Moisture.
Moisture is a very important factor when it comes to the growth of bacteria and if the environment condition were dry then most bacteria would die. Some bacteria’s, on the other hand, can survive in dry condition for month or even years, but for the majority of bacteria’s they cannot survive at all without moist conditions. This is why bathrooms and kitchens are the most prone to bacterial growth, foods that have a high enough water content make good environments for bacterial growth. The microbes most tolerant to low water levels are xerophilic moulds, the next are osmophilic yeasts and the least tolerant microbes to low water levels are halophilic bacteria’s. Because moulds and yeasts are less affected by low water levels this increases the chance of them growing in drier conditions, which means they can carry out their normal functions with less water than bacteria’s could.

Effects of low water levels:
As moisture is required to produce bacteria in the fastest way and most efficient way, so when there is a lack of moisture it effects the production of bacteria in many ways: it slows down the growth rate and increases the lag period, weakens or damages transport, and loses membrane fluidity.
Halophiles are microorganisms that can survive in exceedingly salty conditions, such as lakes (the Great Salt Lake) or seas (the Dead Sea). These microorganisms need excessive amounts of salt that most other microorganisms can’t survive in.
Osmophiles are microorganisms that can survive in exceedingly sugary conditions.
Xerophiles are microorganisms that can survive in exceedingly dry conditions.

Examples:

Staphylococcus aureus is an example of a bacteria that cannot survive in such dry conditions. It is a microorganism that can be found in foods, such as meats and bakery products. It cannot survive in places where the water activity is less than 0.85 and it has a sugar tolerance of 50 to 60% and a salt tolerance of 10 to 20%. Staphylococcus aureus is pathogenic and you can get the disease when food contaminated with toxins from staphylococcus aureus is ingested, symptoms of the disease may include: severe abdominal cramps, nausea, vomiting, or diarrhoea. When this pathogen is ingested it could take up to six hours to notice any symptoms but when symptoms arrive they can last up to two days.

Bacillus Cereus is an example of a bacteria that needs moisture to grow but it can survive in dry conditions for a limited amount of time. It is a spore-forming bacterium that can be found in many foods, such as pastry, rice, pasta, and potatoes. Bacillus cereus has a minimum water activity of 0.95 but can decrease to 0.93 if in contact with glycerol or NaCl and even though the minimum water activity for the growth of bacillus cereus is 0.95 the spores from it can survive for long periods of time in dry foods, such as cereals. Bacillus cereus is pathogenic and can cause such symptoms as vomiting and diarrhoea. There are known to be two kinds of intoxication syndromes associated with bacillus cereus: emetic and diarrhoeal but the recovery rate food both is usually twelve to twenty-four hours. Emetic syndrome symptoms can include: nausea, vomiting, malaise, diarrhoea (in some) and these symptoms can be seen from when the contaminated food is ingested to six hours after ingesting the contaminated food. Diarrhoeal syndrome symptoms can include: abdominal pain, watery diarrhoea, nausea (in some) and theses symptoms can be seen from eight to sixteen hours after the contaminated food is ingested.

Listeria Monocytogenes is an example of a bacteria that needs moisture to be able to growth and is known to be able to survive at lower water activity levels for a limited period of time. It is a bacterium that can be found many foods, including those high in sugars and salts. Listeria monocytogenes has a minimum water activity of 0.90 in glycerol and 0.92 in NaCl/sucrose. The microorganism can grow in conditions with high levels of salt: