GC099 Antimicrobial Resistance
Antimicrobial resistance is the ability of microorganisms to survive and proliferate despite exposure to antimicrobial agents that would normally inhibit or kill them, rendering standard treatments ineffective.
Antimicrobial Resistance
Lecture Map: The Big Idea
This lecture, delivered by Dr. PL Ho (Department of Microbiology, HKU), is the cornerstone GC session on antimicrobial resistance (AMR)—arguably the single most important cross-cutting theme in modern infectious disease medicine. Every clinical specialty encounters AMR, and the in-house summative exam consistently tests the concepts, metrics, and clinical decision-making frameworks presented here.
Why does this matter? Antibiotics are the foundation on which all of modern medicine rests—surgery, chemotherapy, transplantation, and critical care all depend on our ability to treat infections. When resistance erodes that ability, mortality skyrockets and treatment options vanish. The lecture frames AMR as a three-way interaction between the antimicrobial agent, the micro-organism, and the patient (host), and systematically addresses how resistance arises, how to measure it, what the "big three" superbugs are in Hong Kong, and how to contain the problem. [1]
1. Define multidrug-resistant organisms (MDROs) 2. List the leading multidrug-resistant bacterial pathogens 3. Select suitable numerator and denominator in construction of metrics for MDROs based on the purpose of the metrics 4. Describe the main strategies for containment of antimicrobial resistance
This lecture links directly to:
- GC 106 (Practical issues in antibiotic use) – how to actually prescribe
- GC 098 (Antibiotic prophylaxis) – preventing surgical site infections without fuelling AMR
- GC 104 (Infection outbreak / infection control) – MDRO outbreak management
- GC 105 (Medically important microbes) – which organisms matter
- GC 059 (High fever low BP / Sepsis) – empirical therapy and de-escalation
- GC 102 (Fever after chemotherapy) – neutropenic fever and standby antibiotics
- GC 210 (UTI) – ESBL E. coli and local resistance patterns
The Triangle: Patient – Micro-organism – Antimicrobial Agent
The lecture opens with the classic triangle showing host-microbe interaction (virulence vs. host defense) modified by the antimicrobial agent. [1]
Think of this triangle every time you approach an infection:
- Host factors: Is the patient immunocompromised? What is their age, comorbidity burden, and organ function?
- Microbe factors: What organisms are likely? What is their virulence? What resistance mechanisms do they carry?
- Drug factors: What is the spectrum? What is the local susceptibility? Can the drug penetrate the site of infection?
AMR disrupts this triangle by rendering the antimicrobial side ineffective, leaving the patient's own defense as the only barrier—often insufficient in the critically ill.
The lecture provides a comprehensive antibiotic classification slide that is essential context for understanding resistance. [1]
| Class | Key Agents | Spectrum Notes |
|---|---|---|
| Penicillins | Penicillin G (strep), Cloxacillin (staph), Piperacillin (Pseudomonas) | Natural penicillins = narrow; antistaphylococcal penicillins = staph; antipseudomonal = broad |
| Cephalosporins | 1GC: cephalexin; 2GC: cefuroxime (+Gram neg); 3GC: ceftriaxone (++Gram neg), ceftazidime (+non-fermenters); 4GC: cefepime (+Gram pos) | As generation ↑, Gram-negative coverage ↑ but Gram-positive coverage ↓ (except 4GC which restores some Gram pos) |
| Anti-MRSA cephalosporin | Ceftaroline | Unique: only beta-lactam active against MRSA |
