Overview of Antibiotic Resistance Mechanisms
Aminoglycoside Modifying Enzymes
Beta lactamase
Porins
Efflux Pumps
The van Operons
Ribosomal Methyltransferases

Overview of Antibiotic Resistance Mechanisms

Introduction

Ever since Alexander Fleming’s famous discovery of penicillin in 1928, antibiotics have continued to revolutionize medicine, allowing the treatment of infections and making it possible for patients to have surgical procedures with full and speedy recovery. However, as the threat of antimicrobial resistance (AMR) continues to grow, the benefits of using antibiotics are diminishing. A report published in 2022 estimated that in 2019, 1.27 million deaths globally were directly attributable to AMR (Antimicrobial Resistance Collaborators, 2022).

Why is Antimicrobial Resistance significant?

When a bacterium is resistant to the effects of a certain antibiotic, it will not die even in the presence of that antibiotic. So if a physician prescribes an antibiotic to treat a bacterial infection, but the bacteria are resistant to the antibiotic, the infections will persist and patient management can become significantly complicated.

Staphylococcus aureus is one of the most prevalent causes of bacterial infection in humans, and approximately 60% of these infections are resistant to a drug called methicillin, causing an estimated 11,000 deaths per year (Ali et al., 2018). These numbers will only continue to rise with the continued misuse of antibiotics, and medicine will regress significantly as these drugs lose their utility over time (Ali et al., 2018). The goal now is to control the dangers of AMR and preserve the function of clinical antibiotic use.

Understanding the mechanisms of antibiotic resistance is critical for developing ways to combat AMR and to develop new antibiotics to uphold the modern practice of medicine.

The Source of Resistance

While antibiotic resistance is seemingly a modern issue, this crisis predates the market use of antibiotics beginning in the 1940s (Munita and Arias, 2016). Resistance genes to β-lactams, tetracyclines, and glycopeptide antibiotics were identified in 30,000-year-old Beringian permafrost sediments found in Yukon, Canada (D’Costa et al., 2011). Why were such genes present at a time before humans began using antibiotics? This is because organisms, such as fungi, have been coexisting with bacteria for millennia, and in order to survive they evolved mechanisms, such as producing antibiotic molecules, in order to kill the bacteria that they competed with for nutrients. In response bacteria, with their high degree of genetic plasticity, also evolved ways to survive in these circumstances. In an environment now filled with antibiotic molecules from organisms like fungi, bacteria developed various resistance mechanisms to ensure their survival (Figure 1). These mechanisms are encoded as genes found in bacterial DNA, the same way hair and eye color are encoded in human DNA. These genes can be spread from commensal and environmental species to pathogenic bacteria, giving them an "acquired" resistance (von Wintersdorff et al., 2016).

Figure 1. Depiction of steps in natural selection that cause resistant bacteria to emerge and susceptible bacteria to be eliminated. This is due to antibiotic molecules synthesized by organisms, such as fungi, which share the same niche as bacteria.
Figure 1. Depiction of steps in natural selection that cause resistant bacteria to emerge and susceptible bacteria to be eliminated. This is due to antibiotic molecules synthesized by organisms, such as fungi, which share the same niche as bacteria.

Acquired resistance genes can come about through mutations or spread through horizontal gene transfer (HGT, Figure 2). Among the mechanisms of HGT, conjugation has the greatest influence on the dissemination of resistance genes. Conjugation is the transfer of mobile genetic elements, such as plasmids and transposons, through a pili structure that assembles between 2 adjacently located bacteria. Transduction is the HGT mechanism in which bacterial DNA is transferred via bacteriophages that infect one bacterium, take up bacterial DNA, and transfer the DNA to another bacteriophage susceptible bacteria. Finally, transformation is the acquisition of foreign DNA into a competent bacterial cell (von Wintersdorff et al., 2016).

