I have a very short window of collecting the water samples (50ml) and wont be able to culture them for a month. For how long I can store them and and is there anyway to increase the storage time.
I would suggest enrich the sample with appropriate enrichment broth and store as glycerol stock. when you to work spread them on plate and isolate your desired microbes.
The bacterial culture methods that I am familiar with required storage on ice and processing within 24 hours (fecal and total coliform, fecal strep.). There were times when collecting and testing took incredible measures to accomplish. At times, I loaded lab equipment and took to work at various field or temporary housing locations. Today there may be more options for remote power sources through solar or batteries to run equipment, but the expense may not be acceptable. I used Millipore Filter Company equipment, and where equipment was too expensive, help from our electronics technician developed water baths to maintain incubation temperature and ultraviolet sterilizer. Unsure if the delivery of iced water samples from remote sites could be facilitated by using drone to deliver samples to lab. Most would not consider helicopter transport for timely delivery to lab, but an option.
If I was going to seriously try your long term storage before analyzing, I would take large samples from the same or similar water sampling sources or locations, and then run subsamples at 1, 2, 4, 7, 14, 21, 28 and 35 days, plotting the analysis results over time. If you try alternative preservation measures, test their response also. Although most assume that there would be a bacterial decline over time, but important to document the changes, and possible differences with water sources sampled. Going to alternative methods from standards sometimes requires substantial added work and documentation, reporting.
If you really need to wait a month then I would run the tests that William F. Hansen suggested, otherwise you really will not for certain about the accuracy of your data. However this is likely to be true if your interest is only in total count. If you are seeking to determine the presence or absence of specific microbes, they might be very different in their survival in storage in water. While some will undoubtedly survive for a month, I strongly suspect that many others won't.
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