Unit 10: Antibiotic sensitivity of microorganism - Subjective Questions

Antibiotic susceptibility testing (AST) is a laboratory procedure used to determine whether a bacterial isolate is susceptible, intermediate, or resistant to specific antimicrobial agents.

Objectives:

• To select the most effective antibiotic for treatment.
• To detect antimicrobial resistance in a microorganism.
• To determine the susceptibility pattern or antibiogram of an isolate.
• To support surveillance of resistance trends.
• To guide hospital antibiotic policies.

Clinical importance:

• Promotes targeted antimicrobial therapy.
• Reduces treatment failure and complications.
• Prevents unnecessary use of broad-spectrum antibiotics.
• Helps control the emergence and spread of resistant organisms.

The Kirby-Bauer disc diffusion method is based on the diffusion of an antibiotic from an impregnated paper disc into an agar medium inoculated with the test microorganism.

• A standardized bacterial suspension is spread uniformly over Mueller-Hinton agar to produce a lawn culture.
• Antibiotic discs are placed on the agar surface.
• Each antibiotic diffuses radially through the medium and forms a concentration gradient.
• If the organism is inhibited by an antibiotic, a clear zone of inhibition develops around the disc.
• The diameter of the zone is measured in millimetres.
• The result is compared with current standard breakpoint tables, such as those issued by CLSI or EUCAST.
• The isolate is then categorized as susceptible, intermediate or susceptible with increased exposure, or resistant, depending on the interpretive system used.

A larger zone generally suggests greater susceptibility, but zone sizes must always be interpreted using standardized breakpoints rather than by visual comparison alone.

Procedure:

1. Prepare the inoculum: Select well-isolated colonies from a fresh pure culture and suspend them in sterile saline or broth.
2. Standardize the suspension: Adjust turbidity to a 0.5 McFarland standard.
3. Inoculate the plate: Dip a sterile swab into the suspension, remove excess liquid, and streak the entire Mueller-Hinton agar surface in multiple directions to obtain a uniform lawn.
4. Dry the surface: Allow the inoculated plate to stand briefly with the lid closed.
5. Apply antibiotic discs: Place appropriate discs on the agar using sterile forceps or a disc dispenser. Press each disc gently to ensure complete contact.
6. Incubate: Invert the plate and incubate under the conditions recommended for the organism and standard being followed, commonly around for approximately 16–18 hours for rapidly growing non-fastidious bacteria.
7. Measure zones: Measure the diameter of each complete zone of inhibition in millimetres.
8. Interpret: Compare measurements with current CLSI or EUCAST breakpoint tables.
9. Report: Record each agent using the appropriate category and include any relevant resistance alerts.

Quality-control strains should be tested at recommended intervals to verify that the medium, discs, incubation conditions, and technique are satisfactory.

The 0.5 McFarland standard is a turbidity reference used to standardize the density of a bacterial suspension before susceptibility testing. For many bacterial species, it approximates an inoculum of about colony-forming units per millilitre, although the exact viable count varies among organisms.

Role in AST:

• Ensures that approximately the same quantity of bacteria is used in each test.
• Improves reproducibility between laboratories.
• Prevents falsely small zones caused by an excessively heavy inoculum.
• Prevents falsely large zones caused by an excessively light inoculum.

The suspension may be adjusted visually against the standard under proper lighting or measured using a turbidity instrument. It should be used promptly because bacterial numbers can change during prolonged standing.

Mueller-Hinton agar is the standard medium commonly used for disc diffusion testing of rapidly growing, non-fastidious bacteria.

Desirable properties:

• It supports satisfactory growth of many clinically important bacteria.
• Its loose agar structure permits reproducible diffusion of antibiotics.
• It contains relatively low concentrations of substances that antagonize sulfonamides, trimethoprim, and tetracyclines.
• Its composition can be standardized between batches.
• Its calcium and magnesium concentrations can be controlled because these ions influence the activity of certain antibiotics.
• It produces clear, measurable zones of inhibition.
• It can be supplemented when standardized testing of certain fastidious organisms is required.

The medium must have the recommended depth, pH, composition, and surface moisture. Deviations may produce inaccurate zone diameters.

Minimum inhibitory concentration (MIC): The lowest concentration of an antimicrobial agent that prevents visible growth of a microorganism after standardized incubation.

Minimum bactericidal concentration (MBC): The lowest antimicrobial concentration that kills a defined proportion of the original bacterial population, commonly at least under standardized test conditions.