| BLBLI (β-lactam / β-lactamase inhibitor) | Amox-clav*, Pip-tazo*, Ceftazidime-avibactam* | Reverse resistance mediated by some β-lactamases; * = good anti-anaerobic |
| Carbapenems* | Imipenem, Meropenem | Broadest Gram pos/neg spectrum – the "biggest guns" among beta-lactams |
| Fluoroquinolones* | Levofloxacin, Ciprofloxacin, Moxifloxacin (better Gram pos) | Broad; moxifloxacin = respiratory FQ |
| Aminoglycosides | Gentamicin, Amikacin | Synergy in combination; nephro/ototoxic |
| Oxazolidinone | Linezolid | Resistant Gram-positive organisms (MRSA, VRE) |
| Tetracyclines | Doxycycline, Minocycline | Atypicals, CA-MRSA |
| Glycylcyclines* | Tigecycline | Broad-spectrum including MDROs |
| Macrolides | Erythromycin, Clarithromycin, Azithromycin | Atypicals; high macrolide resistance locally |
| Lincosamides* | Clindamycin | Gram pos + anaerobes |
| Glycopeptides | Vancomycin, Teicoplanin | Gram pos only (MRSA drug of choice) |
| Others | Chloramphenicol, Fosfomycin, Nitrofurantoin (UTI only), Rifampicin (combination only), Metronidazole* (anaerobes) | Niche roles |
Asterisks () denote agents with good anti-anaerobic activity per the lecture slide.*
β-lactamase inhibitors: Sulbactam, Clavulanate, Tazobactam—these have no intrinsic antibacterial activity but inhibit β-lactamase enzymes, "rescuing" the partner β-lactam. [1]
Why This Table Matters for AMR
You cannot understand resistance without first understanding what each drug class covers. Every MDRO is defined by which class it has defeated. MRSA = defeated all beta-lactams; ESBL = defeated cephalosporins; CRE = defeated carbapenems. The drugs of choice for each superbug come from remaining active classes.
Types of Antibiotic Resistance
A trait of the bacterial Genus/species. ALL members in the Genus/species are resistant. [1]
Why does intrinsic resistance exist? Because the bacterium never had the target the antibiotic acts on, or has a naturally impermeable outer membrane, or has a constitutive efflux pump. It's not "acquired"—it's a fundamental feature of the organism's biology.
| Organism | Intrinsically Resistant To | Why |
|---|---|---|
| Salmonella | Cefuroxime (active in vitro but not in vivo) | In-vitro susceptibility is misleading; the drug fails at the site of intracellular infection [2] |
| Morganella morganii | Nitrofurantoin | Insufficient drug concentrations achieved |
| Providencia spp. | Nitrofurantoin | Same as above |
| Proteus mirabilis/vulgaris | Nitrofurantoin | Same |
| Listeria | 3rd generation cephalosporins | Listeria lacks PBPs targeted by 3GC; must use ampicillin |
Some intrinsic resistance can be difficult to detect and may require special laboratory techniques. [1]
Clinical implication: The lab should never report susceptibility for intrinsically resistant combinations. If such a result appears, it is edited to "resistant." This is called interpretive reading of susceptibility data. [1]
Exam Trap: Salmonella + Cefuroxime
Salmonella may appear susceptible to cefuroxime on disc testing, but cefuroxime is clinically ineffective. This is a classic example of in-vitro/in-vivo discordance. The lab should automatically edit this to resistant. Similarly, Listeria meningitis must NOT be treated with ceftriaxone—use ampicillin.
Certain antibiotic/organism combinations are prone to rapid mutational resistance developing DURING treatment. [1]
Why does this happen? When a large bacterial population is exposed to a single drug, spontaneous mutants with altered drug targets or upregulated efflux pumps have a survival advantage. The drug kills susceptible bacteria, selecting for the resistant mutant (selective pressure).
