Why doesn't bacteria grow on honey?

Question

I did a science expirement in class and i need to write a conclusion. We put honey on the bread and mold didn't grow, but it did grow on the bread without honey. Our teacher mentioned something about honey not letting bacteria grow on it???? please give more info, and also if u can, sites so that i can look it up also. thanks!

Answer 1

I think it is because honey is sweet and provides an infavorable environment for the bacteria.  Cells need to be in an environment that prevents them from losing water through osmosis.  If an animal cell is put into pure water, it will burst because water rushes into the cell from the area of high water concentration to low water concentration.  In the case of mold and bacteria, the cell wall prevents them from bursting as easily, however, if they are put in a solution with high concentrations of salt or sugar, water rushes out of the cell to the area of lower water concentration--the environment.  Honey is an antiseptic because it provides an infavorable environment for the bacteria and mold to retain water.

Answer 2

The unique chemical composition of low water content and relatively high acidic level in honey creates a low pH (3.2-4.5) environment that makes it very unfavourable for bacteria or other micro-organism to grow. So, when properly stored, honey has no expiry date.

Answer 3

Honey has several antibacteria properties. One of these is osmolarity. That is, the concentration of solute, particularly sugar, is very high. Living cells have to maintain an equilibrium with the environment around them. According to basic physics, the solution will seek an equilibrium (that is, the solute will eventually disperse through the solvent until the concentration is uniform). Here is the catch: cell membranes are what are called semi-permeable. The water can pass through, but larger solute molecules (like sugar) can't. There are special transporters for these things, but they tend to be slow and require energy expenditure. Plus, the concentration of certain solutes in the cell is important for biochemistry. Thus the cells have some ability to adapt to changes in the solute concentration (osmolarity) of their environment, but when this adaptive ability is exceeded their growth is stunted, or they may die. If cells are placed in a hypotonic solution (one with less solute than the cytoplasm) they will absorb water until enough pressure builds up to burst the cell membrane, which is called lysis. If, on the other hand, the solution is hypertonic (more solute than the cytoplasm) the water is drawn out of the cells until they shrivel and die, which is called crenation. This is what happens to bacteria that try to grow in honey.

As an example to picture this process, chop some strawberries and put them in two bowls. Sprinkle one with plenty of sugar and stir them up well. After a few minutes, examine the difference between the two. See how the ones with the sugar become so juicy? See how the strawberry pieces have shriveled? The high concentration of sugar outside the cells has drawn fluid out.

The ironic and slightly confusing thing here is that sugar is generally credited with killing the bacteria. But don't bacteria need sugar for food? In fact, almost all organisms metabolize sugars for energy. What makes the sugar lethal to the microbes  is the concentration. If honey is diluted sufficiently, it can be fermented to produce an alcoholic beverage known as mead. The sugars in honey can, in fact, be food for microorganisms.

There are, however, other factors in some honeys that enhance their antimicrobial effects. One of these is the generation of hydrogen peroxide. Honey can actually give off low concentrations of hydrogen peroxide overtime, contributing to the antibacterial effect.

Also, since honey is made from plants, it may have additional properties from the plants the bees visited, including phytochemical antimicrobials. Most notably, manuka honey contains antibacterial compounds.

Overall, osmolarity is the classical view of honey's antibacterial properties and it is a basic principle of physics that most students are expected to learn in high school. The roles of hydrogen peroxide and phytochemicals are more complex and less well known, but they definitely play a significant role. Honey out performs sugar solutions as an antibacterial agent, and fact that some honeys are far more potent antibacterials than others is explained by these factors.

As a practical note, honey has been used to treat wounds for centuries. Common table honey is probably at least as effective as neosporin for preventing infection and promoting healing of cuts and scrapes. Highly potent antibacterial honeys are being investigated for treating drug resistant wound infections and promoting healing of serious injuries and burns.