TORONTO — The rate of antibiotic prescriptions written in Canada rose marginally last year, a report released Thursday showed. While the increase was slight, experts expressed concern that with rising usage comes greater risk that bacteria will develop resistance to these essential drugs.

Canadian doctors wrote 63.5 prescriptions for every 100 Canadians in 2005, up from 61.9 per 100 in 2004, the annual report from the National Information Program on Antibiotics revealed. It's not clear what drove the increase.

"This may not seem like a major change but it's the first time in 10 years that we have seen an increase in antibiotic use. And this needs to be followed up closely," said Dr. Francois Boucher, a pediatric infectious disease specialist at the Centre hospitalier universitaire de Quebec in Quebec City.

"It should serve as a wake-up call to everyone -- physicians, pharmacists and patients -- that we need to be more judicious about our use of antibiotics.''

The annual report on antibiotic resistance is a project mounted by a coalition of concerned organizations under the umbrella of the National Information Program on Antibiotics. Their efforts to track resistance rates and educate the public about the threat of resistance receives financial support from drug maker Pfizer Canada.

Dr. Donald Low, one of the country's leading microbiologists, said rates of antibiotic resistant bacteria in Canada remain relatively low in comparison to those of other countries. But he suggested slight upticks on rates of Streptococcus pneumonia that are resistant to antibiotics in the macrolide class and some other important bacterial pathogens bear watching.

He also warned it may be only a matter of time before a worrisome bug causing persistent infections in the U.S. makes bigger inroads in Canada. Cases of methicillin resistant Staphylococcus aureus acquired in the community have soared in the United States. That has confounded and disturbed experts as these pathogens are normally acquired in hospital.

"This is something that we just don't understand yet," Low said of the phenomenon.

"But (it's) important that we learn because it's having a major impact on patients, individuals and also it's going to have a major impact on how we use antibiotics and on costs to the health care system."

These trends highlight the need to drive home to the public that there are risks attached each time someone takes a course of antibiotics -- drugs the public typically views as medical wonders that cure myriad bacterial infections, Low suggested.

Every use of antibiotics increases one's risk of acquiring an infection with a bacterium that is resistant to some antibiotics, he said, regardless of whether one uses it entirely appropriately or stops in mid course, against doctors's advice.

A study Low and some colleagues published last summer showed exactly that.

"It didn't matter if you were taking an antibiotic for a skin infection or a urinary tract infection, it affected the flora of all of your body," said Low, chief microbiologist at Toronto's Mount Sinai Hospital.

"So that if within the next three months you had an inner ear infection, those bacteria were more likely to be resistant to that antibiotic you were on three months ago. There's a price to pay."

A perfect example of this use-risk relationship can be found in soaring rates of Clostridium difficile or C. difficile infections in hospitals, Low said.

C. difficile outbreaks have plagued a number of hospitals across North America -- notably in parts of Quebec -- and in Britain. Hospital patients on antibiotics for other ailments can develop debilitating C. difficile diarrhea which can in severe cases lead to colitis and even death.