Aminoglycoside resistance US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Aminoglycoside resistance. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Aminoglycoside resistance US Medical PG Question 1: A 37-year-old woman with a history of anorectal abscesses complains of pain in the perianal region. Physical examination reveals mild swelling, tenderness, and erythema of the perianal skin. She is prescribed oral ampicillin and asked to return for follow-up. Two days later, the patient presents with a high-grade fever, syncope, and increased swelling. Which of the following would be the most common mechanism of resistance leading to the failure of antibiotic therapy in this patient?
- A. Intrinsic absence of a target site for the drug
- B. Use of an altered metabolic pathway
- C. Production of beta-lactamase enzyme (Correct Answer)
- D. Altered structural target for the drug
- E. Drug efflux pump
Aminoglycoside resistance Explanation: ***Production of beta-lactamase enzyme***
- The patient's symptoms of a rapidly worsening infection despite ampicillin treatment suggest the presence of a **beta-lactamase producing organism**. Ampicillin is a **beta-lactam antibiotic** that is inactivated by these enzymes.
- Anorectal abscesses and rapidly progressing soft tissue infections are often caused by **polymicrobial flora**, including staphylococci and enterococci, many of which can produce **beta-lactamase**.
*Intrinsic absence of a target site for the drug*
- While some bacteria inherently lack the target site for certain drugs (e.g., mycoplasma lacking a cell wall, thus being resistant to beta-lactams), this is less likely to be the **most common mechanism of acquired resistance** leading to treatment failure in a typical perianal infection.
- The rapid progression and failed initial treatment point towards an **acquired mechanism of resistance** rather than an intrinsic one.
*Use of an altered metabolic pathway*
- This mechanism, such as altered **folate synthesis pathways** in resistance to trimethoprim-sulfamethoxazole, is less common as the primary mechanism for ampicillin resistance.
- Ampicillin's mechanism of action primarily targets the **bacterial cell wall**, not a metabolic pathway in the same way.
*Altered structural target for the drug*
- This involves modifications to the **penicillin-binding proteins (PBPs)**, which are the targets of beta-lactam antibiotics like ampicillin. While a valid mechanism (e.g., in MRSA), the **production of beta-lactamase** is generally a more widespread and common cause of ampicillin failure, especially in infections involving mixed flora from the perianal region.
- Given the abrupt failure of ampicillin, **beta-lactamase inactivation** is a more immediate and common cause than a rapid mutational change in PBPs.
*Drug efflux pump*
- **Efflux pumps** actively remove antibiotics from the bacterial cell, contributing to resistance against various drug classes.
- While efflux pumps can play a role, the dominant mechanism for resistance to **ampicillin** in many common perianal pathogens is the **enzymatic degradation by beta-lactamases**.
Aminoglycoside resistance US Medical PG Question 2: An 18-year old college freshman presents to his university clinic because he has not been feeling well for the past two weeks. He has had a persistent headache, occasional cough, and chills without rigors. The patient’s vital signs are normal and physical exam is unremarkable. His radiograph shows patchy interstitial lung infiltrates and he is diagnosed with atypical pneumonia. The patient is prescribed azithromycin and takes his medication as instructed. Despite adherence to his drug regimen, he returns to the clinic one week later because his symptoms have not improved. The organism responsible for this infection is likely resistant to azithromycin through which mechanism?
- A. Mutation in topoisomerase II
- B. Methylation of ribosomal binding site
- C. Presence of a beta-lactamase
- D. Decreased binding to RNA polymerase
- E. Insertion of drug efflux pumps (Correct Answer)
Aminoglycoside resistance Explanation: ***Insertion of drug efflux pumps***
- **Azithromycin** is a macrolide antibiotic that inhibits bacterial protein synthesis by binding to the **50S ribosomal subunit**.
- In **Mycoplasma pneumoniae** (the most common cause of atypical pneumonia in young adults), the **most common** mechanism of macrolide resistance is through **efflux pumps**, particularly the **mef genes**.
- These efflux pumps actively transport macrolides out of the bacterial cell, reducing intracellular drug concentration and conferring resistance.
- This mechanism is responsible for the majority of macrolide-resistant *M. pneumoniae* isolates worldwide.
*Methylation of ribosomal binding site*
- **Methylation** of the ribosomal binding site (specifically the **23S rRNA** via erm genes) does prevent azithromycin from binding effectively.
