R elements have two

essential properties: (1) they will replicate independently

of the bacterial chromosome; subsequently, a cell can include many

copies; and (2) they are often transferred not solely to cells of the

similar species, but in addition to different species and genera. Observe that

this conjugal switch is below the management of the genes of the R

plasmid and never of the F (fertility) plasmid, which governs the

switch of the bacterial chromosome (see Chapter 4).

R elements exist in two broad dimension classes: giant plasmids,

with molecular weights of about 60 million, and small ones,

with molecular weights of about 10 million. The massive plasmids

are conjugative R elements, which include the additional DNA to code

for the conjugation course of. The small R elements should not conjuga-

tive and include solely the resistance genes.

Along with conveying antibiotic resistance, R elements

impart two different traits: (1) resistance to metallic ions (e.g., they

code for an enzyme that reduces mercuric ions to elemental

mercury) and (2) resistance to sure bacterial viruses by cod-

ing for restriction endonucleases that degrade the DNA of the

infecting bacteriophages.

Transposon-Mediated Resistance

Transposons are genes which might be transferred both inside or

between bigger items of DNA such because the bacterial chromosome

and plasmids. A typical drug resistance transposon consists

of three genes flanked on each side by shorter DNA sequences,

normally a sequence of inverted repeated bases that mediate the

interplay of the transposon with the bigger DNA (see Determine

2–7). The three genes code for (1) transposase, the enzyme that

catalyzes excision and reintegration of the transposon; (2) a

repressor that regulates synthesis of the transposase; and (3) the

drug resistance gene.

SPECIFIC MECHANISMS OF

RESISTANCE

Penicillins & Cephalosporins—There are a number of mechanisms

of resistance to those medication. Cleavage by β-lactamases (penicil-

linases and cephalosporinases) is by far a very powerful (see

Determine 10–3). β-Lactamases produced by varied organisms

have completely different properties. For instance, staphylococcal penicil-

linase is inducible by penicillin and is secreted exterior of the

bacterium. In distinction, some β-lactamases produced by a number of

gram-negative rods are constitutively produced, are positioned

within the periplasmic area close to the peptidoglycan, and should not

secreted exterior of the bacterium.

The β-lactamases produced by varied gram-negative rods

have completely different specificities: some are extra lively in opposition to cepha-

losporins, others in opposition to penicillins. Clavulanic acid, tazobac-

tam, sulbactam, and avibactam are penicillin analogues that

bind strongly to β-lactamases and inactivate them. Combina-

tions of those inhibitors and penicillins (e.g., clavulanic acid

plus amoxicillin [Augmentin] and piperacillin plus tazobactam

[Zosyn]) can overcome resistance mediated by many however not all

β-lactamases.

Prolonged-spectrum β-lactamases (ESBLs) inactivate

extended-spectrum cephalosporins (second- and third-

era cephalosporins), similar to ceftriaxone, cefotaxime,

and ceftazidime, in addition to penicillins and first-generation

cephalosporins. They’re produced by a number of enteric micro organism,

notably E. coli, Klebsiella, Enterobacter, and Proteus. ESBLs endow

the micro organism with resistance to all penicillins, cephalosporins,

and monobactams, similar to aztreonam. Carbapenems, similar to

imipenem, are the drug of option to deal with infections prompted

by ESBL-producing micro organism. Nonetheless, some ESBL-producing

micro organism have acquired resistance to carbapenems (through car-

bapenemases, see beneath) and might be handled solely with colistin, a

polypeptide antibiotic that doesn’t have a β-lactam ring.

In 2009, a brand new pressure of extremely resistant Klebsiella was

remoted in India carrying a plasmid that encoded New Delhi

metallo-β-lactamase (NDM-1). This plasmid confers high-

degree resistance to many antibiotics together with carbapenems and

has unfold from Klebsiella to different members of the Enterobac-

teriaceae. Resistant Enterobacteriaceae carrying NDM-1 have

emerged in lots of international locations, together with the USA.

Resistance to penicillins can be brought on by adjustments within the

penicillin-binding proteins (PBPs) within the bacterial cell mem-

brane. These adjustments account for each the low-level and high-

degree resistance exhibited by S. pneumoniae to penicillin G and for

the resistance of S. aureus to nafcillin and different β-lactamase–

resistant penicillins. The resistance of MRSA to nearly all β-lactams

is attributed to the presence of PBP2a, which is discovered notably

in MRSA. The relative resistance of Enterococcus faecalis to

penicillins could also be as a result of altered penicillin-binding proteins.