Determination of MIC:

• Prepare serial dilutions of the antibiotic in broth or agar.
• Inoculate each concentration with a standardized quantity of the organism.
• Include growth and sterility controls.
• Incubate under recommended conditions.
• Identify the lowest concentration showing no visible growth.

Determination of MBC:

• Take samples from clear tubes or wells at and above the MIC.
• Subculture them onto antibiotic-free agar.
• Incubate and examine for surviving colonies.
• The lowest concentration producing the defined bactericidal endpoint is recorded as the MBC.

MIC guides susceptibility interpretation and dosing, whereas MBC may be useful in selected serious infections where bacterial killing is especially important.

Conclusion: Disc diffusion is convenient for routine testing, whereas broth dilution is preferred when a precise MIC is required. Both methods must follow standardized procedures and interpretive breakpoints.

Broth dilution determines the MIC by exposing a standardized bacterial inoculum to a series of antibiotic concentrations.

Broth macrodilution:

• Antibiotic dilutions are prepared in test tubes.
• A standardized inoculum is added to each tube.
• The tubes are incubated and inspected for turbidity.
• The lowest concentration without visible growth is the MIC.

Broth microdilution:

• The same principle is applied in microtitre plate wells.
• Small volumes permit many organisms and antibiotics to be tested efficiently.
• Results may be read visually or by an automated instrument.

Essential controls:

• A growth-control well containing organism but no antibiotic.
• A sterility-control well containing uninoculated medium.
• Recommended quality-control organisms.

Accurate antibiotic preparation, inoculum density, medium composition, incubation, and endpoint reading are essential for reliable MIC results.

The gradient diffusion method uses a plastic strip containing a continuous gradient of an antimicrobial agent and a printed concentration scale.

Principle:

• The drug diffuses from the strip into inoculated agar.
• An elliptical zone of inhibition forms.
• The MIC is read where the edge of the ellipse intersects the concentration scale on the strip.

Procedure:

1. Prepare a standardized bacterial suspension.
2. Inoculate the recommended agar to produce a uniform lawn.
3. Apply the gradient strip firmly to the agar surface.
4. Incubate under standardized conditions.
5. Read the MIC at the point of intersection between the inhibition ellipse and the strip scale.
6. Interpret the MIC using current breakpoint tables.

Advantages:

• Produces a numerical MIC.
• Is easier to perform than manual dilution methods.
• Is useful when testing individual isolates against selected agents.
• Can help assess organisms or antibiotics for which a precise MIC is clinically useful.

Its disadvantages include higher cost per test and possible difficulty in reading irregular or indistinct endpoints.

Measured zone diameters or MIC values must be compared with current interpretive breakpoints issued by a recognized standard-setting body, such as CLSI or EUCAST.

Common categories:

• Susceptible: A high likelihood of therapeutic success when the agent is used with the recommended regimen.
• Intermediate: A category used in some systems to indicate uncertain response, possible efficacy at sites where the drug concentrates, or the need for increased exposure.
• Susceptible, increased exposure: In EUCAST terminology, success is likely when drug exposure is increased through dosing adjustment or concentration at the infection site.
• Resistant: A high likelihood of therapeutic failure even when increased exposure is used.

Reporting principles:

• Report the organism, tested drugs, MIC or zone values when appropriate, and interpretive categories.
• Use breakpoints appropriate to the organism, drug, method, and infection context.
• Apply expert rules for intrinsic resistance and important resistance mechanisms.
• Avoid reporting agents that are clinically inappropriate for the infection site.
• Promptly communicate critical resistance findings.

Breakpoints are updated periodically, so outdated tables should not be used.

The zone diameter can be influenced by several technical and biological factors:

• Inoculum density: Heavy inocula produce smaller zones; light inocula produce larger zones.
• Agar depth: Excessively thick agar reduces zone size, whereas thin agar may enlarge it.
• Medium pH: Abnormal pH can alter antibiotic activity.
• Medium composition: Cation concentration, thymidine content, and supplements may affect results.
• Disc potency: Expired, damaged, or improperly stored discs may give reduced zones.
• Timing of disc application: Delays after inoculation can affect bacterial growth and diffusion.
• Disc spacing: Closely placed discs can produce overlapping zones.
• Incubation temperature and duration: Incorrect conditions alter growth and antibiotic activity.
• Atmosphere: Carbon dioxide or anaerobic incubation can alter medium pH and drug action.
• Organism characteristics: Growth rate, resistance mechanisms, and colony variants influence endpoints.
• Surface moisture: Excess moisture may cause spreading and indistinct zones.

