I am planning on growing bacteria from a seawater sample to do a research project on the relationship between pH (lowered using sulfuric acid) and the bacterial concentration of the water sample (colony forming units per $cm^3$ ). I am planning on using the following procedure: enter image description here

Would I be able to use the same procedure, or would I need to make modifications to the agar nutrient media because the bacteria I am using would be used to a higher salinity? I am using standard plate agar.

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  • $\begingroup$ Normal media are at approx "physiological saline" concentration which is usually about 0.9%. I would have thought that seawater bacteria would need saline to be about what they are used to. $\endgroup$
    – bob1
    Commented Jun 9 at 20:58

1 Answer 1


Bacteria all have unique ranges of environmental conditions that will support their growth. If we assume they are growing in the saltwater and not just existing, Marine bacteria would at least need to be halotolerant, since they're growing in 3.3-3.7% saline. Some of them will be halophiles, meaning they require at least 1.7% salt (~2x physiological saline). Another point of interest is that media specifically made to culture marine bacteria don't just have higher salinity, they use several other salts (besides NaCl) to reach this salinity level. Some of these may just be buffering agents, but they also provided different dissolved ions (magnesium, calcium, potassium, borate, bromine, strontium, etc) that can be found in seawater a varying concentrations, and could be growth promotion factors for some marine life.

The halophiles will not grow on agar media made for non-halophilic bacteria. The halo-tolerant organisms might grow, as long as their other growth requirements are met. But if the salinity is outside of their optimal growth range, they will likely be more sensitive to other environmental variables, like changes in pH.

If you're only interested in testing pH as a variable, you should use a growth medium with salinity similar to that of their natural environment. If you are interested in their overall environmental growth requirements, you could start with a salt tolerance test at a pH close to neutral, and then do the pH testing at various salt concentrations to see how those factors interact.


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