Banana shoot-tip batch with 38% bacterial contamination
A district agricultural laboratory initiates 400 shoot-tip explants of the banana variety Embul on MS medium with 30 g/L sucrose, 5 mg/L BAP and 7 g/L agar. The pH is adjusted to 5.8 before autoclaving, but the pH meter was last calibrated three months ago. Vessels are sealed with Parafilm and kept at 26 plus or minus 2 degrees Celsius under a 16-hour photoperiod at 3000 lux. After three weeks, 38% of cultures show bacterial oozing from the base of the explant, 22% show blackening of the cut surface, and many surviving shoots are glassy with four to five small leaves. The subculture interval is eight weeks. The laboratory plans to rescue the batch by adding 100 mg/L cefotaxime and re-cutting the shoots, then sell 2000 acclimatised plantlets within three months to a commercial grower.
1. Rank the most probable causes of the three symptoms and decide whether to attempt rescue of the batch or discard it.
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The three symptoms have different causes and should not be treated as one problem.
Bacterial oozing from the base (38%) points to systemic or endogenous contamination, not casual handling error. At 38%, the mother plants were probably not indexed, or explants were too large and carried vascular tissue with latent bacteria. Once bacteria are inside the explant, surface sterilisation cannot reach them.
Blackening (22%) is phenolic oxidation from wounded tissue, worsened by large explants, light exposure and slow subculture.
Glassy shoots indicate hyperhydricity: 5 mg/L BAP is high, Parafilm limits gas exchange, and high humidity inside the vessel plus low light favour it.
Decision: discard all vessels showing oozing. Cefotaxime at 100 mg/L will not cure systemic contamination; it suppresses visible growth, is phytotoxic, and risks selecting resistant bacteria that then enter the nursery. Retaining visibly clean vessels is defensible only if they are indexed and held in quarantine, but the safer exam-answer judgement is to re-initiate from indexed mother plants using 1 to 2 mm meristems. The cost of carrying latent contamination into 2000 plantlets is far higher than restarting.
2. Specify the medium and environment changes you would make for the re-initiation, with reasons.
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Changes should target hyperhydricity, browning and gas exchange:
- Lower BAP to 2 to 3 mg/L. High cytokinin drives the glassy, condensed morphology; reducing it is the single most effective change.
- Increase agar to 8 g/L or use vented lids with a membrane. Both reduce the water film and internal humidity that cause hyperhydricity.
- Replace Parafilm with gas-permeable sealing film, or use filter vents, to allow ethylene escape.
- Add 0.5 to 1 g/L activated charcoal or an antioxidant such as 100 mg/L ascorbic acid plus citric acid, and keep new cultures in the dark for the first 3 to 5 days to limit phenolic oxidation.
- Calibrate the pH meter against fresh pH 4 and 7 buffers, and check pH after autoclaving, because it typically falls by 0.2 to 0.4 units. A wrong pH affects gelling and nutrient availability.
- Shorten subculture to four weeks and reduce light to about 2000 lux initially.
Reasoning: each change matches a demonstrated cause rather than adding antibiotics or more激素. Antibiotics are not a substitute for aseptic technique and correct culture environment.
3. Can the laboratory credibly deliver 2000 plantlets in three months? What quality checks are essential?
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The arithmetic is tight. With four-week cycles in three months there are three multiplication cycles. If each clean shoot yields a conservative 3-fold increase, starting from 20 indexed shoots gives 20 x 3 x 3 x 3 = 540 shoots, far short of 2000. Even a 4-fold rate from 30 shoots gives 1920, and that assumes no losses, which is unrealistic. The plan is therefore not credible unless they start with at least 40 to 50 clean, indexed shoots or buy in certified in-vitro material.
Essential checks:
- Indexed mother plants and a quarantine area for incoming material.
- Sterility testing of each batch on tryptic soy agar at 30 degrees Celsius for 7 days, plus periodic molecular or ELISA testing for banana viruses such as BBTV and CMV.
- Visual screening for off-types and hyperhydricity at every subculture.
- Records linking each plantlet to a mother line, and a four-week acclimatisation stage with a survival target of at least 90%.
The judgement is that the timeline should be renegotiated rather than padded with contaminated stock.