Strict standardization is therefore essential for accurate interpretation.

Bacteria may resist antibiotics through intrinsic properties or acquired genetic changes.

Major mechanisms include:

• Enzymatic inactivation: Enzymes such as beta-lactamases destroy or modify antibiotics.
• Alteration of the target: Changes in penicillin-binding proteins, ribosomes, DNA gyrase, or other targets reduce drug binding.
• Reduced permeability: Altered or lost porins restrict antibiotic entry, especially in Gram-negative bacteria.
• Active efflux: Efflux pumps remove antibiotics from the cell.
• Metabolic bypass: Alternative biochemical pathways avoid the inhibited step.
• Target protection or replacement: New proteins protect the original target or provide a resistant substitute.
• Biofilm formation: Biofilms limit drug penetration and contain slowly growing, tolerant cells.

Resistance genes can spread through mutation, conjugation, transformation, or transduction. Mobile genetic elements such as plasmids, transposons, and integrons contribute greatly to multidrug resistance.

Intrinsic resistance is a natural and predictable property of all or nearly all members of a bacterial species.

• It results from inherent structural or functional characteristics.
• It is not dependent on newly acquired resistance genes.
• Example: Mycoplasma species are intrinsically resistant to beta-lactam antibiotics because they lack a cell wall.
• Example: many Gram-negative bacteria are intrinsically resistant to vancomycin because the drug cannot effectively cross the outer membrane.

Acquired resistance develops in a previously susceptible bacterial strain.

• It may arise through mutation or acquisition of resistance genes.
• Resistance genes may be transferred by plasmids, transposons, or bacteriophages.
• Example: acquisition of the mecA or mecC gene in Staphylococcus aureus produces methicillin resistance.
• Example: acquisition of extended-spectrum beta-lactamase genes can confer resistance to many extended-spectrum cephalosporins.

Recognizing intrinsic resistance prevents misleading reports, while detecting acquired resistance is important for therapy and infection control.

Quality control (QC) confirms that the complete testing system is functioning within established limits.

QC monitors:

• Accuracy of inoculum preparation.
• Performance and depth of the culture medium.
• Potency and storage of antibiotic discs or dilution panels.
• Incubation temperature, duration, and atmosphere.
• Measuring and endpoint-reading technique.
• Performance of equipment and reagents.

Common reference strains include:

• Escherichia coli ATCC 25922 for many Gram-negative tests.
• Staphylococcus aureus ATCC 25923 for disc diffusion.
• Staphylococcus aureus ATCC 29213 for MIC testing.
• Pseudomonas aeruginosa ATCC 27853.
• Enterococcus faecalis ATCC 29212.

Measured zones or MICs must fall within the published QC ranges. Out-of-range results require investigation, and patient results should not be released until significant problems are resolved.

Measurement:

• Examine the plate after the recommended incubation period.
• Measure the diameter of the complete inhibition zone in millimetres.
• Include the diameter of the antibiotic disc in the measurement.
• Measure through the back of the plate using reflected light when the standard method recommends it.
• Use a ruler, callipers, or an automated zone reader.
• Measure from one edge of complete inhibition to the opposite edge through the centre of the disc.
• Follow organism- and drug-specific rules for colonies within zones, swarming, or faint growth.

Common errors:

• Measuring the radius instead of the diameter.
• Measuring from the edge of the disc rather than across the full zone.
• Rounding values excessively.
• Ignoring resistant colonies within a clear zone.
• Reading fuzzy edges inconsistently.
• Interpreting swarming as ordinary growth without following special guidance.
• Using reflected or transmitted light incorrectly.
• Comparing zones with outdated breakpoint tables.

Unusual colonies within a zone may require purity checks, repeat testing, or identification of a resistant subpopulation.

Bactericidal antibiotics kill susceptible bacteria under defined laboratory conditions. Bacteriostatic antibiotics mainly inhibit bacterial multiplication, allowing host defences to clear the infection.

General differences:

• Bactericidal activity is evaluated by the reduction in viable bacterial count and may be assessed using the MBC.
• Bacteriostatic activity is demonstrated by inhibition of visible growth, commonly represented by the MIC.
• Cell-wall-active agents are often bactericidal, while many protein-synthesis inhibitors are often bacteriostatic; however, exceptions occur.

