Antibiotic Sensitivity of Local Environmental Isolates by the Kirby-Bauer Method: A QC-Framed Study

What this study does. Antimicrobial resistance (AMR) is usually discussed with hospital data, but resistance determinants circulate just as freely in soil, pond water and market surfaces — anywhere bacteria meet people, animals and antibiotic residues. This project asks a question an undergraduate can actually answer with a lab: do bacteria isolated from ordinary local environments, and which antibiotic classes still control them?

Twelve bacterial isolates — four from garden soil, four from pond/lake water and four from market-table swabs — are purified on nutrient agar, characterised to genus level by colony morphology, Gram reaction and spot biochemical tests, and then subjected to the Kirby-Bauer disc-diffusion method on Mueller-Hinton agar exactly as standardised by Bauer, Kirby, Sherris and Turck (1966) and interpreted with CLSI M100 zone-diameter practice. Six antibiotic discs spanning the WHO AWaRe categories (ampicillin, cotrimoxazole, tetracycline, chloramphenicol, gentamicin, ciprofloxacin) are tested per isolate, in duplicate plates.

The methodological honesty is the point. Every batch carries an ATCC 25922 or ATCC 25923 control plate, and the batch is only read if the control zones fall inside the CLSI acceptance range; agar depth, inoculum turbidity (0.5 McFarland) and disc storage are logged for every plate. Results are analysed as mean ± SD zone diameters, interpreted where CLSI tables cover the organism, and summarised per isolate through the multiple antibiotic resistance (MAR) index — a single number that flags isolates with many resistance traits. No clinical samples, no patient data: a safe, defensible resistance snapshot of the student's own surroundings.

Antimicrobial resistance is reported from hospitals, but the same selection pressure — antibiotic residues and resistant strains — is present in soil, water and market surfaces around any college, and nobody has measured it locally. At the student level, the Kirby-Bauer method is taught as a single demonstration: inoculum is eyeballed, no control strain is run, and zone sizes are read as 'sensitive' or 'resistant' with no reference to the standard's organism-antibiotic tables, producing numbers that cannot be compared across batches or defended in a viva.

There is a need for a small, methodologically complete study that (a) isolates and characterises bacteria from ordinary local environments under proper biosafety, (b) performs disc-diffusion testing with the inoculum control, medium discipline and ATCC control-strain acceptance checks the method actually requires, and (c) converts zone diameters into interpretable, comparable results — per-antibiotic statistics, CLSI-based interpretation where valid, and a MAR index per isolate — so that the undergraduate can say something defensible about resistance patterns in their own surroundings rather than merely demonstrating a technique.

Abstract

This single-member laboratory study profiles antibiotic sensitivity in twelve environmental bacterial isolates from three source types (soil, pond water, market-table swabs) in the vicinity of the college. Isolates are purified, characterised to genus level by phenotype, and tested by the Kirby-Bauer disc-diffusion method on Mueller-Hinton agar against six antibiotics from different classes and AWaRe categories. A quality-control frame — ATCC reference-strain plates with CLSI acceptance limits per batch, controlled inoculum turbidity, fixed agar depth, disc-potency logging and duplicate plates — makes the zone diameters reportable rather than anecdotal. Results are summarised as per-antibiotic zone statistics, interpreted against CLSI tables where the organism is covered, and condensed into a per-isolate MAR index, which is compared across source types.

Introduction

Antibiotics are flushed, excreted and discarded into drains that end in soil and surface water, so environmental bacteria live under a low, permanent antibiotic pressure. India's own surveillance (NCDC's NARS-Net reports) shows high resistance in clinical isolates, making the environmental side a natural undergraduate question. Disc diffusion on a standardised medium is sixty years old, cheap, and taught in every microbiology lab.

Existing knowledge and the gap

College practicals demonstrate the Kirby-Bauer method once on one organism and stop. What is usually missing is everything that makes an antibiogram publishable: a controlled inoculum, medium-depth discipline, disc-potency records, control-strain acceptance limits per batch, duplicate plates, and an explicit statement of which results may be interpreted against CLSI tables and which may only be reported as raw zones. Student reports also rarely convert individual zones into a summary statistic. The gap this project fills is a complete, QC-framed antibiogram set with the MAR index, comparing three environmental source types under one protocol.

Proposed study

1. Select the three sampling sites, obtain permissions, and collect samples in sterile containers with a chain-of-custody register.
2. Enrich and streak on nutrient agar; pick twelve well-separated, morphologically distinct isolates; purify and maintain on slants.
3. Characterise each isolate: colony morphology, Gram stain, catalase, oxidase, motility — genus-level identification only, honestly labelled as such.
4. Standardise the AST setup: Mueller-Hinton agar poured to 4 mm, verified pH, 0.5 McFarland inoculum, six discs per plate with written positions.
5. Run AST in duplicate per isolate, with an ATCC control plate per batch; measure zones with calipers after 16–18 h at 35 ± 2 °C.
6. Analyse: zone mean ± SD per antibiotic, CLSI interpretation where applicable, MAR index per isolate, source-type comparison, AWaRe-category summary.

Feasibility

• Technical: media, discs, control strains and incubation are standard B.Sc microbiology lab stock; the whole protocol fits 8 h/week.
• Economic: discs and media for 26 plates plus controls cost roughly ₹800–1,200; the rest is lab glassware.
• Operational: the binding constraints are incubator access and the 16–18 h reading window, so plating days are fixed in advance with the lab in-charge. All cultures are autoclaved before disposal. The main risks — weak control zones and confluent lawns — are what the QC frame catches.