Figure 2. Depiction of the fundamental methods of horizontal gene transfer. Cartoons created using Servier Medical Art.
Figure 2. Depiction of the fundamental methods of horizontal gene transfer. Cartoons created using Servier Medical Art.

Along with these horizontal gene transfer mechanisms, mutations provide a way for new bacterial genotypes to form (Thomas and Nielsen, 2005). With a means for resistance genes to spread, the increased selection pressure from anthropological antibiotic use creates the perfect environment for resistant bacteria to dominate. From 2000 to 2010, global human antibiotic use increased by 36%, reaching nearly 74 billion standard units (von Wintersdorff et al., 2016). Antibiotic use is even greater in animals, as it is used to increase growth and lengthen life in farming conditions. This incessant use of antibiotics allows for strains with resistance genotypes to proliferate and dominate, increasing the pool of resistant bacteria, while susceptible bacteria selectively die off. This increases the prevalence of resistance, makes infections more difficult to treat with antibiotics, and can even lead to the development of superbugs. Learn more about the development of antimicrobial resistance and why we should care about it.

Resistance Mechanisms

It is established that genes are the source of not only bacterial resistance but the dissemination of resistance as well. But how do these genes stop bacterial cells from dying? Resistance genes encode proteins that carry out the functions needed for the resistance mechanisms. Resistance to one antibiotic class can be accomplished through multiple pathways, and one bacterium can harbor multiple mechanisms by having multiple resistance genes. The major mechanisms of antibiotic resistance are classified into the following categories (Munita and Arias, 2016) and summarized in Figure 3:

Figure 3: Depiction of the different resistance mechanism types. The green layer between the inner and outer membranes is the peptidoglycan layer. The outer membrane shown in the figure is only present in gram-negative bacteria; gram-positive bacteria have the green peptidoglycan layer as their outermost layer. Cartoons created using Servier Medical Art.
Figure 3: Depiction of the different resistance mechanism types. The green layer between the inner and outer membranes is the peptidoglycan layer. The outer membrane shown in the figure is only present in gram-negative bacteria; gram-positive bacteria have the green peptidoglycan layer as their outermost layer. Cartoons created using Servier Medical Art.

Modifications of Antimicrobial Molecule

Modifying or chemically altering the antibiotics can render them ineffective. Resistant bacteria may possess enzymes/mechanisms to modify antibiotics in the following two ways:
* Adding certain chemical groups to the antibiotic molecule
* Breaking bonds within the antibiotic molecule

Adding Chemical Groups to Antibiotics

Chemical reactions that involve transferring chemical groups to the antibiotic drug molecule change the physio-chemical properties of the antibiotic molecule and/or change its shape. The modified antibiotic may now have altered properties (change in charge or solubility) or be too big to bind to its target due to steric hindrance. Either way modification of the antibiotic renders it ineffective in killing bacteria (Munita and Arias, 2016). Some examples of resistance caused by adding chemical groups to the antibiotic molecule are listed here.

Aminoglycoside modifications

Aminoglycoside modifying enzymes (AMEs) modify Aminoglycosides by adding chemical groups at specific positions in the antibiotic molecule (see Figure AC1). Three types of chemical modifications are catalyzed by these enzymes (Ramirez and Tolmasky, 2010):
* acetylation by acetyltransferases (AACs)
* phosphorylation by phosphotransferases (APH) and
* adenylation by nucleotidyl transferases (ANTs).

Learn more about Aminoglycoside Modifying Enzymes.

Figure AC1. Examples of chemical transformations catalyzed by the aminoglycoside enzymes. The figure was created using ChemDraw.
Figure AC1. Examples of chemical transformations catalyzed by the aminoglycoside enzymes. The figure was created using ChemDraw.

Fosfomycin modifications

The FomA and FomB enzymes are kinases that phosphorylate Fosfomycin, adding one or two phosphate groups to inactivate the antibiotic (Figure AC2). Learn more about the structure of FomA.