Why the distinction is not absolute:

• Activity depends on the organism, drug concentration, growth phase, and test conditions.
• The same drug may be bactericidal against one species but bacteriostatic against another.
• Laboratory activity does not always predict clinical outcome.
• Host immunity, drug penetration, infection site, dosage, and pharmacokinetics also determine therapeutic success.

A bactericidal agent may be preferred in selected serious infections, but antibiotic choice should primarily be guided by evidence, susceptibility, infection site, and patient factors.

Multidrug-resistant organisms (MDROs) are microorganisms that show non-susceptibility to multiple antimicrobial agents or antimicrobial categories. Precise definitions vary by organism and surveillance standard.

An antibiogram is a summarized report of the antimicrobial susceptibility patterns of bacterial isolates collected in a healthcare facility or defined population during a specified period.

Uses of an antibiogram:

• Guides empirical antibiotic selection before individual AST results become available.
• Shows local resistance rates and trends over time.
• Helps identify increases in multidrug resistance.
• Supports antimicrobial stewardship policies.
• Assists in preparing hospital formularies and treatment guidelines.
• Provides evidence for infection-prevention interventions.
• Helps compare resistance patterns among wards or patient populations when sufficient data are available.

An antibiogram does not replace patient-specific culture and susceptibility testing. It should be prepared using standardized inclusion, analysis, and reporting criteria.

Extended-spectrum beta-lactamases (ESBLs) are enzymes that hydrolyse many extended-spectrum penicillins and cephalosporins and are commonly inhibited in vitro by beta-lactamase inhibitors such as clavulanate.

Screening:

• Reduced susceptibility to selected indicator cephalosporins may suggest ESBL production.
• Screening agents and zone or MIC criteria must follow the current laboratory standard.

Phenotypic confirmation:

• Test a cephalosporin alone and the same cephalosporin combined with clavulanate.
• A significant increase in activity in the presence of clavulanate indicates synergy and supports ESBL production.
• A combination-disc or gradient-strip format may be used, depending on validated procedures.

Importance:

• ESBL-producing isolates may cause treatment failure with affected beta-lactam agents.
• Detection supports suitable therapy, surveillance, and infection-control measures.

Current reporting rules differ among standards and may rely primarily on clinical breakpoints rather than routine confirmatory testing. Laboratories must therefore follow their current CLSI, EUCAST, or national guidance.

AST measures antimicrobial activity under controlled laboratory conditions, but clinical success depends on additional factors.

Limitations include:

• The drug may not reach an effective concentration at the infection site.
• Absorption, distribution, metabolism, and excretion vary among patients.
• Renal or hepatic impairment may alter drug exposure.
• Biofilms, abscesses, necrotic tissue, and foreign bodies may reduce effectiveness.
• Host immunity strongly influences the outcome.
• Mixed infections may not be represented by testing one isolate.
• Resistance may emerge during therapy.
• Inoculum effects can produce different activity in infections with large bacterial burdens.
• Standard laboratory media do not fully reproduce conditions within the human body.
• Breakpoints are organism-, drug-, dosing-, and infection-dependent.
• Technical errors or use of inappropriate methods may produce misleading results.

Therefore, AST results must be interpreted together with the patient's condition, infection site, dosage regimen, microbiological findings, and clinical guidelines.

Biosafety and good laboratory practices include:

• Treat all clinical isolates as potentially pathogenic.
• Wear a laboratory coat, gloves, and appropriate eye or face protection when splash risk exists.
• Perform procedures in accordance with the risk assessment and assigned biosafety level.
• Use a biological safety cabinet for procedures likely to generate infectious aerosols.
• Avoid mouth pipetting and minimize splashing or aerosol production.
• Disinfect work surfaces before and after testing.
• Sterilize or discard contaminated cultures, swabs, plates, and disposables in approved biohazard containers.
• Wash hands after removing gloves and before leaving the laboratory.
• Label cultures and antibiotic plates clearly to prevent sample mix-ups.
• Verify culture purity and maintain proper documentation.
• Store antibiotic discs as recommended, protected from moisture and temperature fluctuations.
• Decontaminate spills promptly using the approved spill procedure.
• Report exposure incidents immediately and follow institutional protocols.

These practices protect staff, preserve specimen integrity, and improve the reliability of susceptibility results.