Resistance to penicillin can be brought on by poor permeability

of the drug. For instance, low-level resistance of N. gonorrhoeae

to penicillin is attributed to poor permeability of the drug.

Excessive-level resistance is as a result of presence of a plasmid coding

for penicillinase.

Some isolates of S. aureus show yet one more type of

resistance, known as tolerance, through which progress of the organism

is inhibited by penicillin however the organism shouldn’t be killed. This

is attributed to a failure of activation of the autolytic enzymes,

murein hydrolases, which degrade the peptidoglycan.

Carbapenems—Resistance to carbapenems, similar to imipe-

nem, is brought on by carbapenemases that degrade the β-lactam

ring. This enzyme endows the organism with resistance to

penicillins and cephalosporins as properly. Carbapenemases are

produced by many enteric gram-negative rods, particularly

Klebsiella, Escherichia, and Pseudomonas. Carbapenem-resistant

strains of Klebsiella pneumoniae are an necessary reason behind

hospital-acquired infections and are proof against nearly all

identified antibiotics.

Vancomycin—Resistance to vancomycin is brought on by a

change within the peptide part of peptidoglycan from d-alanyl-

d-alanine, which is the conventional binding website for vancomycin,

to d-alanine-d-lactate, to which the drug doesn’t bind. Of

the 4 gene loci mediating vancomycin resistance, VanA is

a very powerful. It’s carried by a transposon on a plasmid

and offers high-level resistance to each vancomycin and

teicoplanin. (Teicoplanin is utilized in Europe however shouldn’t be accredited

in the USA.) The VanA locus encodes these enzymes

that synthesize d-alanine-d-lactate in addition to a number of regulatory

proteins.

Vancomycin-resistant strains of enterococci (VRE) have been

recovered from medical specimens. Uncommon isolates of S. aureus that

exhibit resistance to vancomycin have additionally been recovered from

affected person specimens. Uncommon isolates of S. pneumoniae that exhibit

tolerance to vancomycin have been recovered as properly.

Aminoglycosides—Resistance to aminoglycosides happens

by three mechanisms: (1) modification of the medication by

plasmid-encoded phosphorylating, adenylylating, and acetylat-

ing enzymes (a very powerful mechanism); (2) chromo-

somal mutation (e.g., a mutation within the gene that codes for the

goal protein within the 30S subunit of the bacterial ribosome); and

(3) decreased permeability of the bacterium to the drug.

Tetracyclines—Resistance to tetracyclines is the results of failure

of the drug to succeed in an inhibitory focus contained in the micro organism.

This is because of plasmid-encoded processes that both scale back the

uptake of the drug or improve its transport out of the cell.

Chloramphenicol—Resistance to chloramphenicol is due

to a plasmid-encoded acetyltransferase that acetylates the drug,

thus inactivating it.

Erythromycin—Resistance to erythromycin is due pri-

marily to a plasmid-encoded enzyme that methylates the 23S

rRNA, thereby blocking binding of the drug. An efflux pump

that reduces the focus of erythromycin throughout the

bacterium causes low-level resistance to the drug. An esterase

produced primarily by enteric gram-negative rods cleaves the

macrolide ring, which inactivates the drug.

Sulfonamides—Resistance to sulfonamides is mediated pri-

marily by two mechanisms: (1) a plasmid-encoded transport

system that actively exports the drug out of the cell, and (2) a chro-

mosomal mutation within the gene coding for the goal enzyme

dihydropteroate synthetase, which reduces the binding affinity

of the drug.

Trimethoprim—Resistance to trimethoprim is due primar-

ily to mutations within the chromosomal gene that encodes dihy-

drofolate reductase, the enzyme that reduces dihydrofolate to

tetrahydrofolate.

Quinolones—Resistance to quinolones is due primarily to

chromosomal mutations that modify the bacterial DNA gyrase.

Rifampin—Resistance to rifampin is because of a chromosomal

mutation within the gene encoding the bacterial RNA polymerase,

leading to ineffective binding of the drug. As a result of resistance

happens at excessive frequency (10–5), rifampin shouldn’t be prescribed alone

for the therapy of infections. It’s used alone for the prevention

of sure infections as a result of it’s administered for under a brief

time (see Desk 10–8).

Isoniazid—Resistance of M. tuberculosis to isoniazid is due

to mutations within the organism’s catalase–peroxidase gene. Cata-

lase or peroxidase enzyme exercise is required to synthesize the

metabolite of isoniazid that truly inhibits the expansion of

M. tuberculosis.

Ethambutol—Resistance of M. tuberculosis to ethambutol

is because of mutations within the gene that encodes arabinosyl trans-

ferase, the enzyme that synthesizes the arabinogalactan within the

organism’s cell wall.

