by Enterobacteriaceae constitutes a family of Gram-negative, rod-shaped bacteria within the order Enterobacterales, belonging to the phylum Pseudomonadota. These bacteria commonly reside in the intestines of various animals, including humans, as commensals. However, some strains have the potential to cause significant infections in humans.
They are implicated in various infectious diseases, including urinary tract infections (UTIs), respiratory tract infections (RTIs), bloodstream infections (BSIs), gastrointestinal infections, as well as wounds and soft tissue infections. The primary choice for treating infections associated with Enterobacterales are β-lactam antibiotics. Additionally, tetracyclines, colistin, and aminoglycosides are employed when β-lactams display reduced efficacy.
Unfortunately, the rise of antibiotic resistance has rendered many of the commonly used and preferred treatments ineffective against Enterobacterales. Critical antibiotic resistance genes such as Beta-lactamase genes (bla genes), colistin-resistant genes, tetracycline-resistant genes, and aminoglycoside-resistant genes play a pivotal role in this challenge.
Beta-lactamase (bla) Genes
Beta-lactamase (bla) genes encode for enzymes known as beta-lactamases. These enzymes are produced by some bacteria and are responsible for conferring resistance to beta-lactam antibiotics. Beta-lactam antibiotics include a wide range of drugs such as penicillins, cephalosporins, and carbapenems, which are commonly used to treat bacterial infections.
The beta-lactamase enzymes work by breaking the beta-lactam ring structure present in these antibiotics. This structural change renders the antibiotic ineffective against the bacteria, allowing them to survive and multiply.
There is a wide variety of bla genes, each associated with different types of beta-lactamases. They can be found on bacterial chromosomes or on plasmids (small, mobile pieces of DNA). The presence of bla genes can lead to antibiotic resistance, making infections caused by bacteria carrying these genes more difficult to treat.
To combat beta-lactamase-mediated resistance, combination therapies with beta-lactamase inhibitors (such as clavulanic acid, sulbactam, or tazobactam) are often used. These inhibitors bind to the beta-lactamase enzyme, preventing it from inactivating the antibiotic.
The presence and prevalence of bla genes in bacterial populations is a significant concern in healthcare settings, as it can limit treatment options for bacterial infections and lead to the spread of antibiotic-resistant strains.
blaTEM Genes
The blaTEM genes are a group of beta-lactamase genes that encode for a type of beta-lactamase enzyme known as TEM (named after the Temoniera isolate from Greece). Beta-lactamases are enzymes produced by bacteria that confer resistance to beta-lactam antibiotics, which include penicillins and early-generation cephalosporins.
The TEM beta-lactamase enzymes work by hydrolyzing the beta-lactam ring structure present in these antibiotics. This enzymatic activity inactivates the antibiotics, rendering them ineffective against the bacteria.
The blaTEM genes are commonly found in Enterobacteriaceae family of bacteria, which includes species like Escherichia coli and Klebsiella pneumoniae. They can also be carried on plasmids, which are mobile genetic elements that can be transferred between different bacterial strains and species. This mobility allows blaTEM genes to spread rapidly among bacterial populations.
The presence of blaTEM genes in bacterial populations is a significant concern in healthcare settings, as it limits treatment options for infections caused by bacteria carrying these genes. This can lead to more severe infections, longer hospital stays, and increased healthcare costs.
Efforts to combat blaTEM-mediated resistance include the development of new beta-lactamase inhibitors, the judicious use of antibiotics, and the implementation of infection control measures to prevent the spread of resistant strains.
Mechanism of Conferring Resistance of blaTEM Genes
The production of TEM beta-lactamase by bacteria carrying blaTEM genes is a significant mechanism of antibiotic resistance. It underscores the importance of understanding the genetic basis of resistance and developing strategies to combat the spread of resistant strains.
- Normal Action of Beta-Lactam Antibiotics:
Beta-lactam antibiotics, such as penicillins and early-generation cephalosporins, work by inhibiting bacterial cell wall synthesis. They target enzymes called penicillin-binding proteins (PBPs) involved in the construction of the bacterial cell wall.
- Role of TEM Beta-Lactamase:
When bacteria carry the blaTEM gene, they have the genetic instructions to produce the TEM beta-lactamase enzyme.
- Hydrolysis of Beta-Lactam Ring:
The TEM beta-lactamase enzyme possesses the ability to hydrolyze (break apart) the beta-lactam ring structure present in beta-lactam antibiotics. This enzymatic activity cleaves the crucial beta-lactam ring.
- Inactivation of Antibiotics:
The beta-lactam antibiotics become chemically modified and inactivated as a result of the hydrolysis. This means they can no longer effectively bind to the penicillin-binding proteins (PBPs) on the bacterial cell wall.
