Environmental Engineering Reference
In-Depth Information
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08
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04
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48
No. of Freeze -Thaw Cycles
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48
No. of Freeze -Thaw Cycles
Fig. 2.2 Viability of microbial cultures as a function of the number of freeze-thaw cycles in
the cryocycler. Upper histograms: viable cells (colony forming units or CFU/ml) in mixed
cultures derived from one of the soils tested (first white bar in each cycle set) were compared to
pure cultures of P. chlororaphis (hatched, second bar in each cycle set) or E. coli (black, third
bar in the cycle sets) in 48 freeze-thaw cycles. Lower histograms: viability of single colony
isolates after serial freeze-thaw cycles. Microbes that survived two consecutive 48 freeze-thaw
cycles were used to initiate cultures from single colonies of Chryseobacterium sp. C14 (first
white bar in each set) and Enterococcus sp. (grey, third bar in each cycle set), and these were
subjected to further freeze-thaw treatments. E. coli (black, second bar in each cycle set) was
again used as a control. Error bars represent standard deviations (modified from [50])
Unexpectedly some microbes, such as Enterococcus sp., survived freeze-thaw
cycles as part of a consortium, but were less freeze-thaw resistant in isolation
(Fig. 2.2 and not shown). Further investigation revealed that their survival was
influenced by the presence of another bacterium, Chryseobacterium sp. C14. When
Enterococcus sp. and this Chryseobacterium strain were cultured together, both
showed freeze-thaw resistance [27]. Furthermore, when pelleted Enterococcus sp.
cells were resuspended in the cell-free, spent medium obtained fromChryseobacterium
sp. cultures, their survival increased 1000-fold. Thus, these selection experiments also
appear to reveal something about the dependence and interaction of certain microbes
in their communities.
