# How does osmolarity affect bacterial growth?

May 23, 2015

For most bacteria, increasing osmolarity decreases bacterial growth rate. Some bacteria have a preferred osmolarity for maximum growth.

Most bacteria do not need to regulate their internal osmolarity with precision because their cell walls can sustain a considerable osmotic pressure.

The availability of water is the critical factor that affects the growth of cells.

The availability of water is measured by the water activity, ${A}_{\text{w}}$. For pure water, $\text{A"_"w}$ is 1.00.

As the osmolarity increases, the ${A}_{\text{w}}$ decreases. Some typical $\text{A"_"w}$ values are

• blood, 0.99
• seawater, 0.99
• maple syrup, 0.90
• Great Salt Lake 0.75

The most common solute in nature is salt, $\text{NaCl}$.

We can classify bacteria according to their growth response to salt.

Nonhalophiles such as E. coli, Pseudomonas and Aquaspirillum serpens cannot tolerate even low levels of osmolarity.

Halotolerant bacteria such as Staphylococcus aureus grow best in the absence of NaCl but can tolerate moderate levels of osmolarity.

Mild Halophiles such as Alteromonas haloplanktis and Pseudomonas marina can tolerate 1-6% salt (sea water is 3% salt).

Moderate Halophiles such as Paracoccus halodenitrificans and Vibrio costicolus grow best in a medium that contains 6-15 % salt.

Extreme Halophiles such as Halococcus morrhuae, Halobacterium salinarium, and Pediococcus halophilus thrive in a medium that contains 15-36 % salt.

From left to right, the graph above shows the growth rate of a nonhalophile, a halotolerant bacterium, and an extreme halophile.