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The next cause of resistance is antibiotic promoted resistance. When an antibiotic attacks a colony of bacterial cells, those that are more susceptible to the drug will be destroyed first. But the cells which have some resistance from the start, or which acquire it later, through mutation or gene exchange, may survive, especially if too little drug is used to completely overwhelm the cells that are present. This is often the case when people stop taking the drugs after the symptoms of the disease have been alleviated and fail to complete the full course of treatment. The survivors may reproduce to form a 'reformed' population of bacterial cells, replacing the previous one. Since some bacteria may reproduce every 20 minutes, even a single survivor is sufficient to give rise to a new resistant population which is immune to the effects of the antibiotic.
Even when antibiotics are used in the required amounts, they may lead to resistance. When the antibiotics attack disease-causing bacteria, they also affect benign bacteria forming part of the natural flora of the human body. These bacteria normally check the population of harmful ones by competing with them for resources, thus keeping their numbers low. With the 'good' ones gone, the resistant virulent bacterial population would proliferate with greater ease. Even if the harmless bacteria are only replaced by non-pathogenic ones, the indirect consequence is an increase in the reservoir of resistance traits in the bacterial population as a whole. This raises the probability that resistance traits will spread to pathogens by any of the three processes already dealt with earlier. In other words, overexposure to antibiotics also brings about resistance. Overuse or overexposure may result from different circumstances and settings. To get an idea about the extent of the use of antibiotics nowadays, the total amount of penicillin available for use in clinical trials in 1941 was less than the amount one would receive in a single shot today! (Brown, 1995).
Prolonged and/or frequent use of antibiotics in the treatment of diseases is one of the most common causes of antibiotic resistance. Some strains of streptococcus pneumoniae have been seen to show resistance to fluoroquinolone, although the latter is one of the most recently introduced drugs with only 15 years of clinical use. A study carried out in Canada showed that resistance is most common among the elderly and in Ontario where antibiotic prescription is the highest (Mitchell, 1999). Researchers at the Centre for Disease Control (Atlanta, USA) have estimated that some 50 million of the 150 million prescriptions for antibiotics every year are unneeded (Levy, 1999).
