Probiotic bacteria help to reduce overgrowth of harmful yeasts and bacteria in your intestines in a number of ways. They produce lactic acid, acetic acid and hydrogen peroxide which lower intestinal pH and discourage reproduction of less acid-tolerant, harmful bacteria. They also secrete natural antibiotics (bacteriocins such as acidophiline and bulgarican) and stimulate production of interferon - a substance which helps to protect against viral infections. Probiotics also compete with harmful bacteria for available nutrients and for attachment sites on intestinal cell walls. If these sites are already occupied by friendly bacteria, potentially harmful microbes cannot gain a foothold in your intestines so easily and are more likely to get flushed out.
As a result of all these beneficial actions, probiotic bacteria have a powerful, protective action against a number of intestinal infections - especially those responsible for travellers´ diarrhoea such as Bacillus cereus, Salmonella typhi, Shigella dysenteriae, Escherichia coli and Staphylococcus aureus. Even if you don´t take probiotics at any other time, you should definitely consider them when travelling abroad.
Probiotics can also reduce the well-known intestinal side effects of broad-spectrum antibiotics which, as well as killing harmful bacteria, will also deplete your natural population of healthy, probiotic microbes.