- While this is a valid macrolide resistance mechanism seen in organisms like *Streptococcus pneumoniae* and *Streptococcus pyogenes*, it is **less common** in *Mycoplasma pneumoniae*.
- Efflux pumps (mef) are the predominant mechanism in *M. pneumoniae* resistant strains.
*Mutation in topoisomerase II*
- **Topoisomerase II** (DNA gyrase) is the target of **fluoroquinolone antibiotics**, not macrolides.
- Mutations in this enzyme lead to resistance against fluoroquinolones, such as **ciprofloxacin**.
*Presence of a beta-lactamase*
- **Beta-lactamase enzymes** inactivate **beta-lactam antibiotics** (e.g., penicillin, cephalosporins) by hydrolyzing their beta-lactam ring.
- Additionally, *Mycoplasma pneumoniae* **lacks a cell wall**, making it inherently resistant to all beta-lactam antibiotics regardless of beta-lactamase production.
*Decreased binding to RNA polymerase*
- **RNA polymerase** is the target for antibiotics like **rifampin**, which inhibits bacterial transcription.
- Decreased binding to RNA polymerase would lead to rifampin resistance, not azithromycin resistance.
Aminoglycoside resistance US Medical PG Question 3: A 64-year-old female with type 2 diabetes mellitus comes to the physician because of a 1-week history of painful red swelling on her left thigh. Examination shows a 3- x 4-cm, tender, fluctuant mass. Incision and drainage of the abscess are performed. Culture of the abscess fluid grows gram-positive, coagulase-positive cocci that are resistant to oxacillin. Which of the following best describes the mechanism of resistance of the causal organism to oxacillin?
- A. Degradation of the antibiotic
- B. Decreased uptake of the antibiotic
- C. Decreased activation of the antibiotic
- D. Altered target of the antibiotic (Correct Answer)
- E. Acetylation of the antibiotic
Aminoglycoside resistance Explanation: ***Altered target of the antibiotic***
- The organism described (gram-positive, coagulase-positive cocci, oxacillin-resistant) is **methicillin-resistant *Staphylococcus aureus* (MRSA)**.
- MRSA achieves oxacillin (and other beta-lactam) resistance by acquiring the ***mecA* gene**, which encodes for a **modified penicillin-binding protein (PBP2a)** with reduced affinity for beta-lactam antibiotics.
*Degradation of the antibiotic*
- This mechanism, primarily through the production of **beta-lactamase enzymes**, can degrade beta-lactam antibiotics.
- While *Staphylococcus aureus* can produce beta-lactamases, oxacillin (a **penicillinase-resistant penicillin**) is specifically engineered to be stable against these enzymes.
*Decreased uptake of the antibiotic*
- Reduced permeability of the bacterial cell wall can lead to decreased uptake, a mechanism more commonly associated with **gram-negative bacteria** due to their outer membrane.
- This is not the primary mechanism of resistance for MRSA to oxacillin.
*Decreased activation of the antibiotic*
- Some antibiotics are prodrugs that require activation by bacterial enzymes, and resistance can arise from mutations affecting this activation.
- Oxacillin is active in its administered form and does not require bacterial activation.
*Acetylation of the antibiotic*
- **Enzymatic modification**, such as acetylation, adenylylation, or phosphorylation, is a common mechanism of resistance, particularly against **aminoglycoside antibiotics**.
- This specific mechanism is not responsible for oxacillin resistance in MRSA.
Aminoglycoside resistance US Medical PG Question 4: A scientist is studying the mechanisms by which bacteria become resistant to antibiotics. She begins by obtaining a culture of vancomycin-resistant Enterococcus faecalis and conducts replicate plating experiments. In these experiments, colonies are inoculated onto a membrane and smeared on 2 separate plates, 1 containing vancomycin and the other with no antibiotics. She finds that all of the bacterial colonies are vancomycin resistant because they grow on both plates. She then maintains the bacteria in liquid culture without vancomycin while she performs her other studies. Fifteen generations of bacteria later, she conducts replicate plating experiments again and finds that 20% of the colonies are now sensitive to vancomycin. Which of the following mechanisms is the most likely explanation for why these colonies have become vancomycin sensitive?