- Preventing Cell Wall Synthesis Inhibition:
Since the antibiotic’s mechanism of action relies on inhibiting cell wall synthesis, the modified antibiotic is no longer able to effectively interfere with this process.
- Resistance to Beta-Lactam Antibiotics:
As a consequence of TEM beta-lactamase activity, bacteria carrying blaTEM genes become resistant to the action of beta-lactam antibiotics. The antibiotics are unable to effectively inhibit cell wall formation, allowing the bacteria to survive and replicate.
Detection Method of blaTEM Genes
| Detection Method | Principle | Advantages | Limitations |
| Polymerase Chain Reaction (PCR) | Amplifies and detects specific DNA sequences of blaTEM genes | Highly sensitive and specific | Requires access to a PCR machine and DNA analysis equipment |
| Real-Time PCR (qPCR) | Quantitative PCR that allows for real-time monitoring of DNA amplification | Provides quantitative information on gene presence | Requires specialized qPCR equipment and expertise |
| Multiplex PCR | Amplifies multiple target genes, including blaTEM, simultaneously | Allows for simultaneous detection of multiple genes | Requires primer design and optimization for multiple targets |
| Loop-Mediated Isothermal Amplification (LAMP) | Amplifies DNA at a constant temperature, eliminating the need for a thermal cycler | Rapid and less equipment-dependent | Requires specialized LAMP primer design |
| Next-Generation Sequencing (NGS) | Sequences the entire bacterial genome, allowing for detection of blaTEM genes | Comprehensive, can identify multiple resistance genes | Requires advanced sequencing technology and bioinformatics expertise |
| Reverse Transcription PCR (RT-PCR) | Amplifies RNA molecules transcribed from blaTEM genes | Useful for detecting gene expression | Requires conversion of RNA to cDNA prior to amplification |
| Multiplex Ligation-dependent Probe Amplification (MLPA) | Uses probe hybridization and ligation to detect specific DNA sequences | Allows for multiplex detection of genes | Requires specialized probes and equipment |
| DNA Microarrays | Utilizes probes on a microarray chip to hybridize with specific gene sequences | High-throughput, can detect multiple genes simultaneously | Requires specialized microarray technology and analysis tools |
blaCTX-M Genes
The blaCTX-M genes are a group of beta-lactamase genes that encode for a type of extended-spectrum beta-lactamase (ESBL). ESBLs are enzymes produced by bacteria that confer resistance to a broad range of beta-lactam antibiotics, including penicillins, cephalosporins, and monobactams.
The term “CTX-M” stands for cefotaximase-Munich, as the first CTX-M enzyme was identified in Munich, Germany, and had a specific affinity for cefotaxime, a third-generation cephalosporin.
The mechanism of resistance conferred by blaCTX-M genes is similar to other beta-lactamase genes. The enzymes produced by these genes work by hydrolyzing the beta-lactam ring structure present in beta-lactam antibiotics. This enzymatic activity inactivates the antibiotics, rendering them ineffective against the bacteria.
The blaCTX-M genes are commonly found in Enterobacteriaceae family of bacteria, including Escherichia coli and Klebsiella pneumoniae. They can also be carried on plasmids, which are mobile genetic elements that can be transferred between different bacterial strains and species. This mobility allows blaCTX-M genes to spread rapidly among bacterial populations.
The presence of blaCTX-M genes in bacterial populations is a significant concern in healthcare settings, as it limits treatment options for infections caused by bacteria carrying these genes. This can lead to more severe infections, longer hospital stays, and increased healthcare costs.
Efforts to combat blaCTX-M-mediated resistance include the development of new beta-lactamase inhibitors, the judicious use of antibiotics, and the implementation of infection control measures to prevent the spread of resistant strains.
Mechanism of Conferring Resistance of blaCTX-M Genes
The blaCTX-M genes confer resistance to beta-lactam antibiotics through the production of an extended-spectrum beta-lactamase (ESBL) enzyme known as CTX-M. The production of CTX-M ESBL by bacteria carrying blaCTX-M genes is a significant mechanism of antibiotic resistance. It underscores the importance of understanding the genetic basis of resistance and developing strategies to combat the spread of resistant strains.
- Normal Action of Beta-Lactam Antibiotics:
Beta-lactam antibiotics, such as cephalosporins and penicillins, work by inhibiting bacterial cell wall synthesis. They target enzymes called penicillin-binding proteins (PBPs) involved in the construction of the bacterial cell wall.
- Role of CTX-M ESBL:
When bacteria carry the blaCTX-M gene, they have the genetic instructions to produce the CTX-M ESBL enzyme.
