How Bacteria becomes Resistance to Antibiotic Drugs!

 ANTIBIOTIC RESISTANCE PRINCIPLES

Drug-resistant microorganisms are mediated by four main pathways. (1) Drug-inactivating enzymes are produced by bacteria, such as β-lactamases, which cleave the β-lactam ring of penicillins and cephalosporins, rendering them inactive. (2) Bacteria create altered targets that the antibiotic no longer affects as much (for example, a methylation 23S rRNA can cause erythromycin resistance, while a mutant protein in the 30S ribosomal subunit can cause streptomycin resistance). (3) Drug permeability is decreased by bacteria, preventing the drug from reaching an effective intracellular concentration (e.g., alterations in porins might reduce the quantity of penicillin entering the bacterium). (4) A "multidrug-resistant pump" (also known as the "efflux" pump) is a mechanism by which bacteria actively export medications.

The majority of medication resistance results from a genetic alteration in the organism, which can be caused by a transposon or plasmid acquisition or chromosomal mutation. Page 90 discusses nongenetic alterations, which are less significant.

Antibiotic resistance that cannot be overcome by raising the dose is referred to as high-level resistance. An alternative antibiotic is utilized, typically belonging to a different medication class. Because all of the medicine is destroyed, resistance mediated by enzymes like β-lactamases frequently results in high-level resistance. Resistance that is surmountable with an increase in the antibiotic's dosage is referred to as low-level resistance. Since the modified target can still bind part of the medication, resistance caused by mutations in the gene encoding a drug target is frequently low level. 

Antibiotic-resistant bacteria are a major contributing factor to hospital-acquired infections compared to community-acquired infections. This is particularly true for hospital infections brought on by enteric gram-negative rods such Pseudomonas aeruginosa and Escherichia coli, as well as Staphylococcus aureus. Since the extensive use of antibiotics in hospitals tends to favor these germs, antibiotic-resistant pathogens are common in the hospital setting. Moreover, hospital germs frequently exhibit resistance to several different drugs. The acquisition of plasmids containing several genes encoding the enzymes mediating resistance is typically the cause of this resistance.

GENETIC RESISTANCE BASIS

Resistance Mediated by Chromosomes

A mutation in the gene that codes for the drug's target or the membrane transport system that regulates the drug's absorption causes chromosomal resistance. Spontaneous mutations often occur between 10–7 and 10–9 times/year, which is far less frequent than resistance plasmid acquisition. As a result, plasmid-mediated resistance poses a greater clinical risk than chromosomal resistance.

The following idea underpins the use of two or more medications in the treatment of specific infections. Should a bacterium mutate to become resistant to antibiotic A at a frequency of 10–7 (1 in 10 million) and to antibiotic B at a frequency of 10–8 (1 in 100 million), the likelihood that the bacterium will develop resistance to both antibiotics (assuming that the antibiotics function through distinct mechanisms) is equal to the product of the two probabilities, or 10–15. Therefore, the likelihood of the bacteria developing resistance to both medicines is extremely low. Put another way, even if an organism is resistant to one antibiotic, it's still likely to respond well to the other.

Resistance Mediated by Plasmids

From a therapeutic perspective, plasmid-mediated resistance is crucial for three reasons:

(1) It is present in a wide variety of species, particularly in gram-negative rods.

(2) Drug resistance is often mediated by plasmids.

(3) Plasmids are highly mobile, moving quickly from one cell to another, typically through conjugation.

Resistance plasmids, also known as resistance factors or R factors, are circular, double-stranded, extrachromosomal DNA molecules that contain the genes for a wide range of enzymes that can alter membrane transport systems and break down antibiotics.

R factors can have one or more of these genes, or they can have more than one antibiotic resistance gene. The first and most obvious medical consequence of a plasmid carrying multiple resistance genes is that a bacterium harboring that plasmid can be resistant to more than one class of antibiotics (e.g., penicillins and aminoglycosides).

.. For instance, using penicillin will favor the development of an organism resistant to tetracyclines, aminoglycosides (such as streptomycin and gentamicin), erythromycin, chloramphenicol, and penicillin if the organism has the R plasmid.

R factors are capable of producing drug resistance in addition to two other crucial characteristics: (1) they can replicate independently of the bacterial chromosome, allowing a cell to contain multiple copies; and (2) they can spread not only between cells in the same species but also between different species and genera. Keep in mind that the genes of the R plasmid, not the F (fertility) plasmid, which controls the transfer of the bacterial chromosome, are in charge of this conjugal transfer.

R factors can be broadly classified into two groups: giant plasmids, which have molecular weights of approximately 60 million, and tiny plasmids, which have molecular weights of approximately 10 million. Conjugative R factors, or big plasmids, are those that carry the additional DNA needed to code for the conjugation process. The resistance genes are the only things found in the tiny R factors, which are not conjugative.

R factors transmit not only antibiotic resistance but also resistance to metal ions (they encode an enzyme that breaks down mercuric ions into elemental mercury, for example) and resistance to specific bacterial viruses by encoding restriction endonucleases that break down the DNA of the infecting bacteriophages.