- A. Point mutation
- B. Gain of function mutation
- C. Viral infection
- D. Plasmid loss (Correct Answer)
- E. Loss of function mutation
Aminoglycoside resistance Explanation: ***Plasmid loss***
- The initial **vancomycin resistance** in *Enterococcus faecalis* is often mediated by genes located on **plasmids**, which are extrachromosomal DNA.
- In the absence of selective pressure (vancomycin), bacteria that lose the plasmid (and thus the resistance genes) have a **growth advantage** over those that retain the energetically costly plasmid, leading to an increase in sensitive colonies over generations.
*Point mutation*
- A **point mutation** typically involves a change in a single nucleotide and could lead to loss of resistance if it occurred in a gene conferring resistance.
- However, since there was no selective pressure for loss of resistance, it is less likely that 20% of the population would acquire such a specific point mutation to revert resistance.
*Gain of function mutation*
- A **gain of function mutation** would imply that the bacteria acquired a *new* advantageous trait, not the *loss* of resistance.
- This type of mutation would not explain why some colonies became sensitive to vancomycin after the drug was removed.
*Viral infection*
- **Viral infection** (bacteriophages) can transfer genes through transduction or cause bacterial lysis, but it's not the primary mechanism for a widespread reversion of resistance in the absence of antibiotic pressure.
- It would not explain the observed increase in vancomycin-sensitive colonies due to evolutionary pressure.
*Loss of function mutation*
- While a **loss of function mutation** in a gene conferring resistance could lead to sensitivity, it's generally less likely to explain a 20% shift without selective pressure than **plasmid loss**.
- Plasmids are often unstable and are easily lost in the absence of selection, whereas a specific gene mutation causing loss of function would need to arise and become prevalent in the population.
Aminoglycoside resistance US Medical PG Question 5: A researcher is studying a new antituberculosis drug. In the laboratory, the drug has been shown to be effective against mycobacteria located within phagolysosomes of macrophages, but it is also significantly less effective against extracellular tuberculoid bacteria. The characteristics of this drug are most similar to which of the following agents?
- A. Isoniazid
- B. Pyrazinamide (Correct Answer)
- C. Ethambutol
- D. Streptomycin
- E. Rifampin
Aminoglycoside resistance Explanation: ***Pyrazinamide***
- Pyrazinamide is unique among antituberculosis drugs for its efficacy in the **acidic environment of phagolysosomes**, where dormant mycobacteria reside.
- It is **less effective against actively replicating extracellular bacteria** at neutral pH, aligning with the drug's described characteristics.
*Isoniazid*
- Isoniazid is primarily effective against **rapidly dividing, extracellular *M. tuberculosis*** by inhibiting mycolic acid synthesis.
- While it can penetrate macrophages, its activity is not specifically enhanced or limited by the acidic phagolysosomal environment as described.
*Ethambutol*
- Ethambutol primarily inhibits **arabinogalactan synthesis**, affecting the cell wall of growing mycobacteria, both intracellular and extracellular.
- Its efficacy is not selectively focused on the acidic intracellular environment.
*Streptomycin*
- Streptomycin is an **aminoglycoside antibiotic** that inhibits protein synthesis and is active against extracellular mycobacteria.
- It has limited penetration into cells and is not particularly effective against intracellular organisms, nor is its activity pH-dependent.
*Rifampin*
- Rifampin is highly effective against both **extracellular and intracellular mycobacteria** by inhibiting DNA-dependent RNA polymerase.
- It exhibits strong sterilizing activity across various environments, which contradicts the described drug's selective efficacy.
Aminoglycoside resistance US Medical PG Question 6: A 61-year-old woman who recently emigrated from India comes to the physician because of a 2-month history of fever, fatigue, night sweats, and a productive cough. She has had a 5-kg (11-lb) weight loss during this period. She has a history of type 2 diabetes mellitus and poorly controlled asthma. She has had multiple asthma exacerbations in the past year that were treated with glucocorticoids. An x-ray of the chest shows a cavitary lesion of the posterior apical segment of the left upper lobe with consolidation of the surrounding parenchyma. The pathogen identified on sputum culture is found to be resistant to multiple drugs, including streptomycin. Which of the following mechanisms is most likely involved in bacterial resistance to this drug?
- A. Alteration in the sequence of gyrA genes
- B. Upregulation of arabinosyl transferase production
- C. Upregulation of mycolic acid synthesis
- D. Alteration in 30S ribosomal subunit (Correct Answer)
- E. Inhibition of bacterial synthesis of RNA
Aminoglycoside resistance Explanation: ***Alteration in 30S ribosomal subunit***
- Streptomycin is an **aminoglycoside antibiotic** that acts by binding to the **16S rRNA of the 30S ribosomal subunit**, which interferes with bacterial protein synthesis.
- **Resistance to streptomycin** most commonly arises from mutations in the genes encoding ribosomal proteins (e.g., *rpsL*) or the 16S rRNA that alter the drug's binding site on the 30S ribosomal subunit, preventing its inhibitory effect.
*Alteration in the sequence of gyrA genes*
- Mutations in the *gyrA* gene typically confer resistance to **fluoroquinolone antibiotics**, such as ciprofloxacin and levofloxacin.
- Fluoroquinolones target **DNA gyrase (topoisomerase II)**, which is encoded by *gyrA*, not the ribosomes.
*Upregulation of arabinosyl transferase production*
- **Arabinogalactan**, a major component of the mycobacterial cell wall, is synthesized by **arabinosyl transferases** (e.g., EmbB).
- Resistance to **ethambutol** is often associated with mutations or upregulation of these enzymes, leading to increased synthesis of the arabinogalactan layer.
*Upregulation of mycolic acid synthesis*
- **Mycolic acid** is a crucial component of the mycobacterial cell wall, and its synthesis is inhibited by drugs like **isoniazid**.
- Upregulation of mycolic acid synthesis or mutations in genes related to its production (e.g., *kasA*) can lead to **isoniazid resistance**, but not directly to streptomycin resistance.
*Inhibition of bacterial synthesis of RNA*
- **Rifampin** is an antibiotic that inhibits bacterial RNA synthesis by binding to the **DNA-dependent RNA polymerase**.
- While resistance to rifampin often involves mutations in the *rpoB* gene, this mechanism is specific to rifampin and not streptomycin.
Aminoglycoside resistance US Medical PG Question 7: A 62-year-old woman with type 2 diabetes mellitus is brought to the emergency department by her husband because of fever, chills, and purulent drainage from a foot ulcer for 2 days. Her hemoglobin A1c was 15.4% 16 weeks ago. Physical examination shows a 2-cm ulcer on the plantar surface of the left foot with foul-smelling, purulent drainage and surrounding erythema. Culture of the abscess fluid grows several bacteria species, including gram-negative, anaerobic, non-spore-forming bacilli that are resistant to bile and aminoglycoside antibiotics. Which of the following is the most likely source of this genus of bacteria?
- A. Stomach
- B. Oropharynx
- C. Vagina
- D. Colon (Correct Answer)
- E. Skin
Aminoglycoside resistance Explanation: ***Colon***
- The description of the bacteria—**gram-negative, anaerobic, non-spore-forming bacilli** that are **resistant to bile** and **aminoglycoside antibiotics**—is highly characteristic of the genus *Bacteroides*, especially *Bacteroides fragilis*.
- *Bacteroides fragilis* is a prominent component of the normal **colonic microflora** and is frequently implicated in infections originating from breaches in the gastrointestinal tract, such as a diabetic foot ulcer with a mixed infection.
*Stomach*
- The stomach's highly acidic environment generally limits significant bacterial colonization, and it is not a primary source of mixed anaerobic infections as described.
- While *Helicobacter pylori* can colonize the stomach, it does not fit the described microbiological characteristics.
*Oropharynx*
- The oropharynx contains a diverse microbiota, including anaerobes like **Peptostreptococcus** and **Fusobacterium**, but it is not the typical source for *Bacteroides fragilis* or the specific resistance profile mentioned.
- Oropharyngeal anaerobes are more commonly associated with head and neck infections, aspiration pneumonia, or dental abscesses.
*Vagina*
- The vaginal flora includes various anaerobes such as **Gardnerella vaginalis** and some *Bacteroides* species, but it is not the most common or primary source of widespread mixed anaerobic infections matching this description.
- Infections originating from the vagina would typically be linked to pelvic or genitourinary conditions.
*Skin*
- The skin surface predominantly harbors **aerobic** and **facultative anaerobic bacteria** like **Staphylococcus** and **Streptococcus** species.
- While skin breaches can lead to infections, the described **anaerobic, gram-negative, bile-resistant** profile points away from the typical skin flora as the primary source for the specific bacterial characteristics given.
Aminoglycoside resistance US Medical PG Question 8: A 29-year-old pregnant woman with no prior antibiotic exposure presents with gonorrhea. Culture of Neisseria gonorrhoeae shows resistance to penicillin, tetracycline, and fluoroquinolones. Genetic testing reveals she has a strain with chromosomal mutations in penA (mosaic allele), mtrR promoter, and gyrA. She reports her partner recently returned from Southeast Asia. Apply epidemiologic and resistance mechanism knowledge to determine the most appropriate management and public health action.
- A. Treat with ceftriaxone alone and report to local health department
- B. Treat with dual therapy (ceftriaxone plus azithromycin) and initiate partner notification with travel history documentation (Correct Answer)
- C. Perform cephalosporin susceptibility testing before treatment initiation
- D. Treat with azithromycin monotherapy due to pregnancy
- E. Initiate spectinomycin therapy and routine partner notification only
Aminoglycoside resistance Explanation: ***Treat with dual therapy (ceftriaxone plus azithromycin) and initiate partner notification with travel history documentation***
- The presence of the **mosaic penA allele** and **mtrR promoter mutations** signifies significant resistance potential; **dual therapy** with ceftriaxone and azithromycin remains critical for ensuring cure and slowing further resistance in highly resistant strains.
- Given the partner's travel to **Southeast Asia**, a region known for emerging **extensively drug-resistant (XDR)** gonorrhea, documenting travel and notification is vital for public health **surveillance**.
*Treat with ceftriaxone alone and report to local health department*
- While ceftriaxone is the backbone of treatment, using **monotherapy** for a strain already exhibiting multiple chromosomal resistance mutations (mosaic penA) increases the risk of selecting for **cephalosporin resistance**.
- This approach is less robust than dual therapy in the context of suspected **imported resistant strains** from high-risk geographic regions.
*Perform cephalosporin susceptibility testing before treatment initiation*
- Delaying treatment for **Neisseria gonorrhoeae** waiting for susceptibility results is inappropriate as it allows for ongoing **transmission** and potential progression to **pelvic inflammatory disease**.
- Clinical guidelines recommend **empiric treatment** based on local prevalence and travel history while simultaneously performing surveillance cultures.
*Treat with azithromycin monotherapy due to pregnancy*
- **Azithromycin monotherapy** is contraindicated for gonorrhea treatment because it has a low barrier to resistance and would fail to cover chromosomal mutations affecting **efflux pumps (mtrR)**.
- While both drugs are safe in **pregnancy**, azithromycin must be used in **combination** with ceftriaxone to prevent treatment failure.
*Initiate spectinomycin therapy and routine partner notification only*
- **Spectinomycin** is a second-line agent that is currently not readily available in the United States and has poor efficacy for **pharyngeal infections**.
- Focusing only on routine notification ignores the critical **epidemiologic significance** of the Southeast Asian travel history associated with highly resistant strains.
Aminoglycoside resistance US Medical PG Question 9: A 67-year-old woman with persistent Enterococcus faecium bacteremia despite appropriate vancomycin therapy undergoes repeat culture. The isolate now shows vancomycin MIC of 128 μg/mL (previously 2 μg/mL). PCR testing reveals the presence of vanA gene cluster. Hospital epidemiology traces potential sources. What is the most likely mechanism by which this organism acquired high-level vancomycin resistance?
- A. Spontaneous chromosomal mutation during therapy
- B. Transposon-mediated transfer from vancomycin-resistant enterococci (Correct Answer)
- C. Transformation with DNA from lysed resistant bacteria
- D. Increased vancomycin efflux pump expression
- E. Alteration in cell wall synthesis without genetic acquisition
Aminoglycoside resistance Explanation: ***Transposon-mediated transfer from vancomycin-resistant enterococci***
- High-level vancomycin resistance in Enterococcus is primarily mediated by the **vanA gene cluster**, which is carried on the **Tn1546 transposon** and spread via **conjugation**.
- This mechanism involves the replacement of the terminal **D-Ala-D-Ala** of peptidoglycan precursors with **D-Ala-D-Lac**, resulting in a 1000-fold decrease in vancomycin binding affinity.
*Spontaneous chromosomal mutation during therapy*
- While mutations can cause resistance to some antibiotics (like rifampin), **high-level vancomycin resistance** in enterococci is not caused by single-point mutations.
- A sudden jump in MIC from 2 to 128 μg/mL is characteristic of **horizontal gene transfer** rather than the gradual accumulation of chromosomal mutations.
*Transformation with DNA from lysed resistant bacteria*
- **Transformation** (uptake of naked DNA) is less common in enterococci compared to **conjugation** for the transfer of large, complex gene clusters like **vanA**.
- The epidemiological tracing implied in the scenario is classic for the spread of **plasmids** and **transposons** between colonized patients in a hospital setting.
*Increased vancomycin efflux pump expression*
- **Efflux pumps** are significant for resistance against drugs like tetracyclines or fluoroquinolones, but they are not the mechanism for **vancomycin resistance**.
- Vancomycin is a large **glycopeptide molecule**; resistance is achieved through **structural modification** of its target (cell wall precursors) rather than active expulsion.
*Alteration in cell wall synthesis without genetic acquisition*
- Vancomycin-intermediate S. aureus (**VISA**) involves cell wall thickening without new gene acquisition, but this results in a **smaller, gradual MIC increase**.
- The detection of the **vanA gene** by PCR confirms that the resistance is due to **acquired genetic material** rather than a purely metabolic or adaptive physiological change.
Aminoglycoside resistance US Medical PG Question 10: A 31-year-old man with acute myeloid leukemia develops neutropenic fever during chemotherapy. Blood cultures grow Pseudomonas aeruginosa resistant to all tested antibiotics including polymyxins (colistin). Genetic analysis shows mutations in pmrA and pmrB genes, as well as arnB gene. What mechanism best explains this organism's resistance to polymyxins, and what therapeutic implication does this have?
- A. Efflux pump overexpression; combination therapy with efflux inhibitors
- B. Lipopolysaccharide modification reducing drug binding; limited treatment options remain (Correct Answer)
- C. Porin loss preventing drug entry; carbapenem combination therapy indicated
- D. Target site mutation; higher doses of polymyxins can overcome resistance
- E. Enzymatic drug inactivation; polymyxin derivatives remain effective
Aminoglycoside resistance Explanation: ***Lipopolysaccharide modification reducing drug binding; limited treatment options remain***
- Mutations in **pmrAB** and **arnB** cause the addition of **L-ara4N** (aminoarabinose) to **Lipid A**, which reduces the negative charge of the lipopolysaccharide (LPS) and prevents the positively charged **polymyxins** from binding.
- This mechanism results in **high-level resistance** to last-resort agents like **colistin**, leaving patients with extremely limited options such as **cefiderocol** or experimental combination therapies.
*Efflux pump overexpression; combination therapy with efflux inhibitors*
- While **MexAB-OprM** efflux pumps contribute to resistance against **fluoroquinolones** and **beta-lactams**, they are not the primary mechanism for **polymyxin** resistance.
- **Efflux inhibitors** are currently not a standard of care for treating systemic infections caused by pan-drug resistant **Pseudomonas aeruginosa**.
*Porin loss preventing drug entry; carbapenem combination therapy indicated*
- **OprD** porin loss is a specific mechanism for **carbapenem** resistance in Pseudomonas, but it does not physically block the large **polymyxin** molecules which target the cell surface.
- Using **carbapenems** is ineffective if the isolate is already identified as resistant to all tested antibiotics including **carbapenems**.
*Target site mutation; higher doses of polymyxins can overcome resistance*
- Unlike some antibiotics where higher doses overcome low-level resistance, structural modification of the **LPS** target creates a complete lack of affinity that **increased dosing** cannot bypass.
- Higher doses of **polymyxins** are severely limited by significant **nephrotoxicity** and **neurotoxicity** risks.
*Enzymatic drug inactivation; polymyxin derivatives remain effective*
- Resistance to **polymyxins** via enzymatic destruction is not the recognized mechanism; structural modification of the **cell membrane** is the primary driver.
- There are currently no clinically available **polymyxin derivatives** that can overcome resistance mediated by the **arn** operon and **lipid A** modification.
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