- Hydrolysis of Beta-Lactam Ring:
The CTX-M ESBL enzyme possesses the ability to hydrolyze (break apart) the beta-lactam ring structure present in beta-lactam antibiotics. This enzymatic activity cleaves the crucial beta-lactam ring.
- Inactivation of Antibiotics:
The beta-lactam antibiotics become chemically modified and inactivated as a result of the hydrolysis. This means they can no longer effectively bind to the penicillin-binding proteins (PBPs) on the bacterial cell wall.
- Preventing Cell Wall Synthesis Inhibition:
Since the antibiotic’s mechanism of action relies on inhibiting cell wall synthesis, the modified antibiotic is no longer able to effectively interfere with this process.
- Resistance to Beta-Lactam Antibiotics:
As a consequence of CTX-M ESBL activity, bacteria carrying blaCTX-M genes become resistant to the action of beta-lactam antibiotics. The antibiotics are unable to effectively inhibit cell wall formation, allowing the bacteria to survive and replicate.
Detection Method of blaCTX-M Genes
| Detection Method | Principle | Advantages | Limitations |
| Polymerase Chain Reaction (PCR) | Amplifies and detects specific DNA sequences of blaCTX-M genes | Highly sensitive and specific | Requires access to a PCR machine and DNA analysis equipment |
| Real-Time PCR (qPCR) | Quantitative PCR that allows for real-time monitoring of DNA amplification | Provides quantitative information on gene presence | Requires specialized qPCR equipment and expertise |
| Multiplex PCR | Amplifies multiple target genes, including blaCTX-M, simultaneously | Allows for simultaneous detection of multiple genes | Requires primer design and optimization for multiple targets |
| Loop-Mediated Isothermal Amplification (LAMP) | Amplifies DNA at a constant temperature, eliminating the need for a thermal cycler | Rapid and less equipment-dependent | Requires specialized LAMP primer design |
| Next-Generation Sequencing (NGS) | Sequences the entire bacterial genome, allowing for detection of blaCTX-M genes | Comprehensive, can identify multiple resistance genes | Requires advanced sequencing technology and bioinformatics expertise |
| Reverse Transcription PCR (RT-PCR) | Amplifies RNA molecules transcribed from blaCTX-M genes | Useful for detecting gene expression | Requires conversion of RNA to cDNA prior to amplification |
| Multiplex Ligation-dependent Probe Amplification (MLPA) | Uses probe hybridization and ligation to detect specific DNA sequences | Allows for multiplex detection of genes | Requires specialized probes and equipment |
| DNA Microarrays | Utilizes probes on a microarray chip to hybridize with specific gene sequences | High-throughput, can detect multiple genes simultaneously | Requires specialized microarray technology and analysis tools |
blaSHV Genes
The blaSHV genes are a group of beta-lactamase genes that encode for a type of beta-lactamase enzyme known as SHV (named after a hospital in Richmond, Virginia, where it was first identified). Beta-lactamases are enzymes produced by bacteria that confer resistance to beta-lactam antibiotics, which include penicillins and cephalosporins.
The SHV beta-lactamase enzymes work by hydrolyzing the beta-lactam ring structure present in these antibiotics. This enzymatic activity inactivates the antibiotics, rendering them ineffective against the bacteria.
The blaSHV genes are commonly found in Enterobacteriaceae family of bacteria, including species like Escherichia coli and Klebsiella pneumoniae. They can also be carried on plasmids, which are mobile genetic elements that can be transferred between different bacterial strains and species. This mobility allows blaSHV genes to spread rapidly among bacterial populations.
The presence of blaSHV genes in bacterial populations is a significant concern in healthcare settings, as it limits treatment options for infections caused by bacteria carrying these genes. This can lead to more severe infections, longer hospital stays, and increased healthcare costs.
Efforts to combat blaSHV-mediated resistance include the development of new beta-lactamase inhibitors, the judicious use of antibiotics, and the implementation of infection control measures to prevent the spread of resistant strains.
Mechanism of Conferring Resistance of blaSHV Genes
- Normal Action of Beta-Lactam Antibiotics:
Beta-lactam antibiotics, such as cephalosporins and penicillins, work by inhibiting bacterial cell wall synthesis. They target enzymes called penicillin-binding proteins (PBPs) involved in the construction of the bacterial cell wall.
- Role of SHV Beta-Lactamase:
When bacteria carry the blaSHV gene, they have the genetic instructions to produce the SHV beta-lactamase enzyme.
- Hydrolysis of Beta-Lactam Ring:
The SHV beta-lactamase enzyme possesses the ability to hydrolyze (break apart) the beta-lactam ring structure present in beta-lactam antibiotics. This enzymatic activity cleaves the crucial beta-lactam ring.