Clinical implications (directly from lecture):
1. Use alternative agents to treat infections caused by these organisms 2. Never use these agents as MONOTHERAPY for those organisms [1]
| Organism | Drug Where Mutational Resistance Readily Develops |
|---|---|
| Staphylococci | Fusidic acid – never use as monotherapy; always combine (e.g., with rifampicin) [2] |
| Pseudomonas | Single-agent fluoroquinolones, carbapenems |
| Enterobacter | 3GC (due to AmpC de-repression) |
| M. tuberculosis | Any single anti-TB drug → hence RIPE combination |
Transposable elements are "jumping genes" with an ability to change their genomic or plasmid positions. [1]
This is the most clinically threatening mechanism because:
- Plasmids can carry multiple resistance genes simultaneously → a single transfer event can make a bacterium resistant to many drug classes at once
- Integrons can capture and express multiple resistance gene cassettes
- Transposons ("jumping genes") can insert into chromosomes or plasmids, spreading resistance within and between species
- Transfer occurs via conjugation (plasmid transfer via pilus), transformation (uptake of free DNA), or transduction (bacteriophage-mediated)
This is why ESBL-producing E. coli often show co-resistance to cotrimoxazole, fluoroquinolones, and aminoglycosides—the resistance genes travel together on the same mobile genetic element. [1]
Detection of inducible clindamycin resistance in Streptococcus and Staphylococcus aureus – the D-test. [1]
The mechanism: Some staphylococci and streptococci carry the erm gene, which encodes methylation of the 23S ribosomal RNA. This methylation confers resistance to MLS_B antibiotics (Macrolides, Lincosamides, Streptogramin B). In some strains, this gene is inducible—it is only "switched on" when a macrolide (e.g., erythromycin) is present. Such strains appear erythromycin-resistant but clindamycin-susceptible on standard disc testing.
The D-test: Place an erythromycin disc adjacent to a clindamycin disc. If the clindamycin zone is flattened on the side facing erythromycin (forming a "D" shape), the erm gene is inducible → clindamycin will fail clinically → report as erythromycin-R, clindamycin-R, D-test positive. [1] [2]
If no flattening occurs → true clindamycin susceptibility → report as erythromycin-R, clindamycin-S, D-test negative. [1]
D-Test Clinical Significance
If you are considering clindamycin for a staphylococcal or streptococcal infection and the lab reports erythromycin-resistant, always check the D-test result. A positive D-test means clindamycin will be ineffective in vivo despite appearing susceptible on initial testing. This is a favourite exam question topic.
BIG GUN = Exit doors when clinicians are faced with uncertainties. [1]
The lecture makes the critical point that "big gun" is not an absolute label—it depends on:
- Whether you're treating CAI (community-acquired infection) vs HAI (hospital-acquired infection)
- The usual vs unusual pathogens for that site
- Local antimicrobial resistance patterns
- Patient risk factors (immunosuppression, prior antibiotics, prior MDROs)
No single big gun is good for all resistant infections. The choice depends on the site, the likely organisms, and historical susceptibility data. [1]
Why this matters: A junior doctor might reflexively prescribe meropenem for everything. But meropenem does NOT cover MRSA, and it does NOT cover atypical pathogens. The "educated guess" for empirical therapy requires knowledge of:
- The infection site (meningitis, pneumonia, UTI, bacteremia, etc.)
- The likely organisms for that site
- The local antibiogram
Antimicrobial coverage data must be LOCAL. [1]
High Yield Concept
"Nonsusceptible = Intermediate + Resistant." When reading an antibiogram, % susceptible is the inverse of % nonsusceptible. The clinical threshold for choosing an empirical agent is typically ≥80-90% susceptibility for that drug-organism pair in local data. [1]
30 years without a new chemical class, new mode of action, new bacterial target, and no cross-resistance. [1]
The lecture emphasizes that the antibiotic pipeline has been drying up since the 1990s. Most "new" antibiotics are modifications of existing classes—they share cross-resistance with their parent compounds. Truly novel mechanisms of action are desperately needed.