Figure AC2. Modification of fosfomycin, catalyzed by the fosfomycin modifying enzymes (FomA and FomB). The figure is redrawn based on information from Castañeda-García, 2013, and created using ChemAxon.
Figure AC2. Modification of fosfomycin, catalyzed by the fosfomycin modifying enzymes (FomA and FomB). The figure is redrawn based on information from Castañeda-García, 2013, and created using ChemAxon.

Lincosamide modification

Several organisms have been found to posess lin genes, whose product is an enzyme that inactivates lincosamides via adenylylation. In this family, the LinA catalyzed enzymes and their variants (LnuC and LnuD) are capable of adenylylation at either 3′- or 4′-OH lincosamides. On the other hand, the LinB enzyme adenylates at the 3' end (Bozdogan et al., 1999, Figure AC3).

Learn more about clindamycin resistance.

Figure AC3. Modification of clindamycin, catalyzed by the Lin enzymes (linA and linB). The figure was drawn using ChemAxon.
Figure AC3. Modification of clindamycin, catalyzed by the Lin enzymes (linA and linB). The figure was drawn using ChemAxon.

Macrolide modifications

There are two types of enzymes that add chemical groups to macrolides to modify their structure and functions (Golkar et al., 2018):
* Macrolide glycosyltransferase - uses UDP-glucose as the donor for adding a glucose moiety to one of the sugars in the macrolide
* Macrolide phosphotransferase - uses GTP as a source for a phosphate group, also on the 2' of the sugar linked to the C5 position of the macrolide (Figure AC4)

Figure AC4. 2D drawing of Erythromycin showing the location of phosphorylation and glycosylation. The figure was drawn using ChemAxon.
Figure AC4. 2D drawing of Erythromycin showing the location of phosphorylation and glycosylation. The figure was drawn using ChemAxon.

Learn more about macrolide-inactivating enzymes.

Rifamipicin modifications

There are four types of rifampicin-deactivating enzymes that modify the chemical structure of the drug:
* Phosphotransferases (Rph)
* ADP-ribosyltransferases (Arr)
* Glycosyltransferases (Rgt)
* Monooxygenases (Rox)

In the 2D structure of Rifampicin (Figure AC5), the Rph enzyme acts on the C21 hydroxyl, while Arr and Rgt can act on the C23 hydroxyl. These regions of the drug are crucial for antibiotic action. The enzyme Rox acts on the N2’ atom of the antibiotic, which is involved in van der Waals interactions with RNAP (Liu et al., 2016). All of these chemical modifications by rifampicin-deactivating enzymes significantly decrease the binding of the drug to RNA Polymerase and neutralize its bactericidal activity.

Learn more about rifampicin binding.

Figure AC5. The chemical structure of rifampicin is shown to highlight where the rifampicin-deactivating enzymes act. The figure was created using ChemDraw.
Figure AC5. The chemical structure of rifampicin is shown to highlight where the rifampicin-deactivating enzymes act. The figure was created using ChemDraw.

Learn about the structures of selected rifampicin-deactivating enzymes.

Tetracycline modification

The TetX and other related enzymes modify the tetracycline family of antibiotics by adding a hydroxyl group to C11 between rings B and C (Figure AC6). Learn more about Tetracyclines.

These TetX enzymes are NADPH-dependent monooxygenases that function in the presence of FADs to modify tetracyclines. The enzymes utilize NADPH and O2 to add the hydroxyl group. The new hydroxylated drug has different chemical properties which prevent it from exerting its antimicrobial properties. Specifically, the hydroxylation, which is located near atoms that interact with the Mg2+ ion, affects the magnesium coordination between the drug and the 16S rRNA. This ultimately reduces the drug’s binding affinity for the ribosome.

TetX can only operate in aerobic conditions because O2 is required as a substrate (Nguyen et al., 2014). Figure AC6 shows the reaction pathway for hydroxylation of tetracycline. Learn more about the structure of TetX in action.