- Inactivation of Antibiotics:
The beta-lactam antibiotics become chemically modified and inactivated as a result of the hydrolysis. This means they can no longer effectively bind to the penicillin-binding proteins (PBPs) on the bacterial cell wall.
- Preventing Cell Wall Synthesis Inhibition:
Since the antibiotic’s mechanism of action relies on inhibiting cell wall synthesis, the modified antibiotic is no longer able to effectively interfere with this process.
- Resistance to Beta-Lactam Antibiotics:
As a consequence of SHV beta-lactamase activity, bacteria carrying blaSHV genes become resistant to the action of beta-lactam antibiotics. The antibiotics are unable to effectively inhibit cell wall formation, allowing the bacteria to survive and replicate.
Detection Method of blaSHV Genes
| Detection Method | Principle | Advantages | Limitations |
| Polymerase Chain Reaction (PCR) | Amplifies and detects specific DNA sequences of blaSHV genes | Highly sensitive and specific | Requires access to a PCR machine and DNA analysis equipment |
| Real-Time PCR (qPCR) | Quantitative PCR that allows for real-time monitoring of DNA amplification | Provides quantitative information on gene presence | Requires specialized qPCR equipment and expertise |
| Multiplex PCR | Amplifies multiple target genes, including blaSHV, simultaneously | Allows for simultaneous detection of multiple genes | Requires primer design and optimization for multiple targets |
| Loop-Mediated Isothermal Amplification (LAMP) | Amplifies DNA at a constant temperature, eliminating the need for a thermal cycler | Rapid and less equipment-dependent | Requires specialized LAMP primer design |
| Next-Generation Sequencing (NGS) | Sequences the entire bacterial genome, allowing for detection of blaSHV genes | Comprehensive, can identify multiple resistance genes | Requires advanced sequencing technology and bioinformatics expertise |
| Reverse Transcription PCR (RT-PCR) | Amplifies RNA molecules transcribed from blaSHV genes | Useful for detecting gene expression | Requires conversion of RNA to cDNA prior to amplification |
| Multiplex Ligation-dependent Probe Amplification (MLPA) | Uses probe hybridization and ligation to detect specific DNA sequences | Allows for multiplex detection of genes | Requires specialized probes and equipment |
| DNA Microarrays | Utilizes probes on a microarray chip to hybridize with specific gene sequences | High-throughput, can detect multiple genes simultaneously | Requires specialized microarray technology and analysis tools |
blaOXA Genes
The blaOXA genes are a group of beta-lactamase genes that encode for a type of beta-lactamase enzyme known as OXA (Oxacillinase). Beta-lactamases are enzymes produced by bacteria that confer resistance to beta-lactam antibiotics, which include penicillins, cephalosporins, and carbapenems.
The OXA beta-lactamase enzymes work by hydrolyzing the beta-lactam ring structure present in these antibiotics. This enzymatic activity inactivates the antibiotics, rendering them ineffective against the bacteria.
The blaOXA genes are commonly associated with resistance to carbapenem antibiotics, which are considered as a last-line treatment for severe bacterial infections. Carbapenem-resistant bacteria carrying blaOXA genes pose a significant challenge in healthcare settings, as it limits treatment options for infections caused by these strains.
The blaOXA genes can be found in various species of bacteria, including Acinetobacter baumannii and Pseudomonas aeruginosa, which are known for their ability to cause opportunistic infections in hospitalized patients.
Efforts to combat blaOXA-mediated resistance include the development of new antibiotics, the judicious use of existing antibiotics, and the implementation of infection control measures to prevent the spread of resistant strains. Additionally, research into novel treatment strategies, such as combination therapy, is ongoing.
Mechanism of Conferring Resistance of blaOXA Genes
- Normal Action of Beta-Lactam Antibiotics:
Beta-lactam antibiotics, including penicillins, cephalosporins, and carbapenems, work by inhibiting bacterial cell wall synthesis. They target enzymes called penicillin-binding proteins (PBPs) involved in the construction of the bacterial cell wall.
- Role of OXA Beta-Lactamase:
When bacteria carry the blaOXA gene, they have the genetic instructions to produce the OXA beta-lactamase enzyme.
- Hydrolysis of Beta-Lactam Ring:
The OXA beta-lactamase enzyme possesses the ability to hydrolyze (break apart) the beta-lactam ring structure present in beta-lactam antibiotics. This enzymatic activity cleaves the crucial beta-lactam ring.
- Inactivation of Antibiotics:
The beta-lactam antibiotics become chemically modified and inactivated as a result of the hydrolysis. This means they can no longer effectively bind to the penicillin-binding proteins (PBPs) on the bacterial cell wall.
- Preventing Cell Wall Synthesis Inhibition:
Since the antibiotic’s mechanism of action relies on inhibiting cell wall synthesis, the modified antibiotic is no longer able to effectively interfere with this process.