LPAD (Limited Population Antibacterial Drug) pathway: An FDA fast-track approval pathway for drugs treating serious/life-threatening infections with few treatment options. These drugs can be approved based on smaller clinical trials (but with no change in evidence standard). [1]
The 10 × 20 Initiative (IDSA, 2010): A call to develop 10 new antibiotics by 2020. [1]
Three connected antimicrobial (resistance) ecosystems – human health, animal health/agriculture, and the environment. [1]
AMR bacteria don't respect boundaries. Resistance genes flow between:
- Human medicine (hospitals, community)
- Animal husbandry (growth promoters, veterinary antibiotics)
- The environment (wastewater, soil)
This is why antibiotic stewardship must extend beyond hospitals—reducing agricultural antibiotic use is equally critical. The WHO and Hong Kong's AMR Action Plan both adopt this One Health framework. [1]
MDRO: Definition, Metrics, and Burden
"Multi" in Multidrug-Resistant Organism (MDRO) means resistance to "one" critically important class. [1]
This is a subtle but exam-critical point. "Multi" does NOT necessarily mean resistant to many individual drugs—it means resistant to one class that was the key therapeutic option, rendering many drugs in that class useless.
| MDRO | Defining Resistance | Drug of Choice for Serious Infections |
|---|---|---|
| MRSA | Total beta-lactam failure | Vancomycin (alternatives: linezolid, daptomycin; or ceftaroline—the only anti-MRSA beta-lactam) [1] [2] |
| VRE | Vancomycin failure | Linezolid, daptomycin [1] [2] |
| ESBL-producing Enterobacteriaceae | Cephalosporin failure | Carbapenem [1] [2] |
| CRE | Total beta-lactam failure (including carbapenems) | "Bad bugs, no drugs" – limited options (colistin, ceftazidime-avibactam, etc.) [1] |
| CRAB | Carbapenem-resistant A. baumannii | Similar dire situation |
Exam Favourite: MDRO Definition
"Multi" refers to resistance to one CRITICALLY IMPORTANT antimicrobial class, not necessarily multiple individual drugs. However, ESBL organisms OFTEN have co-resistance to cotrimoxazole, fluoroquinolones, and aminoglycosides due to plasmid-borne resistance genes travelling together. [1]
Metrics for MDROs
Metrics for superbugs must be interpreted with care. [1]
The lecture devotes considerable time to this because it is both a learning objective and an e-learning question topic.
| Metric | Purpose | Example |
|---|---|---|
| Hospital antibiogram | Monitoring susceptibility patterns | % susceptible of E. coli to ciprofloxacin [3] |
| Point prevalence rate, admission prevalence rate | Estimating exposure burden | How many patients carry MRSA on admission? |
| Incidence of hospital-onset bacteremia | Estimating infection burden | New MRSA bacteremia per 1,000 patient-days |
From the E-learning set: Hospital antibiogram serves the purpose of monitoring susceptibility patterns (not estimating exposure burden or infection burden). [3]
Antibiogram % can be calculated in many different ways, and the calculation method confounds the result. [1]
The 2-patient, 7-isolate example from the lecture is a classic teaching case:
- Patient 1 has multiple cultures (some susceptible, some resistant)
- Patient 2 has one culture (susceptible)
| Calculation Method | Result |
|---|---|
| All isolates included | 3/7 = 43% susceptible |
| First isolate per patient | 2/2 = 100% susceptible |
| First isolate per episode | 2/3 = 67% susceptible |
Why does this matter?
- Using all isolates over-represents patients with multiple positive cultures (often the sickest, most resistant cases) → biases downward
- Using first isolate per patient may miss resistance that develops during treatment
- The CLSI/SHEA recommendation is typically first isolate per patient per species per analysis period for antibiogram construction
In general, antibiogram % should NOT be calculated if the sample size (n) is < 30. [1]
The lecture shows the example: with n = 10, the 95% CI for 50% susceptibility is 21–78% (essentially useless), whereas with n = 300, the 95% CI narrows to 71–78% (clinically meaningful). [1]
Percentage can be misleading. [1]
The lecture provides the HKW cluster 2009 data as illustration:
| Organism | % Resistant (misleading) | Absolute Number of Patients | Bacteremia Patients |
|---|---|---|---|
| MRSA | 46% | 600 | 71 (1-2/week) |
| ESBL-EC | 27% | 744 | 142 (2-3/week) |
| CRAB | 43% | 106 | 10 (1/month) |
| VRE | < 0.1% | rare | 1 patient |
The insight: CRAB has a high resistance %, but the absolute number of patients affected is much smaller than ESBL. For a clinician, knowing that you'll see an ESBL bacteremia 2-3 times per week is more actionable than knowing "27% resistance." [1]
Incidence rates (per 1,000 admissions, per bed-day, per 100,000 population) are useful for epidemiologists but difficult for clinicians to interpret intuitively. Numbers are easy to understand. [1]
"The Big Three" MDROs in Hong Kong
The lecture identifies ESBL-producing Enterobacteriaceae, MRSA, and Carbapenem-resistant Acinetobacter (CRA/CRAB) as the three most important superbugs in the Hong Kong Hospital Authority system. [1]
1 superbug every 17 minutes. 1 blood infection every 3 hours. [1]
| 2013 (patients) | 2016 (patients) | Change | 2016 Bacteremia | |
|---|---|---|---|---|
| ESBL | 13,280 | 14,213 | +7% | 1,677 |
| MRSA | 12,462 | 13,001 | +4% | 816 |
| CRA | 2,684 | 3,191 | +19% | 84 |
| Total | 28,426 | 30,405 | +7% | 2,577 |
A group of enzymes produced by some Enterobacteriaceae: E. coli, Klebsiella pneumoniae, Proteus, Enterobacter, Shigella, Salmonella. [1]
What do ESBLs do? They hydrolyze the β-lactam ring of cephalosporins (including 3GC like ceftriaxone) and monobactams (aztreonam), rendering them ineffective. ESBLs are typically plasmid-mediated (TEM, SHV, CTX-M types), meaning they spread horizontally.
Why is ESBL a problem beyond cephalosporin resistance?
ESBL organisms are often multidrug-resistant – co-resistant to cotrimoxazole (Septrin), fluoroquinolones (levofloxacin, ciprofloxacin), and aminoglycosides (gentamicin). [1]
This is because the same plasmid carrying the ESBL gene often carries resistance genes for other drug classes.
Drug of choice for serious ESBL infections: CARBAPENEM (meropenem, imipenem). [1] [2]
Epidemiology:
Half of the in-patient ESBL burden occurs in aged ≥75 years. There has been a 3× increase in ESBL-EC from 2003 to 2016 in the HKW cluster, with seasonal surges. [1]
MRSA acquires the SCCmec element carrying the mecA gene, which encodes PBP2a (an altered penicillin-binding protein with low affinity for beta-lactams). This makes MRSA resistant to ALL beta-lactam antibiotics, with the exception of anti-MRSA beta-lactams such as ceftaroline. [1]
Key distinction: Methicillin itself is no longer clinically used. It was a laboratory agent for detection. The name persists historically. [1]
MSSA = virulent. MRSA = virulent AND resistant. [1]
Annual MRSA burden in HA (estimation):
200 deaths → 600 bacteremia → 3,000+ infections → 10,000+ colonizations [1]
This pyramid illustrates the iceberg effect: for every MRSA death, there are many more infections and even more colonized patients who serve as reservoirs for transmission.