Figure AC6. Reaction pathway for the hydroxylation of tetracycline as mediated by TetX. The figure was created using ChemDraw.
Figure AC6. Reaction pathway for the hydroxylation of tetracycline as mediated by TetX. The figure was created using ChemDraw.

Breaking Bonds of Antibiotics

Breaking β-lactams

β-lactam antibiotics are named after their 4 membered amide ring which is critical to their function in killing bacteria (Fernandes, Amador, and Prudêncio, 2013). The β-Lactamase enzymes inactivate β-Lactam antibiotics by breaking the amide bond (shown in red in Figure BB1). Breaking this bond opens the β-Lactam ring which takes away its potential to kill the bacteria. The discovery of β-Lactamase mediated resistance initiated the development of new β-Lactam drugs that preserved bactericidal functions despite these enzymes. However, the creation of new drugs was always accompanied by the evolution of new β-Lactamases which could destroy them. This arms race between synthetic chemists and bacteria has resulted in many β-Lactamase enzymes and β-Lactam drugs.

Learn more about β-Lactamases

Figure BB1. The structural formula of Penicillin G, a β-lactam, and the resulting destroyed molecule after enzymatic destruction of the amide bond. The amine ring is colored red and blue. The amide bond which breaks is colored in red. The figure was created using ChemDraw.
Figure BB1. The structural formula of Penicillin G, a β-lactam, and the resulting destroyed molecule after enzymatic destruction of the amide bond. The amine ring is colored red and blue. The amide bond which breaks is colored in red. The figure was created using ChemDraw.

Explore β-Lactamases that break the β-Lactam rings in Penicillin G, Methicillin, Ampicillin, Amoxicillin, Piperacillin, Cefoxitin, Ceftazidime, Cefepime, Aztreonam, and Meropenem.

Breaking fosfomycin

Various plasmid-encoded genes produce Fosfomycin modifying enzymes e.g., FosA, FosB, and FosX that belong to the glyoxalase superfamily (Castañeda-García et al., 2013). The Glutathione S-transferase encoded by FosA inactivates fosfomycin adding the sulphydryl group of the cysteine of tripeptide glutathione (GSH) to C1 of the epoxide ring of fosfomycin and opening the antibiotic epoxide group. The FosB and FosX enzymes use l-cysteine and water, respectively, to open the epoxide group and render the antibiotic inactive (see Figure BB2).

Learn more about the structure of FosA and its function.

Figure BB2: Example chemical transformations catalyzed by the fosfomycin modifying enzymes FosA, FosB, and FosX. The figure was redrawn based on Castañeda-García, 2013, and created using ChemAxon.
Figure BB2: Example chemical transformations catalyzed by the fosfomycin modifying enzymes FosA, FosB, and FosX. The figure was redrawn based on Castañeda-García, 2013, and created using ChemAxon.

Breaking macrolides

One way in which macrolide antibiotics may be rendered inactive is by hydrolytic cleavage of the macrocyclic lactone ring by erythromycin esterases (Eres) (Morar et al., 2012). The most frequently identified Ere enzyme is EreA, but other enzymes EreB, EreC, etc. are also commonly found. The catalysis breaks macrolactone found in macrolides e.g., erythromycin and azithromycin (Figure BB3).

Learn more about macrolide inactivating enzymes.

Figure BB3: A 2D drawing showing the breaking of the macrolactone ring of macrolides by Erythromycin esterases (Morar et al., 2012). The figure was created using ChemAxon.
Figure BB3: A 2D drawing showing the breaking of the macrolactone ring of macrolides by Erythromycin esterases (Morar et al., 2012). The figure was created using ChemAxon.

