Troubleshooting a Contaminated Banana Tissue-Culture Batch
At a banana tissue-culture unit, 60 shoot-tip cultures are initiated from field-collected suckers. After 10 days, 18 jars show a cloudy halo in the medium, 11 show bacterial ooze from the explant base, and 7 show white cottony mycelium. Contamination appears only in jars from one autoclave run. Records show the autoclave reached 121 °C for 15 min, but the 20-L chamber was loaded with 12 tightly packed 1-L jars and the run started with cold water. Medium pH was 5.7 before autoclaving and 4.9 after; sucrose is 30 g L−1; hormone stocks were filter-sterilized. The lab has a laminar-flow hood, an autoclave with a validated 121 °C cycle, and no antibiotic stock. The manager must decide whether to discard all 60 cultures, re-autoclave the medium, or change the loading and cycle, and needs a root-cause explanation before the next batch.
1. Separate the contamination symptoms into likely bacterial and fungal causes and explain why the distribution across jars points to the autoclave run rather than the hood technique.
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The cloudy halo in the medium and the bacterial ooze from the explant base are consistent with bacterial contamination. The white cottony mycelium is consistent with a fungus. The key clue is that contamination is confined to jars from one autoclave run. If the laminar-flow hood or operator technique were the source, contamination would be scattered across runs and would not cluster by autoclave load. The fact that 24 jars remained clean also fits incomplete sterilisation within a load, where some jars reached temperature and others did not. The pH drop from 5.7 to 4.9 is not proof of contamination; it can result from heat-induced changes in the medium. The pattern therefore points to the autoclave load as the root cause, not the hood.
2. Explain why starting with cold water and tightly packing jars can defeat a 121 °C/15 min cycle even when the gauge reaches the set point. Propose two verifiable corrections.
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An autoclave sterilises with saturated steam, not with the chamber gauge reading alone. Starting with cold water creates a long lag before steam quality is established, and tightly packed jars trap air and create cold spots. Air is an insulator, so the centre of a jar or the centre of the load may not reach 121 °C for the full 15 minutes even though the chamber probe or gauge shows the set point. The gauge measures the chamber atmosphere, not the coldest point inside the load. Two corrections are: preheat and vent the chamber properly before the sterilisation cycle, and load jars with space between them, preferably in perforated baskets, so steam can circulate. Validate the change with a biological indicator such as Geobacillus stearothermophilus spores placed in the centre of the load, and use thermocouples to confirm the coldest point reaches 121 °C for the full holding time.
3. The manager wants to salvage the 60 cultures. Recommend a course of action and state one risk. Include a practical test to confirm the cause before the next batch.
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Discard the obviously contaminated jars; do not attempt to rescue them with antibiotics, because antibiotics can mask latent contamination and are not a substitute for sterilisation. The 24 clean-looking jars can be retained only as indexed cultures, subcultured separately, and watched for delayed contamination. Do not re-autoclave medium that already contains cultures; prepare fresh medium and sterilise it with the corrected cycle. Before the next batch, run the autoclave with biological indicator vials in the centre and at the corners of a representative load, incubate the indicators, and also hold a sample of sterilised medium at 28 °C for 7 days as a sterility check. The main risk of keeping the clean jars is latent contamination that appears after subculture; the main risk of changing the cycle without validation is either repeated contamination or heat damage to the medium.