Drug of choice for serious MRSA infections: Vancomycin (alternatives: linezolid, daptomycin). [1] [2]
Community-Associated MRSA (CA-MRSA)
CA-MRSA affects persons WITHOUT traditional healthcare risk factors. They carry unique genotypic features (e.g., PVL-positive) and remain susceptible (>95%) to cotrimoxazole and minocycline. [1]
7-fold increase in CA-MRSA in Hong Kong was documented, with a decline during COVID-19. [1]
CA-MRSA typically causes skin and soft tissue infections (boils, abscesses, necrotizing fasciitis) and can be devastating in young, otherwise healthy individuals. The lecture illustrates this with Case 2 (F/35y with a boil progressing despite oral cloxacillin). [1]
Less virulent; infections are usually hospital-acquired. Causes a wide range of infections, especially pneumonia. Intrinsic resistance to a wide range of drugs – very limited treatment options. [1]
Key features:
- Acquired resistance occurs readily DURING treatment (a consequence of the organism's ability to rapidly incorporate resistance genes)
- Carbapenem WAS the treatment of choice before widespread resistance emerged
- Some isolates are resistant to ALL available antibiotics including colistin [1]
Surveillance definitions:
- CRAB = Carbapenem-resistant A. baumannii
- MRAB = Multidrug-resistant A. baumannii (definition not standardized)
MDR/XDR/PDR classification (proposed but not standardized):
| Category | Definition |
|---|---|
| MDR | Nonsusceptible to ≥1 agent in ≥3 antimicrobial classes |
| XDR | Nonsusceptible to ≥1 agent in all but ≤2 classes |
| PDR | Nonsusceptible to ALL agents listed |
"Bad bugs, no drugs" – CRE and CRAB represent infections for which no reliable treatment may exist. [1]
Impact on Patient Care
Traditional approach: TAT 4 days (2 days culture → 1-2 days identification → 1-2 days susceptibility). MALDI-TOF can reduce identification to 30 minutes. Even so, among 232 patients who died, test results were reported AFTER death in 35%. [1]
This statistic is sobering. It means that for many patients, the definitive answer about which antibiotic to use arrives too late. This is precisely why empirical antibiotic therapy—educated guessing based on the site of infection, likely organisms, and local resistance data—is essential.
The Escalation vs. De-escalation Strategy
The lecture presents a fundamental antibiotic algorithm for inpatient infections, dividing patients into low-risk and high-risk categories based on age, underlying disease, infection source, severity (e.g., APACHE score), prior antibiotic history, and recent culture results. [1]
This category includes the great majority of infections. Start narrow; escalate if not responding. [1]
Reasons for escalation strategy:
- Excessive antibiotic therapy is harmful – antibiotic budget implications, but more importantly:
- Ecological consequence – broad-spectrum antibiotics disrupt the hospital flora, increasing:
- Yeast infections (Candida)
- Clostridioides difficile infection
- Selection of more problematic superbugs (S. maltophilia, KPC-producing organisms, CRAB)
- Minimize the "vicious cycle" of broad antibiotics → more resistance → broader antibiotics
- Accepted trade-off: some patients will receive discordant (initially inadequate) treatment [1]
Start broad (cover the worst-case scenario); narrow once culture results return. [1]
Why de-escalation for high-risk infections?
- Meta-analysis evidence (Schwaber MJ et al., JAC 2007): ESBL bacteremia has significantly higher mortality than non-ESBL bacteremia. Inadequate initial therapy is a major contributor to this excess mortality. [1]
- Each hour of delay in antimicrobial administration in severe sepsis is associated with an average decrease in survival of ~8%. [1]
This is why the Surviving Sepsis Campaign recommends antibiotics within 1 hour of recognition of sepsis/septic shock.
The Coroner's Case – Neutropenic Fever
The lecture presents a real Hong Kong Coroner's case where a patient with post-chemotherapy neutropenic fever died because of delayed antibiotic administration. The Coroner recommended:
- Standby Emergency Antibiotic Kits (e.g., Augmentin, Tazocin, Meropenem, Ceftriaxone, Cefotaxime) stocked in designated locations
- Door-to-antibiotic time within ONE HOUR for neutropenic fever
- Chemotherapy Alert Cards for patients receiving chemotherapy
- Blood culture BEFORE starting antibiotics [1]
This case illustrates why the de-escalation strategy exists for high-risk infections—delay kills.