Reduced Permeability

Antibiotic drugs with targets inside the cell require mechanisms that allow them to penetrate the cell membrane and access their intracellular target sites. Decreasing this permeability, or the ability of an antibiotic to access the intracellular domain, is another mode of action for resistance. The cell membrane of bacteria is made from a phospholipid bilayer consisting of non-polar, hydrophobic fatty acids forming the interior of the bilayer, and polar, hydrophilic phosphate molecules forming the exterior. This bilayer is relatively impervious to polar, hydrophilic molecules. Thus, hydrophilic antibiotic molecules, such as β-Lactams and tetracyclines, may use aqueous diffusion channels, also known as porin proteins, to cross the cell membrane to access their target (Munita and Arias, 2016). Porins are integral proteins, which means they span the entire length of the cellular membrane, and allow polar hydrophilic molecules (which normally cannot diffuse through the lipid bilayer of the cell) into the cell.

By changing the types of porins that are expressed, or the level of porin expression, certain antibiotics can no longer enter the cell to access their targets, thus creating resistance. For example, in the bacteria Klebsiella pneumoniae, decreased susceptibility to β-Lactam drugs results from a switch in porin expression to OmpK36 which has a smaller channel size, allowing less of the drug to pass into the cell (Dutzler et al., 1999, Figure RP1).

Learn more about porins.

Figure RP1: A monomer of the osmoporin OmpK36 is shown in relative position in the outer cell membrane of a Klebsiella pneumoniae (gram-negative) bacterium. Its smaller channel size leads to decreased β-Lactams permeability [PDB ID: 1OSM] (Dutzler et al., 1999). Cartoons were created using Servier Medical Art.
Figure RP1: A monomer of the osmoporin OmpK36 is shown in relative position in the outer cell membrane of a Klebsiella pneumoniae (gram-negative) bacterium. Its smaller channel size leads to decreased β-Lactams permeability [PDB ID: 1OSM] (Dutzler et al., 1999). Cartoons were created using Servier Medical Art.

Learn about specific examples of reduced permeability to Penicillin G, Ampicillin, Amoxicillin, Cefoxotin, Ceftazidime, and Cefepime.

Antibiotic Efflux

Another mechanism of antimicrobial resistance is decreasing antibiotic permeability into the cellular domain by actively pumping them back out. Efflux pumps are enzymatic “machines” that are capable of extruding antibiotic molecules from the cell. There are 5 major classes of efflux pumps which differ in their structure, energy source, substrate specificity, and distribution among bacteria:
1. Major facilitator superfamily (MFS)
2. Small multidrug resistance family (SMR)
3. Resistance-nodulation-cell-division family (RND)
4. ATP-binding cassette family (ABC)
5. Multidrug and toxic compound extrusion family (MATE)

Learn more about efflux pumps.

Shown below is the AcrAB-TolC system, found in E. coli. This RND efflux pump extrudes antibiotics by the orchestrated functions of 3 separate proteins: AcrB, AcrA, and TolC (Figure AE). The AcrB protein is a protein located in the inner membrane that binds the antibiotic molecules found in the cell. The AcrA protein serves as a linking protein to the TolC protein. TolC is a protein channel located in the outer membrane. Through conformational changes in these proteins, antibiotics are pumped out of TolC and into the extracellular environment (Munita and Arias, 2016).

Figure AE. AcrAB-TolC system shown in relation to an E. coli (gram-negative) cell wall, PDB ID 5O66, (Wang et al., 2017). Cartoons created using Servier Medical Art.
Figure AE. AcrAB-TolC system shown in relation to an E. coli (gram-negative) cell wall, PDB ID 5O66, (Wang et al., 2017). Cartoons created using Servier Medical Art.

Learn about specific efflux pumps that facilitate the resistance of fosfomycin, methicillin, ampicillin, piperacillin, ceftazidime, cefepime, Aztreonam, Meropenem, Vancomycin, moxifloxacin, rifampin, streptomycin, gentamicin, azithromycin, Tetracycline, Tigecycline, Linezolid, and Trimethoprim.