Resistance Mediated via Transposon

Genes known as transposons are inserted either inside or between bigger DNA structures, like plasmids and bacterial chromosomes. Three genes make up a typical drug resistance transposon, which is flanked on both sides by shorter DNA sequences. These shorter DNA sequences, which are often a sequence of repeated bases that are inverted, mediate the transposon's interaction with the larger DNA. 

(1) transposase, which is the enzyme responsible for the excision and reintegration of transposons;

 (2) a repressor that controls transposase synthesis; and

 (3) the drug resistance gene.

PRECISE RESISTANCE MECHANISMS

Cephalosporins and Penicillins—These medications can cause resistance through a variety of routes. By far the most significant is cleavage by β-lactamases, which include penicillinases and cephalosporinases. The features of laccases produced by different species vary. Staphylococcal penicillinase, for instance, is secreted into the medium and is induced by penicillin. On the other hand, several gram-negative rods produce certain constitutively produced β-lactamases that are found in the periplasmic region close to the peptidoglycan and are not secreted into the medium. The specificities of the β-lactamases produced by different gram-negative rods vary.

Some have a greater affinity for cephalosporins, whereas others are more active against penicillins. Analogs of penicillins that bind firmly to β-lactamases and render them inactive are sulbactam and clavulanic acid. Not all β-lactamases can induce resistance, but combinations of these inhibitors plus penicillins (such as clavulanic acid and amoxicillin [Augmentin]) can.

Proteus, Klebsiella, Enterobacter, and E. coli are among the enteric bacteria that produce extended-spectrum β-lactamases, or ESBLs. The bacteria develop resistance to all monobactams, cephalosporins, and penicillins thanks to ESBLs. These bacteria are still susceptible, nevertheless, to mixtures like piperacillin/tazobactam. 2009 saw the isolation of a novel, very resistant strain of Klebsiella in India that carried the plasmid encoding New Delhi metallo-β-lactamase (NDM-1). This plasmid, which has moved from Klebsiella to other Enterobacteriaceae members, confers high-level resistance to numerous drugs.

NDM-1-carrying resistant Enterobacteriaceae have surfaced in numerous nations, including the US.

Alterations in the penicillin-binding proteins (PBPs) within the bacterial cell membrane may also be the cause of resistance to penicillins. These modifications explain why S. aureus is resistant to nafcillin and other β-lactamase-resistant penicillins, as well as why S. pneumoniae exhibits both low- and high-level resistance to penicillin G. The reason behind MRSA's resistance to nearly all β-lactam antibiotics is the existence of PBP2a, which is specific to MRSA. Changes in penicillin-binding proteins may be the cause of Enterococcus faecalis's relative resistance to penicillins. Poor penicillin permeability is the reason for Neisseria gonorrhoeae's low level of resistance to the antibiotic. The existence of a plasmid coding for causes high-level resistance.

Certain S. aureus isolates exhibit a different type of resistance known as tolerance, wherein the organism is not killed by penicillin but its growth is impeded. This is explained by the autolytic enzymes, known as murein hydrolases, failing to activate and breaking down the peptidoglycan.

Carbapenems: The degradation of the β-lactam ring by carbapenemases results in resistance to carbapenems, including imipenem. The bacterium gains resistance to both cephalosporins and penicillins thanks to this enzyme. Numerous enteric gram-negative rods, including Klebsiella, Escherichia, and Pseudomonas, generate carbapenemases. Klebsiella pneumoniae strains that are resistant to carbapenems are a major contributor to hospital-acquired infections and are resistant to practically every antibiotic now in use.

Vancomycin—The peptide component of peptidoglycan changes from D-alanyl-D-alanine, the drug's usual binding site, to D-alanine-Dlactate, to which the medication does not bind. This is the source of resistance to vancomycin. VanA is the most significant of the four gene loci mediating vancomycin resistance. It offers strong resistance to teicoplanin and vancomycin and is transported by a transposon on a plasmid. (Teicoplanin is not authorized in the US, but it is used in Europe.) The enzymes that produce D-alanine-D-lactate as well as several regulatory proteins are encoded by the VanA locus.

From clinical specimens, enterococci that are resistant to vancomycin (VRE) have been found. From patient specimens, uncommon S. aureus isolates that show vancomycin resistance have also been found. uncommon isolates from patient specimens, aureus that is resistant to vancomycin have also been found. Additionally, vancomycin-tolerant S. pneumoniae isolates that are uncommon have been found.

Aminoglycosides: Two main mechanisms cause resistance to aminoglycosides: 

(1) the drugs are modified by plasmid-encoded phosphorylating, adenylylating, and acetylating enzymes (the most significant mechanism); 

(2) chromosomal mutations (e.g., a mutation in the gene that codes for the target protein in the 30S subunit of the bacterial ribosome); and

 (3) the bacterium's permeability to the drug is reduced.

Tetracyclines: Tetracycline resistance arises from the drug's inability to achieve an inhibitory concentration within the bacterium. This is caused by plasmid-encoded mechanisms that either improve the drug's transport out of the cell or decrease its absorption.