In scope

• Twelve isolates from three environmental source types near the college, one sampling round with re-collection allowed for failed samples.
• Phenotypic characterisation to genus level only (no molecular identification).
• Disc-diffusion AST against six antibiotics, duplicate plates, ATCC control per batch, zone diameters in mm.
• Descriptive statistics, CLSI interpretation where covered, MAR index per isolate, source-type comparison.

Out of scope

• MIC determination by dilution methods and MIC-zone correlation — a dilution series is a separate project.
• Molecular detection of resistance genes (PCR) — beyond the college lab.
• Clinical specimens of any kind, and any claim about patient treatment.
• Anaerobes, fastidious organisms requiring supplemented media, and fungi.
• Time-series monitoring of the sites (seasonality noted as a limitation).

Research design: an experimental laboratory study with a fixed, standardised protocol (Kirby-Bauer as per the 1966 single-disc method and CLSI M100 practice), a per-batch QC gate, and pre-planned descriptive analysis. Individual study, 12 weeks at 8 h/week.

Phase 1 — Sampling and isolation (Weeks 1–3). Three sites are chosen: a garden soil plot, a pond/lake edge, and a market vegetable/meat table (surface swab with stall-holder permission). Samples are collected in sterile containers, coded S-, P-, M-01…, logged with date/time/conditions, and processed the same day: enrichment in nutrient broth, streaking on nutrient agar, and repeated re-streaking until pure. Twelve morphologically distinct isolates (four per source) are selected and maintained on agar slants.

Phase 2 — Characterisation (Week 4). Colony morphology (size, edge, elevation, pigment), Gram stain with controls, catalase (3% H₂O₂), oxidase (spot test), and motility by hanging drop. Results are recorded as a characterisation matrix; identification stops at genus level (e.g., 'Gram-negative rod, oxidase-positive' rather than a species claim), because further identification needs a panel the college lab does not run.

Phase 3 — AST setup and running (Weeks 5–9). Mueller-Hinton agar is prepared per manufacturer instructions, pH verified 7.2–7.4, poured to a measured 4 mm depth in 90 mm plates. Inoculum: colonies from a fresh culture suspended to 0.5 McFarland (verified by a meter or by comparison with prepared standards), swabbed in three directions on dry agar. Six discs (ampicillin 10 µg, cotrimoxazole, tetracycline 30 µg, chloramphenicol 30 µg, gentamicin 10 µg, ciprofloxacin 5 µg) placed with sterile forceps at written grid positions within 15 minutes of inoculation. Incubation 16–18 h at 35 ± 2 °C. Duplicate plate per isolate; one ATCC control plate per batch; zone diameters read with vernier calipers against a dark background and recorded raw, including any failed/contaminated plate.

Quality control: control-strain zones must fall inside the CLSI acceptance range for that batch or the whole batch is repeated after tracing the cause (agar depth, disc storage, inoculum); disc vials logged for storage at 2–8 °C with desiccant; each batch's control values tabulated.

Analysis plan (Weeks 10–11): zone mean ± SD per antibiotic across isolates; per-isolate CLSI category (S/I/R) only where M100 covers the organism group, raw zones otherwise; MAR index = number of antibiotics resisted / number tested; source-type comparison of MAR indices and zone distributions (box plots); AWaRe-category summary. Two plates re-measured by a second reader to gauge reading error.

Timeline: W1–3 sampling/isolation; W4 characterisation; W5 method shakedown on lab strains; W6–9 AST batches with controls; W10 re-runs; W10–11 analysis; W12 report, slides, viva.

The study is a sample-to-verdict pipeline in which every zone diameter is traceable to a batch whose control plate passed acceptance. The workflow below is the report's methods spine.

flowchart TD
  A["Sample 3 sites: soil, pond water, market swab"] --> B["Enrich and streak; pick 12 distinct isolates"]
  B --> C["Purify on slants; characterise: Gram, catalase, oxidase, motility"]
  C --> D["Standardise inoculum to 0.5 McFarland"]
  D --> E["Seed Mueller-Hinton plates, 4 mm depth; apply 6 discs per plate"]
  E --> F{"QC gate: ATCC control zones within CLSI range?"}
  F -->|Pass| G["Read zones after 16-18 h at 35 deg C; duplicate plates"]
  F -->|Fail| H["Trace cause: agar depth, discs, inoculum; repeat batch"]
  G --> I["Zone statistics; CLSI interpretation where covered"]
  I --> J["MAR index per isolate"]
  J --> K["Source-type comparison and AWaRe summary"]
  K --> L["Report, resistance-pattern atlas, viva"]

Data organisation

One row per isolate-antibiotic replicate: isolate code, source type, characterisation results, batch number, plate conditions (agar lot, pH check, McFarland verification), disc code and potency, zone diameter in mm, and the same batch's control-zone values. Every computed number regenerates from this table; the MAR index and category assignments are scripted once in Python/Excel, and two hand-calculated isolates are shown in the appendix.

Core calculations (abridged)

MAR index (per isolate) = number of antibiotics with no inhibition zone
                          (or CLSI 'R' where covered) / number of antibiotics tested
Batch acceptance        = all control-zone values inside the CLSI published
                          acceptance range for ATCC 25922 / 25923

Results tables to fill (no numbers are pre-filled)

• Species-level identification through a partner college's biochemical panel (IMViC, sugar utilisation) or a 16S rDNA sequence at a service lab.
• MIC confirmation by broth micro-dilution for the most resistant isolates, linking zones to concentrations.
• Seasonal rounds to test whether monsoon runoff changes pond and soil resistance patterns.
• Gene-level look at a partner lab: PCR screen for a common resistance marker to connect phenotype with mechanism.
• Antibiogram wall poster for the department corridor — the atlas doubles as an AMR-awareness exhibit for Science Day.
• A repeatable site antibiogram booklet handed to the panchayat or market committee with plain-language findings.