Illustrating Clinical Cases from the Lecture
F/20y university student with progressive community-acquired pneumonia despite antibiotics. [1]
- Failed to respond to initial therapy (likely macrolide)
- Given doxycycline → rapid resolution
- NPA PCR positive for M. pneumoniae; serology: < 10 (D5) → 1280 (D21) (diagnostic 4-fold rise)
- Macrolide resistance marker found: A2063G mutation [1]
40% of M. pneumoniae in Hong Kong are macrolide-resistant. [1]
The problem of limited treatment options:
| Antibiotic Class | Agents | Children | Pregnancy |
|---|---|---|---|
| Macrolides | Azithromycin, Clarithromycin | Suitable | Suitable (FDA class B) |
| Tetracyclines | Doxycycline | Not suitable (< 8y) | Not suitable (FDA class D) |
| Fluoroquinolones | Ciprofloxacin, Levofloxacin | Not suitable (< 18y) | Not suitable (FDA class C) |
Options for macrolide-resistant M. pneumoniae infections are limited – this illustrates "the era of untreatable infections." [1]
In a child < 8 years or a pregnant woman with macrolide-resistant M. pneumoniae, there is effectively NO safe antibiotic option. This is a powerful illustration of why AMR matters beyond the hospital setting.
The lecture also presents a severe case (pleural empyema requiring decortication) where M. pneumoniae was resistant to macrolides, augmentin, cefepime, and even levofloxacin, only resolving with doxycycline + surgical drainage. [1]
F/35y with a boil for 2 days, progressing despite oral cloxacillin for 5 days. [1]
- Cloxacillin targets MSSA → ineffective against MRSA
- CA-MRSA should be suspected when SSTI fails standard antistaphylococcal therapy
- CA-MRSA remains susceptible to cotrimoxazole and minocycline (>95%) [1]
- The lecture shows dramatic photos of necrotizing CA-MRSA infection requiring urgent drainage
2-year-old with fever, cough, and SOB, given azithromycin (Zithromax) by GP for 3 days without improvement. [1]
- HKW cluster data shows high rates of erythromycin resistance in S. pneumoniae (used as an indicator for macrolide resistance to clarithromycin and azithromycin) [1]
- This illustrates why macrolides are not first-line monotherapy for serious pneumococcal infections—beta-lactams (amoxicillin, ceftriaxone) are preferred
The lecture aligns with the Hong Kong AMR Action Plan, which follows the WHO framework: [1]
- Surveillance – Tracking resistance trends, local antibiograms
- Infection prevention and control – Hand hygiene, contact precautions, isolation of MDRO patients
- Antimicrobial stewardship – Appropriate prescribing, de-escalation, formulary restrictions, senior endorsement for "big guns"
- Education and public awareness – Including the One Health approach
- Research and development – New antibiotics, diagnostics, vaccines
From the lecture's framing:
- Prescribe narrow-spectrum when possible (escalation strategy for low-risk)
- Use combination therapy when mutational resistance is likely (e.g., fusidic acid + rifampicin for MSSA)
- Never use monotherapy for organisms where resistance readily develops
- Local data drives decisions – antibiograms must be institution-specific
- Rapid diagnostics (MALDI-TOF, PCR) can shorten time to appropriate therapy
| Related Lecture | Key AMR Connection |
|---|---|
| GC 106 (Practical Abx use) | Stewardship principles, IMPACT guidelines, local resistance data for choosing empirical therapy |
| GC 098 (Abx prophylaxis) | Prophylaxis reduces SSI but inappropriate use drives AMR; must be narrow, timely, and brief |
| GC 104 (Infection control) | MDRO outbreak management: contact precautions, cohorting, active surveillance cultures |
| GC 059 (Sepsis) | Hour-1 bundle: broad-spectrum Abx, cultures, lactate, fluids; de-escalate once culture results available |
| GC 102 (Neutropenic fever) | Standby Abx kits, door-to-antibiotic < 1 hour, anti-pseudomonal beta-lactam empirically |
| GC 210 (UTI) | ESBL-EC prevalence in UTI; nitrofurantoin for uncomplicated cystitis (spares carbapenems); avoid empirical FQ due to high local resistance [4] |
| GC 105 (Medically important microbes) | Organism-specific susceptibility patterns |
Past-Paper and Slide-Derived Question Styles
-
MCQ: "Which of the following describes the purpose of a hospital antibiogram?"