Modifications of target site

Bacteria can confer resistance by modifying or changing the target site of the antibiotic. These modifications result in decreased affinity between the target site and the drug. Bacteria can alter their target sites in 4 ways:
* Protection of target site
* Target gene alterations (pre-translational modifications)
* Substitution of the original target
* Enzymatic alterations of target (post-translational modifications)

Protection of target site

Target site protection entails preventing the antibiotic from reaching its target through mechanisms other than decreased permeability. Some examples are included here.

TetM protein

TetM is a ribosome protection protein that dislodges tetracycline antibiotic molecules from their binding site in the ribosome of the bacterial cell. Additionally, TetM prevents rebinding of the drug by altering the ribosomal conformation (Munita and Arias, 2016). Learn more about TetM-mediated tetracycline resistance.

ABC-F subfamily of ATP-binding cassette

The lsa-type ABC-F proteins confer resistance to streptogramin, lincosamide, and pleuromutilin antibiotics through antibiotic target protection (of the ribosome). Learn more about how LsaA binding leads to Clindamycin resistance here.

Another ABC-F protein that confers resistance to macrolides is MsrE. This protein binds to the ribosomal exit site, preventing the binding of macrolide antibiotics (e.g., azithromycin). Learn more about MsrE binding to ribosomes.

Target gene alterations

Bacteria can modify the genes that encode the target site. This is known as pre-translational alterations. By altering the RNA, the translated protein will be modified, and the target site will have less affinity for the drug. These modifications can arise from point mutations in genes. These modifications can arise from point mutations in genes. An example of this is through a point mutation in the rpoB gene which encodes the bacterial RNA polymerase. This genetic alteration changes the target site of the antibiotic rifamycin; rifamycin now has decreased affinity for this new target site, resulting in resistance (Munita and Arias, 2016).

Species-specific variations

The native form of the enzyme or targets in some bacterial species may have different amino acids in the active site, protecting it from antibiotic action. For example, the MurA homolog of Mycobacterium tuberculosis has an Asp in place of the catalytic Cys residue. Thus the antibiotic can not bind to the enzyme. Learn more about this mechanism of fosfomycin resistance.

Substitution of the original target

Replacement of the target site for a less susceptible target (with reduced affinity) while preserving biochemical function is another resistance mechanism that bacteria have evolved. A few examples are included here.

Van type operons

In vancomycin-resistant Enterococci, the susceptible target site is replaced with one that has reduced affinity from vancomycin. Vancomycin binds to terminal D-Alanyl-D-Alanine units of the pentapeptide group of the peptidoglycan cell wall. Resistant bacteria have completely substituted the final D-Alanine unit for other units such as D-Lactate or D-Serine. Vancomycin has reduced affinity for these units, resulting in reduced antibiotic action, and in other words, vancomycin resistance (Munita and Arias, 2016). Learn more about the van operons.

Sul genes

In sulfa-insensitive bacteria, a series of plasmid-borne Sul genes code for modified DHPS enzymes that can select against the antibiotic sulfamethoxazole and bind the natural substrate (Venkatesan et al., 2023). These genes are likely to have evolved from folP (the DHPS gene), in environmental bacteria. The overall shapes of these enzymes (e.g., Sul1, Sul2, Sul3) are similar. However, due to specific differences in these proteins, Sul enzymes are able to discriminate against sulfamethoxazole (the competitive inhibitor of the enzyme) and bind to pABA (the substrate) rendering the bacteria resistant to the antibiotic.

Learn more about their role in Sul enzymes mediated sulfamethoxazole resistance.

Dihydrofolate reductase enzymes

In several clinical samples, plasmid-borne and chromosomal trimethoprim-resistant dihydrofolate reductase enzymes have been identified. The dfrA, dfrB, dfrC, dfrD, dfrE, dfrG, dfrK enzymes may be horizontally transferred spreading resistance to the antibiotic. Learn more about trimethoprim resistance by Dhfr enzymes.

