Yes, bacteria can grow in simple syrup. Despite the short ingredients list, bacteria is still able to grow. You can see when bacteria are growing, because the syrup will turn cloudy. Once this happens the syrup is no longer safe for consumption, and needs to be trashed. The best way to avoid bacteria is to store syrup properly and use it quickly. Keep simple syrup in the fridge for a longer shelf life, and to avoid bacteria growth. The lid should be tightly secured, and avoid dipping anything into the syrup. Even a spoon can introduce bacteria. If you’re making your own syrup, use the hot process method to prolong the use of the syrup, as it kills bacteria better. Store your syrup in sterilized containers. Increasing the sugar to water ratio also helps. The higher the sugar content, the longer lasting the syrup.