Bacteria are everywhere. A few give diseases like tuberculosis (Mycobacterium tuberculosis), but most are very useful. Some lactic acid bacteria transform milk into yogurt; others make cheddar cheese from milk.
One gram of yogurt, for example, has as much as 10 million Lactobacillus delbrueckii bulgaricus bacteria and 100 million Streptococcus thermophilus, which represent for a 100g cup of yogurt a total of 11 billion lactic acid bacteria. These bacterial cultures can also be purchased in small packets in the grocery store to make yogurt at home. Those I bought here in Victoria looked like a white powder that can be added to milk after it has been heated and slowly cooled.
Leaving the inoculated milk in a warm place (I personally use the oven) for few hours allows these lactic acid bacteria to grow by feeding on lactose and casein in the milk, and to produce lactic acid, giving the yogurt its slightly sour taste. Other lactic acid bacteria like Lactobacillus plantarum, for instance, play a key role in the fermentation of vegetables like cabbage (sauerkraut) or green Manzanillo olives that are found in grocery stores.
Although these bacteria are very small, they still can contract a cold-like virus. When you catch a cold, you have been infected with a type of virus scientists call Rhinovirus.
Compared to bacteria, the vast majority of viruses are about 10 times smaller. But unlike bacteria, they cannot divide on their own. They have to be inside a cell to be able to replicate their genes and produce their proteins.
Viruses can also infect the lactic acid bacteria that are so important in the dairy industry. Because the bacterium infected with a virus disappears, these viruses became known as bacteriophages (eater of bacteria), or more simply “phages.” More accurately, phages break up the cells in a process called cell lysis, which releases several copies of the phage that initially infected the bacterium.
There are many types of phages, but they are usually specific to only one type of bacteria. If one type of phage starts infecting the lactic acid bacteria used in a yogurt or cheese factory, it can be catastrophic. It is like an infection in a hospital. It slows down the production of cheese and causes economic losses until the phages are removed.
Phages sometimes may even be the cause of tragic outcome. When a phage called beta phage infects a bacterium called Corynebacterium diphtheria, it produces a very potent toxin and a disease (diphtheria) that can kill people. Today, the diphteria-tetanus-pertussis (DTaP) vaccine contains an inactive toxin, which allows vaccinated children to build immunity against the diphtheria toxin. Before law required pasteurization of milk, raw milk was one of the ways diphtheria was transmitted.
The first to discover phages was a Canadian born in Montreal: Félix d’Hérelle (1873-1949). He found a phage of dysenteric bacteria that could clear a cloudy culture without the phage being retained by a porcelain filter. In time, other phages were discovered and became models in the study of the molecular basis of life.
Today we know that phages may play an important role as a genetic shuttle between different types of bacteria, which is one mechanism that explains the development of resistance of bacteria to antibiotics.
Between 1896 and 1899, before his discovery of bacteriophage, d’Hérelle received a grant from the Canadian Minister of Revenue to develop a method of fermentation of maple syrup for the production of whisky. The minister was Henri-Gustave Joly de Lotbinière (1829-1908), the seventh governor of British Columbia from 1900 to 1906.
Réal Roy is a microbiologist and an assistant professor in the department of biology at the University of Victoria.