- Resistance to Beta-Lactam Antibiotics:
As a consequence of OXA beta-lactamase activity, bacteria carrying blaOXA genes become resistant to the action of beta-lactam antibiotics, particularly carbapenems. The antibiotics are unable to effectively inhibit cell wall formation, allowing the bacteria to survive and replicate.
blaIMP Genes
The blaIMP (Imipenemase) genes belong to a group of beta-lactamase genes responsible for encoding Imipenemase enzymes in bacteria. These genes are prevalent in a diverse range of Gram-negative bacteria that have developed resistance to carbapenem antibiotics. This includes various strains of Enterobacterales, particularly E. coli, Klebsiella pneumoniae, and Serratia marcescens.
Many of these genes are located within the integron of plasmid or chromosomal DNA, enabling them to be transferred between related bacteria. As of now, there are 19 distinct types of Imipenemase enzymes that have been identified, indicating the existence of at least 19 different variants of the blaIMP genes. This diversity underscores the adaptability and complexity of antibiotic resistance mechanisms among bacteria.
Mechanism of Conferring Resistance of blaIMP Genes
- Normal Action of Carbapenem Antibiotics:
Carbapenem antibiotics are a class of beta-lactam antibiotics that are highly effective against a broad spectrum of bacteria. They work by inhibiting bacterial cell wall synthesis, targeting penicillin-binding proteins (PBPs) involved in cell wall construction.
- Role of Imipenemase Enzymes:
When bacteria carry the blaIMP gene, they have the genetic instructions to produce Imipenemase enzymes.
- Hydrolysis of Carbapenem Ring:
Imipenemase enzymes possess the ability to hydrolyze (break apart) the carbapenem ring structure present in carbapenem antibiotics. This enzymatic activity cleaves the crucial carbapenem ring.
- Inactivation of Antibiotics:
The carbapenem antibiotics become chemically modified and inactivated as a result of the hydrolysis. This means they can no longer effectively bind to the penicillin-binding proteins (PBPs) on the bacterial cell wall.
- Preventing Cell Wall Synthesis Inhibition:
Since the antibiotic’s mechanism of action relies on inhibiting cell wall synthesis, the modified carbapenem is no longer able to effectively interfere with this process.
- Resistance to Carbapenem Antibiotics:
As a consequence of Imipenemase activity, bacteria carrying blaIMP genes become resistant to the action of carbapenem antibiotics. The antibiotics are unable to effectively inhibit cell wall formation, allowing the bacteria to survive and replicate.
blaVIM Genes
The blaVIM genes belong to a group of bla genes responsible for encoding Verona integron encoded Metallo-beta-lactamase (VIM) enzymes. These genes are commonly found in various members of the Enterobacteriaceae family, particularly in bacteria like Klebsiella pneumoniae, E. coli, Serratia marcescens, Citrobacter spp., among others. Additionally, they play a significant role in conferring carbapenem resistance in strains of Acinetobacter baumannii and Pseudomonas aeruginosa.
There are more than 40 known variants of VIM enzymes, suggesting the existence of at least 46 different variants of the blaVIM genes. Among these, blaVIM-2 and blaVIM-1 are the most frequently reported variants. This diversity underscores the adaptability and complexity of antibiotic resistance mechanisms among bacteria.
Mechanism of Conferring Resistance of blaVIM Genes
- Normal Action of Carbapenem Antibiotics:
Carbapenem antibiotics are a class of beta-lactam antibiotics that are highly effective against a broad spectrum of bacteria. They work by inhibiting bacterial cell wall synthesis, targeting penicillin-binding proteins (PBPs) involved in cell wall construction.
- Role of VIM Metallo-beta-lactamase:
When bacteria carry the blaVIM gene, they have the genetic instructions to produce VIM enzymes.
- Hydrolysis of Carbapenem Ring:
VIM Metallo-beta-lactamase enzymes possess the ability to hydrolyze (break apart) the carbapenem ring structure present in carbapenem antibiotics. Unlike other beta-lactamases, VIM enzymes use a zinc ion in their active site to cleave the carbapenem ring.
- Inactivation of Antibiotics:
The carbapenem antibiotics become chemically modified and inactivated as a result of the hydrolysis. This means they can no longer effectively bind to the penicillin-binding proteins (PBPs) on the bacterial cell wall.
- Preventing Cell Wall Synthesis Inhibition:
Since the antibiotic’s mechanism of action relies on inhibiting cell wall synthesis, the modified carbapenem is no longer able to effectively interfere with this process.
