Clumping of dehydrated culture media
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Humidity was too high during storage
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Container was left open too long
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Container was not tightly sealed after it had been opened
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Dehydrated culture medium was too old
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pH-shift
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Water was not neutral
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Container was not tightly sealed after it had been opened
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Culture medium was overheated during preparation
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Dehydrated culture medium was too old
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Turbidity, precipitation
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Turbidity of the prepared culture medium should only be considered as an error if it appears in the culture vessel (e.g. Petri dish, test tube etc.). Any turbidity observed in the vessel used for preparation due to the presence of a considerably thicker layer of culture medium is of no consequence. Precipitates which settle out to form sediments indicate, however, that an error has been made. Exception: obligatorily turbid culture media!
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Water was not adequately demineralized
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Vessel used for preparation was not clean
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pH-value was incorrect (see "pH adjustment")
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Culture medium was overheated during preparation
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In the case of self-mixed culture media, the basic components contained precipitating impurities
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Caused by the sample material
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Loss of water of the prepared culture medium due to evaporation
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Solidification point too high
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Important when sample material or heat sensitive substances are to be mixed into the culture media when they are still fluid
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Too much dehydrated culture medium was weighed out
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Agar-agar not suitable
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Gel stability too low
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Insufficient dehydrated culture medium was weighed out
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Dehydrated culture medium was not completely dissolved
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Culture medium was overheated, possibly at a low pH value, during preparation (see "pH adjustment")
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Vessel was not swirled before pouring the plates
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In the case of self-mixed culture media, unsuitable or too little agar-agar
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Acidic culture medium was not prepared under mild conditions (see "Acidic culture media")
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color change
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In the case of culture media containing indicators, the pH was incorrect (see "pH adjustment")
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Culture medium was overheated during preparation: culture medium dark, colored pigments destroyed, sugar caramelized
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Vessel used for preparation was not clean
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Ready-to-use culture medium contaminated
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Inadequate sterilization
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Contaminated after sterilization, e.g. while pouring the plates, contaminated Petri dishes
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Growth too poor
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Residues of growth inhibiting substances present in the vessels used for preparation culture (e.g. detergent) in the water used (e.g. substances from the air), in the sample material
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Microorganisms in the sample material already damaged
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pH shift in the case of culture medium
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In the case of culture medium bases, additives dosed incorrectly
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pH shift caused by acid (or basic) sample material
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Culture medium was overheated during preparation
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In the case of pour-plates, temperature was too high
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Growth too strong
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Culture medium was overheated during preparation causing destruction of selective inhibitors
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In the case of culture medium bases, additives dosed incorrectly
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Culture medium was inoculated with too much sample material
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Colonies liquefy or swarm
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Surface of the culture medium was too moist
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Surface of the culture medium was inoculated with too much sample material
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Culture medium was overheated during preparation causing destruction of inhibitors
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Atypical growth
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Culture medium prepared incorrectly
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Dehydrated culture medium was too old
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Prepared culture medium was too old or unfit for use
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Wrong conditions were used for cultivation
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Residues of foreign substances present in the vessel used for preparation or culture (e.g. detergents), in the water used, in the sample material
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