Pyrazinamide—Resistance of M. tuberculosis to pyrazin-

amide (PZA) is because of mutations within the gene that encodes bacte-

rial amidase, the enzyme that converts PZA to the lively type of

the drug, pyrazinoic acid.

 

NONGENETIC BASIS OF RESISTANCE

There are a number of nongenetic causes for the failure of medication to

inhibit the expansion of micro organism:

(1) Micro organism might be walled off inside an abscess cavity that

the drug can not penetrate successfully. Surgical drainage is there-

fore a vital adjunct to chemotherapy.

(2) Micro organism might be in a resting state (i.e., not rising); they

are subsequently insensitive to cell wall inhibitors similar to penicillins

and cephalosporins. Equally, M. tuberculosis can stay dor-

mant in tissues for a few years, throughout which period it’s insensi-

tive to medication. If host defenses are lowered and the micro organism start

to multiply, they’re once more vulnerable to the medication, indicating

{that a} genetic change didn’t happen.

(3) Underneath sure circumstances, organisms that may ordi-

narily be killed by penicillin can lose their cell partitions, survive as

protoplasts, and be insensitive to cell wall–lively medication. Later,

if such organisms resynthesize their cell partitions, they’re totally

vulnerable to those medication.

(4) The presence of international our bodies makes profitable antibi-

otic therapy tougher. This is applicable to international our bodies such

as surgical implants and catheters in addition to supplies that enter

the physique on the time of penetrating accidents, similar to splinters

and shrapnel.

(5) A number of artifacts could make it seem that the organisms

are resistant (e.g., administration of the incorrect drug or the

incorrect dose or failure of the drug to succeed in the suitable website

within the physique). (An excellent instance of the latter is the poor penetra-

tion into spinal fluid by a number of early-generation cephalospo-

rins.) Failure of the affected person to take the drug (noncompliance,

nonadherence) is one other artifact.

SELECTION OF RESISTANT BACTERIA

BY OVERUSE & MISUSE OF

ANTIBIOTICS

Critical outbreaks of ailments brought on by gram-negative rods resis-

tant to a number of antibiotics have occurred in lots of creating

international locations. In North America, many hospital-acquired infections

are brought on by multidrug-resistant organisms. Three details

of overuse and misuse of antibiotics enhance the probability of

these issues by enhancing the choice of resistant mutants:

(1) Some physicians use a number of antibiotics when one would

be ample, prescribe unnecessarily lengthy programs of antibiotic

remedy, use antibiotics in self-limited infections for which they

should not wanted, and overuse antibiotics for prophylaxis earlier than

and after surgical procedure.

(2) In lots of international locations, antibiotics are bought over-the-counter

to most of the people; this observe encourages inappropriate and

indiscriminate use of the medication.

(3) Antibiotics are utilized in animal feed to stop infections

and promote progress. This selects for resistant organisms within the

animals and will contribute to the pool of resistant organisms

in people.

ANTIBIOTIC SENSITIVITY TESTING

Antibiogram

An antibiogram is the time period used to explain the outcomes of anti-

biotic susceptibility assessments carried out on the micro organism remoted

from the affected person. These outcomes are a very powerful issue

in figuring out the selection of antibiotic with which to deal with the

affected person. Different elements such because the affected person’s renal operate and

hypersensitivity profile should even be thought-about in selecting the

antibiotic.

There are two sorts of assessments used to find out the anti-

biogram: (1) the tube dilution check that determines the minimal

inhibitory focus and (2) the disk diffusion (Kirby-Bauer)

check that determines the diameter of the zone of inhibition (see

following dialogue and Figures 11–2 and 11–3).

Minimal Inhibitory Focus

For a lot of infections, the outcomes of sensitivity testing are impor-

tant within the selection of antibiotic. These outcomes are generally

reported because the minimal inhibitory focus (MIC),

which is outlined because the lowest focus of drug that

inhibits the expansion of the organism. The MIC is decided by

inoculating the organism remoted from the affected person right into a sequence

of tubes or cups containing twofold dilutions of the drug (Determine

11–2). After incubation at 35°C for 18 hours, the bottom concen-

tration of drug that forestalls seen progress of the organism is

the MIC. This offers the doctor with a exact concentra-

tion of drug to information the selection of each the drug and the dose.

