Contamination 

Contamination of bacterial cultures can be very problematic, and may mean the need to reisolate a pure culture. Culture contamination can come from many sources, right from the original sample itself through the process of culturing and even storage. Good aseptic technique can help to avoid contamination of bacterial cultures. 

Overgrowth of some species

Some bacterial species grow easily and vigorously. When attempting to isolate a species from a mixed sample, these vigorous species may overgrow and mask the presence of slower-growing target species. Using selective media and optimal growth conditions for your target species (if known) can help to mitigate this. Try to culture the sample as soon as possible after it has been taken to ensure it is as representative as possible.

Incorrect growth conditions

The use of [inappropriate or suboptimal growth conditions](https://www.umsl.edu/microbes/files/pdfs/cultureproblems.pdf) may impede or completely prevent the growth of your target strain. Be sure to double check growth requirements or if using antibiotic selection ensure the correct antibiotic has been chosen for the resistance gene present.

Non-culturable and slow-growing organisms

Some bacterial species, even now, cannot be grown in the lab. Others, such as mycobacteria, are very slow growing and can take months to culture successfully.