Enzymatic Alterations of Target

Post-translational enzymatic alteration of a target site occurs once the protein has already been translated from the RNA. These alterations can change the properties of the target and the antibiotic-binding interaction. Examples of these include methylation of rRNA (by methyltransferases) altering the antibiotic binding sites and conferring resistance.

16S ribosomal RNA methyltransferase

Methyltransferases that modify the 16S rRNA of bacterial 30S subunit of ribosomes, commonly modify either G1405 or A1408. Both these bases play key roles in binding aminoglycosides. Methylation leads to steric hindrance for antibiotic binding, conferring resistance to this class of drugs.

Learn more about 16S rRNA methyltransferases.

23S ribosomal RNA methyltransferase

These methyltransferases are enzymes that modify the 23S rRNA of the bacterial 50S ribosomes and impact antibiotics that target it (ARO:3004274). Some examples of these enzymes are listed here:

Erm enzymes (e.g., ErmE, ErmC, ErmA, etc.) confer resistance to macrolides and lincosamides. These 23S ribosomal rRNA methyltransferase enzymes can transfer methyl groups to an adenine residue in the bacterial ribosome. Macrolides target the ribosome but the steric hindrance from the methyl groups on the binding site reduces their affinity for its target, leading to resistance (Munita and Arias, 2016).

Cfr methylases methylate A2503 and lead to resistance to lincosamides. Here too, the methylated base leads to steric hindrance for lincosamide binding.

Learn more about 23S rRNA methyltransferases.

Global Cell Adaptations

The final category of resistance mechanism is global cell adaptations, which entails complex cellular changes to resist antibiotic molecules. These mechanisms can be manifested in diverse ways.

Pore Formation

Daptomycin resistance is noteworthy in this category. Daptomycin is a lipopeptide antibiotic that complexes with calcium ions, giving the molecule an overall positive charge. The molecule passes into the membrane, forms oligomers, and creates a pore-like structure in the cell membrane which results in bacterial death (Figure PF, Munita and Arias, 2016). To confer resistance to daptomycin, resistant strains of Enterococci use a mutated regulatory system that modifies the cell membrane to have a positive charge that repels the cationic daptomycin-calcium complexes, resulting in daptomycin resistance (Arias et al., 2011).

Figure PF. The process by which Daptomycin causes cell death is shown, sequenced chronologically from top to bottom. Cartoons created using Servier Medical Art.
Figure PF. The process by which Daptomycin causes cell death is shown, sequenced chronologically from top to bottom. Cartoons created using Servier Medical Art.

Host-dependent Nutrient Acquisition

An energy-coupling factor (ECF) transporter component gene (thfT) enables Group A Streptococcus to acquire reduced folate compounds from the host that it is infecting (Rodrigo et al., 2022). Since these bacteria do not depend on the biosynthesis of reduced folates (TFH), they are resistant. Learn more about host-dependent THF acquisition leading to sulfamethoxazole resistance.

Conclusion

These resistance mechanisms form the basis of the immeasurable threat of AMR. While these classifications may be simple, they encompass a diverse array of mechanisms, many of which require more investigation. Moreover, many mechanisms active in nature have yet to be described in scientific literature. For these reasons, AMR is a difficult crisis for scientists to combat, yet its threat to patients grows every day. For every drug used in treatment, a resistance mechanism exists. With the genetic plasticity of bacteria, there is the potential for these mechanisms to spread, rendering the antibiotics mankind depends on ineffective. The World Health Organization has ranked AMR as one of the top 10 health threats of 2019 (WHO, 2019), for a good reason. The impending post-antibiotic era will result in the collapse of current medical protocols and treatment, and we will be left with no way to manage ailments, that were once treatable.

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April 2025, Sameer Ahmad, Helen Gao, Gauri Patel, Shuchismita Dutta; Reviewed by: Dr. André O. Hudson
https://doi.org/10.2210/rcsb_pdb/GH/AMR/drugs/amr-mech/overview