- Resistance to Carbapenem Antibiotics:
As a consequence of VIM Metallo-beta-lactamase activity, bacteria carrying blaVIM genes become resistant to the action of carbapenem antibiotics. The antibiotics are unable to effectively inhibit cell wall formation, allowing the bacteria to survive and replicate.
blaNDM Genes
The blaNDM genes are a group of beta-lactamase genes that code for the production of New Delhi Metallo-beta-lactamase (NDM) enzymes in bacteria. These genes are associated with high-level resistance to a broad range of beta-lactam antibiotics, including carbapenems, which are considered as a last-line treatment for severe bacterial infections.
NDM enzymes are a type of Metallo-beta-lactamase (MBL) and they are able to hydrolyze (break down) a wide variety of beta-lactam antibiotics. This enzymatic activity inactivates the antibiotics, rendering them ineffective against the bacteria.
The blaNDM genes have been reported in various species of bacteria, including Enterobacteriaceae (such as Escherichia coli and Klebsiella pneumoniae), as well as in other Gram-negative bacteria like Acinetobacter baumannii and Pseudomonas aeruginosa.
The spread of blaNDM genes is a significant concern in healthcare settings, as it limits treatment options for infections caused by bacteria carrying these genes. Efforts to combat blaNDM-mediated resistance include the development of new antibiotics, the judicious use of existing antibiotics, and the implementation of infection control measures to prevent the spread of resistant strains. Additionally, research into novel treatment strategies, such as combination therapy, is ongoing.
Mechanism of Conferring Resistance of blaNDM Genes
- Normal Action of Beta-Lactam Antibiotics:
Beta-lactam antibiotics, including penicillins, cephalosporins, and carbapenems, work by inhibiting bacterial cell wall synthesis. They target enzymes called penicillin-binding proteins (PBPs) involved in the construction of the bacterial cell wall.
- Role of NDM Metallo-beta-lactamase:
When bacteria carry the blaNDM gene, they have the genetic instructions to produce NDM enzymes.
- Hydrolysis of Beta-Lactam Ring:
NDM Metallo-beta-lactamase enzymes possess the ability to hydrolyze (break apart) the beta-lactam ring structure present in beta-lactam antibiotics. Unlike other beta-lactamases, NDM enzymes use a metal ion (often zinc) in their active site to cleave the beta-lactam ring.
- Inactivation of Antibiotics:
The beta-lactam antibiotics become chemically modified and inactivated as a result of the hydrolysis. This means they can no longer effectively bind to the penicillin-binding proteins (PBPs) on the bacterial cell wall.
- Preventing Cell Wall Synthesis Inhibition:
Since the antibiotic’s mechanism of action relies on inhibiting cell wall synthesis, the modified antibiotic is no longer able to effectively interfere with this process.
- Resistance to Beta-Lactam Antibiotics:
As a consequence of NDM Metallo-beta-lactamase activity, bacteria carrying blaNDM genes become highly resistant to a broad spectrum of beta-lactam antibiotics, including carbapenems. The antibiotics are unable to effectively inhibit cell wall formation, allowing the bacteria to survive and replicate.
blaAmpC Genes
The blaAmpC genes are a group of beta-lactamase genes that code for the production of AmpC beta-lactamase enzymes in bacteria. These enzymes are able to hydrolyze (break down) a wide range of beta-lactam antibiotics, including penicillins, cephalosporins, and some extended-spectrum cephalosporins.
The AmpC beta-lactamase enzymes are typically expressed at low levels in many Enterobacteriaceae species. However, when overexpressed or mutated due to genetic changes (such as mutations or gene amplification), they can confer significant resistance to these antibiotics.
One of the challenges posed by AmpC beta-lactamases is their ability to hydrolyze extended-spectrum cephalosporins, which are often used in clinical settings. This can lead to treatment failures and the need for alternative antibiotic therapies.
The blaAmpC genes can be found in various species of bacteria, including Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis. They are often located on plasmids, which can facilitate their spread among different bacterial strains.
Efforts to combat blaAmpC-mediated resistance include the development of beta-lactamase inhibitors that can be used in combination with beta-lactam antibiotics to overcome this type of resistance. Additionally, infection control measures and prudent antibiotic use are crucial in managing infections caused by bacteria carrying blaAmpC genes.
Mechanism of Conferring Resistance of blaAmpC Genes
- Normal Action of Beta-Lactam Antibiotics:
Beta-lactam antibiotics, including penicillins, cephalosporins, and some extended-spectrum cephalosporins, work by inhibiting bacterial cell wall synthesis. They target enzymes called penicillin-binding proteins (PBPs) involved in cell wall construction.
- Role of AmpC Beta-lactamase:
When bacteria carry the blaAmpC gene, they have the genetic instructions to produce AmpC beta-lactamase enzymes.
- Hydrolysis of Beta-Lactam Ring:
AmpC beta-lactamase enzymes possess the ability to hydrolyze (break apart) the beta-lactam ring structure present in beta-lactam antibiotics.