- Answer: Monitoring susceptibility patterns [3]
-
SAQ: "Define MDRO and give 3 examples with their defining resistance and drug of choice."
- MRSA: total beta-lactam failure → vancomycin
- VRE: vancomycin failure → linezolid/daptomycin
- ESBL: cephalosporin failure → carbapenem [1]
-
MCQ: "A patient with Listeria meningitis is treated with ceftriaxone. What type of resistance is this?"
- Answer: Intrinsic (natural) resistance – Listeria is inherently resistant to 3GC
-
SAQ: "Explain the D-test. When is it clinically important?"
- Detects inducible clindamycin resistance mediated by erm gene in staphylococci/streptococci
- Positive D-test: erythromycin-R, clindamycin-R (report as resistant despite apparent susceptibility)
- Clinically important when considering clindamycin therapy for an erythromycin-resistant isolate [1]
-
Minicase: "A 25-year-old previously healthy woman presents with community-acquired pneumonia. She was given azithromycin by GP but worsened. NPA PCR positive for M. pneumoniae. What is the likely cause of treatment failure? How would you manage?"
- Macrolide-resistant M. pneumoniae (A2063G mutation); switch to doxycycline [1]
-
SAQ: "A hospital antibiogram reports 50% susceptibility of E. coli to amoxicillin-clavulanate, based on 10 isolates. Comment on the reliability."
- Sample size < 30 → unreliable; 95% CI is very wide (21-78%); should not be used for clinical decision-making [1]
-
MCQ: "In septic shock, each hour of delay in antibiotic administration is associated with what decrease in survival?"
- ~8% decrease per hour [1]
-
SAQ: "List the main strategies for containment of AMR."
- Surveillance, infection prevention & control, antimicrobial stewardship, education/awareness, R&D [1]
-
MCQ: "Which antibiotic/organism combination has a high risk of mutational resistance developing during monotherapy?"
-
SAQ: "Explain the difference between escalation and de-escalation antibiotic strategies. When is each appropriate?"
- Escalation: low-risk infections; start narrow, broaden if needed; avoids ecological damage
- De-escalation: high-risk infections (bacteremia, sepsis); start broad, narrow once cultures return; minimizes mortality from inadequate initial therapy [1]
High Yield Summary
Antimicrobial resistance is defined by failure of critically important drug CLASSES, not individual drugs. The "big three" MDROs in Hong Kong are ESBL (→ treat with carbapenem), MRSA (→ treat with vancomycin), and CRAB (→ limited options). Resistance is intrinsic, mutational, acquired via horizontal gene transfer, or inducible (D-test for clindamycin). Antibiogram data must be local, use first isolate per patient, and require n ≥ 30. For low-risk infections, use escalation strategy (narrow → broad); for high-risk infections (sepsis, bacteremia, neutropenic fever), use de-escalation (broad → narrow) with antibiotics within 1 hour. Each hour of delay in sepsis reduces survival by ~8%. The One Health approach recognizes AMR as a human-animal-environment problem. There has been no truly new antibiotic class for 30 years—preservation of existing drugs through stewardship is critical.
Active Recall - Lecture Notes
GC098 Antibiotic Prophylaxis : 20260116
Antibiotic prophylaxis is the preventive administration of antibiotics before, during, or shortly after a procedure or exposure to reduce the risk of infection in susceptible individuals.
GC100 Defense Against Microbes
The host defense against microbes encompasses the integrated system of innate and adaptive immune mechanisms—including physical barriers, phagocytes, complement, and lymphocyte-mediated responses—that collectively recognize, contain, and eliminate pathogenic microorganisms.