A second technique of figuring out antibiotic sensitivity is the

disk diffusion technique, through which disks impregnated with varied

antibiotics are positioned on the floor of an agar plate that has

been inoculated with the organism remoted from the affected person

(Determine 11–3). After incubation at 35°C for 18 hours, throughout

which period the antibiotic diffuses outward from the disk, the

diameter of the zone of inhibition is decided. The dimensions of

the zone of inhibition is in contrast with requirements to find out

the sensitivity of the organism to the drug.

Minimal Bactericidal Focus

For sure infections, similar to endocarditis, you will need to

know the focus of drug that truly kills the organism

quite than the focus that merely inhibits progress. This

focus, known as the minimal bactericidal focus

(MBC), is decided by taking a small pattern (0.01 or 0.1

mL) from the tubes used for the MIC assay and spreading it

over the floor of a drug-free blood agar plate (Determine 11–2).

Any organisms that had been inhibited however not killed now have a

likelihood to develop as a result of the drug has been diluted considerably.

After incubation at 35°C for 48 hours, the bottom focus

that has decreased the variety of colonies by 99.9%, in contrast

with the drug-free management, is the MBC. Bactericidal medication usu-

ally have an MBC equal or similar to the MIC, whereas

bacteriostatic medication normally have an MBC considerably greater

than the MIC.

Serum Bactericidal Exercise

Within the therapy of endocarditis, it may be helpful to find out

whether or not the drug is efficient by assaying the flexibility of the drug

within the affected person’s serum to kill the organism. This check, known as the

serum bactericidal exercise, is carried out in a fashion just like

that of the MBC dedication, besides that it’s a serum pattern

from the affected person, quite than a normal drug resolution, that’s

used. After a normal inoculum of the organism has been added

and the combination has been incubated at 35°C for 18 hours, a small

pattern is subcultured onto blood agar plates, and the serum

dilution that kills 99.9% of the organisms is decided. Scientific

expertise has proven {that a} peak1

serum bactericidal exercise of

1:8 or 1:16 is satisfactory for profitable remedy of endocarditis.

β-Lactamase Manufacturing

For extreme infections brought on by sure organisms, similar to S. aureus

and Haemophilus influenzae, you will need to know as quickly as

potential whether or not the organism remoted from the affected person is produc-

ing β-lactamase. For this function, fast assays for the enzyme can

be used that yield a solution in a couple of minutes, versus an

MIC check or a disk diffusion check, each of which take 18 hours.

A generally used process is the chromogenic β-lactam

technique, through which a coloured β-lactam drug is added to a suspension

of the organisms. If β-lactamase is made, hydrolysis of the β-lactam

ring causes the drug to show a unique shade in 2 to 10 minutes.

Disks impregnated with a chromogenic β-lactam may also be used.

USE OF ANTIBIOTIC COMBINATIONS

Most often, the one greatest antimicrobial agent needs to be

chosen to be used as a result of this minimizes unwanted effects. Nonetheless,

there are a number of cases through which two or extra medication are com-

monly given:

(1) To deal with severe infections earlier than the id of the

organism is thought.

(2) To realize a synergistic inhibitory impact in opposition to sure

organisms.

(3) To stop the emergence of resistant organisms. (If bac-

teria turn out to be resistant to at least one drug, the second drug will kill

them, thereby stopping the emergence of resistant strains.)

Two medication can work together in one in every of a number of methods (Determine 11–4).

They’re normally detached to one another (i.e., additive solely).

Typically there’s a synergistic interplay, through which the impact

of the 2 medication collectively is considerably higher than the sum of

the results of the 2 medication appearing individually. Hardly ever, the impact

of the 2 medication collectively is antagonistic, through which the result’s

considerably decrease exercise than the sum of the actions of the

two medication alone.

A synergistic impact may result from a wide range of mecha-

nisms. For instance, the mix of a penicillin and an

aminoglycoside similar to gentamicin has a synergistic motion

in opposition to enterococci (E. faecalis), as a result of penicillin damages the

cell wall sufficiently to reinforce the entry of aminoglycoside.

When given alone, neither drug is efficient. A second instance

is the mix of a sulfonamide with trimethoprim. On this

occasion, the 2 medication act on the identical metabolic pathway, such

that if one drug doesn’t inhibit folic acid synthesis sufficiently,

the second drug offers efficient inhibition by blocking a sub-

sequent step within the pathway.

Though antagonism between two antibiotics is uncommon,

one instance is clinically necessary. This entails using

penicillin G mixed with the bacteriostatic drug tetracycline

within the therapy of meningitis brought on by S. pneumoniae. Antag-

onism happens as a result of the tetracycline inhibits the expansion of the

organism, thereby stopping the bactericidal impact of penicillin

G, which kills solely rising organisms.