- Inactivation of Antibiotics:
The beta-lactam antibiotics become chemically modified and inactivated as a result of the hydrolysis. This means they can no longer effectively bind to the penicillin-binding proteins (PBPs) on the bacterial cell wall.
- Preventing Cell Wall Synthesis Inhibition:
Since the antibiotic’s mechanism of action relies on inhibiting cell wall synthesis, the modified antibiotic is no longer able to effectively interfere with this process.
- Resistance to Beta-Lactam Antibiotics:
As a consequence of AmpC beta-lactamase activity, bacteria carrying blaAmpC genes become resistant to a wide range of beta-lactam antibiotics, including penicillins, cephalosporins, and some extended-spectrum cephalosporins.
aac(6′)-Ib Genes
The aac(6′)-Ib gene is a type of antibiotic resistance gene that encodes for an enzyme known as aminoglycoside N-acetyltransferase. This enzyme is capable of modifying certain aminoglycoside antibiotics, which include drugs like gentamicin, tobramycin, and amikacin.
The aac(6′)-Ib gene is frequently found in various bacterial species, particularly among members of the Enterobacteriaceae family, which includes organisms like Escherichia coli and Klebsiella pneumoniae.
The mechanism of resistance conferred by aac(6′)-Ib involves the enzymatic modification of aminoglycoside antibiotics. Specifically, the enzyme adds an acetyl group to the antibiotic molecule, altering its structure. This modification disrupts the antibiotic’s ability to bind to bacterial ribosomes and inhibit protein synthesis, rendering it less effective.
The presence of aac(6′)-Ib and similar resistance genes poses a challenge in the treatment of bacterial infections, as aminoglycosides are important antibiotics for the management of various infections. Efforts to combat this type of resistance include the development of new antibiotics and combination therapies, as well as the implementation of antibiotic stewardship programs to ensure judicious use of these drugs.
Mechanism of Conferring Resistance of aac(6′)-Ib Genes
- Normal Action of Aminoglycoside Antibiotics:
Aminoglycoside antibiotics, such as gentamicin, tobramycin, and amikacin, work by binding to the bacterial ribosome, disrupting protein synthesis, and ultimately leading to bacterial cell death.
- Role of Aminoglycoside N-acetyltransferase:
When bacteria carry the aac(6′)-Ib gene, they have the genetic instructions to produce the enzyme aminoglycoside N-acetyltransferase.
- Acetylation of Aminoglycoside Molecule:
Aminoglycoside N-acetyltransferase enzymes catalyze the addition of an acetyl group to the aminoglycoside antibiotic molecule. This chemical modification alters the structure of the antibiotic.
- Reduced Binding to Ribosome:
The acetylation of the aminoglycoside antibiotic reduces its affinity for the bacterial ribosome. This means that the antibiotic is less able to effectively bind and disrupt protein synthesis.
- Decreased Antibiotic Efficacy:
As a consequence of this modification, the aminoglycoside antibiotic becomes less effective in inhibiting protein synthesis within the bacterial cell.
- Resistance to Aminoglycoside Antibiotics:
Bacteria carrying the aac(6′)-Ib gene, and producing the aminoglycoside N-acetyltransferase enzyme, are thus resistant to the action of aminoglycoside antibiotics.
aph(3’) Genes
The aph(3′) genes, also known as aminoglycoside phosphotransferase genes, encode enzymes that confer resistance to aminoglycoside antibiotics. Aminoglycosides are a class of antibiotics commonly used to treat bacterial infections, particularly those caused by Gram-negative bacteria.
The enzymes produced by aph(3′) genes work by adding a phosphate group to specific sites on the aminoglycoside molecule. This chemical modification alters the structure of the antibiotic, preventing it from binding effectively to its target in the bacterial cell, and thereby rendering it less effective in inhibiting bacterial growth.
There are several different variants of aph(3′) genes, each associated with specific aminoglycosides. For example, aph(3′)-Ia is associated with resistance to kanamycin, aph(3′)-IIa with resistance to gentamicin, and aph(3′)-IIIa with resistance to tobramycin.
The presence of aph(3′) genes in bacterial populations can contribute to aminoglycoside resistance, making infections caused by these bacteria more challenging to treat. This underscores the importance of judicious antibiotic use and the development of strategies to combat antibiotic resistance.
Mobilized Colistin Resistance (mcr) Genes
Mobilized Colistin Resistance (mcr) genes are a group of genes that encode for a protein responsible for conferring resistance to colistin, an antibiotic of last resort used to treat infections caused by multidrug-resistant Gram-negative bacteria.
