Surface Sterilisation and Contamination in Hibiscus Nodal Explants: A Tissue-Culture Audit

What this study does. Every plant-tissue-culture programme in India — from college teaching labs to commercial banana and orchid units — loses a painful share of its cultures to contamination, and the losses trace back to one step: surface disinfestation of the explant. Too mild, and bacteria and fungal spores survive; too harsh, and the explant dies of chemical burn (excessive browning) before it can sprout. This project turns that trade-off into data with a controlled contamination-rate audit on Hibiscus rosa-sinensis, a hardy garden shrub whose nodal explants micropropagate reliably on Murashige–Skoog (MS) medium.

A two-member team sets up four sterilisation protocols — ethanol pre-treatment followed by either 2 % sodium hypochlorite (10 or 15 minutes) or 0.1 % mercuric chloride (3 or 5 minutes), all with a Tween-20 wetting pre-wash — and runs 36 nodal explants per protocol (three replicate batches of twelve), giving 144 cultures in total on MS basal medium with 3 % sucrose. Cultures are observed weekly for four weeks and every culture is classified as clean, contaminated (bacterial or fungal, by visual diagnosis), or dead from chemical injury.

The analysis is deliberately statistical, not anecdotal: a 2×4 contingency table of clean-versus-contaminated by protocol is tested with scipy.stats.chi2_contingency (Fisher exact where cell counts are small), contamination onset is mapped by week, and browning rates are compared so that a protocol is judged on establishment (clean and alive), not merely cleanliness. The result is a protocol recommendation the department's teaching lab can adopt next semester.

Contamination is the single largest cause of culture loss in introductory plant-tissue-culture teaching labs, and it discourages students before they ever reach the interesting parts of micropropagation. Lab protocols in circulation specify disinfestants by habit — 'use mercuric chloride, it works' — without an honest comparison of effectiveness against chemical injury, without replication, and without any statistical basis for the claim.

There is also a safety dimension: mercuric chloride is acutely toxic and environmentally persistent, so if a chlorine-based protocol achieves statistically comparable establishment, the department has a reason to drop the mercury. Conversely, if chlorine fails in our water and climate conditions, the lab needs documented evidence, not folklore, to justify continuing its use.

This project therefore runs a replicated, randomised, four-protocol contamination audit on Hibiscus nodal explants, records contamination by type and onset week, separates chemical injury from contamination, and tests the protocol differences with chi-square analysis, so that the final recommendation for the teaching lab is evidence-based and auditable.

Abstract

This project quantifies how surface-sterilisation chemistry and exposure time affect contamination and establishment of Hibiscus rosa-sinensis nodal explants in vitro. Four protocols (70 % ethanol dip + Tween-20 wash, then 2 % NaOCl 10 or 15 min, or 0.1 % HgCl₂ 3 or 5 min) were applied to 36 explants each — 144 cultures on MS basal medium — observed weekly for four weeks and classified as clean, contaminated (bacterial/fungal) or chemically injured. Chi-square tests of independence compare protocols on contamination incidence, and a combined establishment metric (clean and viable at week 4) identifies the best protocol.

Introduction

Micropropagation begins with an explant carrying bacteria and fungal spores on its surface and, sometimes, endogenous bacteria within. The disinfestant must kill the first without killing the second — the explant's own living cells. Sodium hypochlorite releases active chlorine and is inexpensive; mercuric chloride is dramatically effective against spore-borne contamination but is a cumulative poison, and over-exposure produces the characteristic glassy, browned, dying explant. Indian college labs use both, usually with doses inherited from a senior student's notebook, not data.

Existing practice and the gap

Standard texts prescribe medium composition and general disinfestation logic but leave dose-and-time to be optimised per species, season and source plant. What student reports almost never contain is replication and a test of significance: 'protocol A gave less contamination' is stated from a handful of cultures. The gap this project fills: a replicated design with 36 explants per protocol, onset recording that separates fast bacterial from slow fungal outbreaks, an explicit chemical-injury category so a 'clean but dead' outcome is not misread as success, and formal chi-square analysis.

Proposed study

1. Source uniform Hibiscus shoots from one hedge (single mother plant block), excise nodal segments (1–1.5 cm, one node each) under wet conditions.
2. Prepare MS basal medium with 3 % sucrose and 0.7 % agar (pH 5.7), autoclaved into culture tubes; prepare disinfestant solutions fresh.
3. Run the four protocols in three replicate batches of 12, randomising protocol order per session to cancel day and batch effects.
4. Incubate at 25 ± 2 °C, 16-hour photoperiod; score weekly for four weeks for contamination (type, onset week), chemical injury and survival.
5. Analysis: chi-square test of clean-versus-contaminated × protocol (Fisher exact where counts are small), onset distribution, browning comparison, and an establishment table.
6. Recommend a protocol for the teaching lab and document the residual risk.

Feasibility

• Technical: the lab needs a laminar-air-flow cabinet, an autoclave, a lit growth rack and routine reagents — all standard in B.Sc biotech teaching labs; explant material comes from any campus hedge with permission.
• Economic: medium components and reagents for 144 tubes cost roughly ₹1,200–1,800.
• Operational: 144 tubes occupy four rack shelves and weekly scoring takes 45–60 minutes; media prep and inoculation sit in weeks 2–4, observation in 5–8, analysis and writing in 9–12. A laminar-flow failure is the main risk, absorbed by running protocol replicates across separate sessions.