Colistin, also known as polymyxin E, is a potent antibiotic that targets the outer membrane of bacteria, disrupting its structure and ultimately leading to cell death. It has been a critical treatment option for infections when other antibiotics fail.
The emergence of mcr genes is a significant concern in the field of antibiotic resistance. These genes are often found on plasmids, which are small, mobile pieces of DNA that can be easily transferred between different bacterial strains and species. This mobility allows mcr genes to spread rapidly among bacterial populations.
The presence of mcr genes in bacteria is particularly worrisome because it limits treatment options for infections, potentially leading to higher mortality rates and increased healthcare costs. Moreover, as colistin is considered one of the last-resort antibiotics, the emergence of resistance to it poses a serious threat to public health.
Efforts are underway globally to monitor and control the spread of mcr genes, including strict infection control measures, surveillance programs, and research into alternative treatment options. Additionally, responsible antibiotic use and stewardship are essential in preventing the further spread of colistin resistance.
Mechanism of Conferring Resistance
The mechanism by which Mobilized Colistin Resistance (mcr) genes confer resistance to colistin involves the production of a protein that modifies the outer membrane of Gram-negative bacteria, preventing colistin from effectively targeting and disrupting bacterial cells.
The acquisition of mcr genes and subsequent modification of the outer membrane is a significant mechanism of antibiotic resistance and poses a serious challenge to public health efforts to combat multidrug-resistant bacteria. Monitoring and controlling the spread of mcr genes are crucial steps in addressing this emerging threat.
- Normal Action of Colistin:
Colistin, a polypeptide antibiotic, functions by binding to the lipopolysaccharides (LPS) in the outer membrane of Gram-negative bacteria. This disrupts the integrity of the outer membrane, leading to leakage of intracellular contents and ultimately bacterial cell death.
- Role of mcr Genes:
When bacteria acquire mcr genes, they gain the ability to produce a protein called phosphoethanolamine transferase. This enzyme catalyzes the addition of a phosphoethanolamine (PEA) molecule to the lipid A portion of the LPS in the bacterial outer membrane.
- Modification of Lipid A:
Lipid A is a component of the LPS molecule that anchors it in the outer membrane. It plays a crucial role in maintaining the stability of the bacterial cell envelope. The addition of PEA to the lipid A alters its structure.
- Impact on Colistin Binding:
The modification of lipid A reduces the affinity of colistin for the bacterial outer membrane. This means that colistin is less effective in binding to and disrupting the bacterial cell envelope.
- Resistance to Colistin:
As a result of this modification, bacteria carrying mcr genes become resistant to the action of colistin. The antibiotic is no longer able to effectively target and kill the bacteria, allowing them to survive and multiply even in the presence of colistin.
- Plasmid-Mediated Transfer:
One of the concerning aspects of mcr genes is that they are often located on plasmids, which are small, mobile pieces of DNA. This allows for the rapid transfer of mcr genes between different bacterial strains and species, facilitating the spread of colistin resistance.
Detection Method:
| Detection Method | Principle | Advantages | Limitations |
| Disk Diffusion | Measures the zone of inhibition around a disk | Simple, inexpensive | Semi-quantitative, influenced by various factors |
| Minimum Inhibitory Concentration (MIC) | Determines the lowest concentration of an antibiotic that inhibits bacterial growth | Quantitative, provides specific concentration values | More time-consuming, requires specialized equipment |
| Agar Dilution | Dilutions of antibiotics are incorporated into agar plates | Accurate, provides precise MIC values | Labor-intensive, requires a range of antibiotic concentrations |
| E-Test | Plastic strip with a gradient of antibiotic concentration is placed on agar | Combines aspects of disk diffusion and agar dilution | Limited range of antibiotics available on strips |
| Broth Microdilution | Antibiotics are serially diluted in a liquid medium, and bacteria are inoculated | Quantitative, high-throughput | Requires specialized equipment and expertise |
| Molecular Methods | PCR-based techniques to detect resistance genes or mutations | Rapid, specific for known genetic markers | Requires knowledge of specific genetic markers |
| Whole Genome Sequencing (WGS) | Analyzes entire bacterial genome for resistance genes and mutations | Comprehensive, identifies novel resistance mechanisms | Requires advanced sequencing technology and bioinformatics expertise |
| MALDI-TOF Mass Spectrometry | Analyzes bacterial proteins to detect resistance mechanisms | Rapid, provides information on multiple antibiotics | Requires specialized equipment and expertise |
| Phenotypic Microarrays | Tests multiple antibiotic concentrations simultaneously using microplates | High-throughput, assesses response to various conditions | Requires specialized equipment and consumables |
| Immunochromatographic Assays | Utilizes antibodies to detect specific resistance proteins | Rapid, easy to use | Limited to specific resistance markers |