In scope

• One species (Hibiscus rosa-sinensis), one explant type (nodal segment), one medium (MS basal + 3 % sucrose), four sterilisation protocols, 36 explants per protocol in three replicate batches.
• Outcomes: contamination (type and onset week), chemical injury, survival and sprout emergence over four weeks.
• Chi-square/Fisher analysis of protocol–outcome association and an establishment comparison.

Out of scope

• Identifying the contaminant organisms to species or testing antibiotics in the medium.
• Endophytic (systemic) bacteria diagnosis and biochemical control.
• Shoot multiplication, rooting and acclimatisation stages beyond the initial establishment response.
• Other species, explant types (leaf, internode) or media variants (hormone-supplemented).

Research design: a laboratory factorial audit — one factor (sterilisation protocol, 4 levels) with replication at two levels (three session-batches × 12 explants per protocol), observed longitudinally for four weeks. Outcome analysis is by contingency-table statistics; onset analysis is descriptive Kaplan-Meier-style tabulation (proportion still clean at each week).

Treatments (all preceded by running-tap wash 10 min, 5 % Tween-20 wash 5 min, and 70 % ethanol dip 30 s):

• P1: 2 % NaOCl (from household bleach, diluted fresh), 10 min
• P2: 2 % NaOCl, 15 min
• P3: 0.1 % HgCl₂, 3 min
• P4: 0.1 % HgCl₂, 5 min

All protocols end with three sterile-distilled-water rinses inside the laminar-air-flow cabinet.

Explant and medium: nodal segments 1–1.5 cm with one axillary bud, cut from the same hedge block; MS basal salts + 3 % sucrose + 0.7 % agar, pH 5.7 before autoclaving (121 °C, 15 psi, 15–20 min), 10 ml per 25 × 150 mm tube, one explant per tube.

Layout and randomisation: 12 explants per protocol per batch session; the order of protocols within a session is randomised by draw, and the medium batch is common to all four protocols within a session. This cancels medium-batch effects and operator fatigue effects across protocols.

Observation (Weeks 5–8 of the project): weekly scoring sheet per tube — contamination present (Y/N), type (bacterial: wet, glistening halo usually within week 1–2; fungal: fuzzy mycelial growth, typically later), browning score (0 none – 3 severe), bud response (green/swollen/sprouted), and date of first sign. Suspect tubes are only observed, never opened outside the cabinet.

Analysis plan: build the 2×4 table (clean at week 4 vs contaminated) and the establishment table (clean and viable vs failed); chi-square test via scipy.stats.chi2_contingency with expected-count check, Fisher's exact fallback for small counts; pairwise protocol comparisons only if the overall test is significant, with a note on multiple comparisons; browning-rate comparison as a second contingency table; onset distribution tabulated by week and type. All computations duplicated once in Excel as a cross-check.

Timeline (12 weeks, team of two): Week 1 literature and protocol drafting, approval from the lab in-charge; Week 2 reagent and medium preparation, sterility checks of the cabinet and medium (48-h blank-tube incubation); Weeks 3–4 explant collection, inoculation sessions (two per week); Weeks 5–8 weekly scoring (alternating lead); Weeks 9–10 analysis; Weeks 11–12 report, slides and viva preparation.

The study is a linear bench pipeline with a statistical decision stage. Every tube is traceable from bush to contingency table.

flowchart TD
  A["Collect Hibiscus shoots from one hedge block"] --> B["Excise 1-1.5 cm nodal segments"]
  B --> C["Tween-20 wash + 70 pct ethanol dip (common pre-treatment)"]
  C --> D["Randomise protocol order per session"]
  D --> E["P1 NaOCl 10 min"]
  D --> F["P2 NaOCl 15 min"]
  D --> G["P3 HgCl2 3 min"]
  D --> H["P4 HgCl2 5 min"]
  E & F & G & H --> I["3 sterile water rinses, transfer to MS tubes in LAF cabinet"]
  I --> J["Growth room 25 +/- 2 degC, 16 h photoperiod"]
  J --> K["Weekly scoring, weeks 1-4: contamination type/onset, browning, bud response"]
  K --> L["2 x 4 contingency tables: clean vs contaminated; established vs failed"]
  L --> M["Chi-square test (scipy chi2_contingency), Fisher exact where counts are small"]
  M --> N["Protocol recommendation for the teaching lab"]

Data organisation

One master sheet, one row per tube: tube ID (protocol, batch, position), treatment details, and a week-by-week score grid. Weekly scores are entered the same day they are taken; photographs are keyed to tube ID. The analysis script builds both contingency tables from the master sheet, so re-classifying one tube updates every statistic.

Safety-critical controls

flowchart LR
  A["Mercuric chloride 0.1 pct"] --> B["Gloves + lab coat + eye protection"]
  B --> C["Dedicated waste jar, never the sink"]
  C --> D["Heavy-metal waste handed to the authorised disposal channel"]
  E["NaOCl 2 pct"] --> F["Fumes: work under LAF / ventilated bench"]
  F --> G["Prepare fresh daily; never mix with acids"]

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

• Contaminant identification: Gram staining and simple biochemical tests on representative bacterial contaminants, or lactophenol cotton blue mounts for fungi, to explain the onset patterns.
• Antibiotic strategies: cefotaxime supplementation trials for endophytic-bacteria-prone species.
• Stage progression: carry the winning protocol's established cultures into shoot multiplication and rooting, closing the full micropropagation loop.
• Seasonality: repeat the audit across pre-monsoon and post-monsoon shoot flushes to quantify how ambient microflora shifts the optimum.
• Mercury-free certification: a larger trial of chlorine-only protocols sized to rule out a practically important difference.
• A reusable departmental protocol card updated each year with the new audit results.