Resistance of bacteria to antibiotics poses a major threat to the prospect of chemotherapy. While by restricting the use of antibiotics in farming and aquaculture, it might be possible to restrain the problem to some extent, it is definitely not possible to get rid of the problem completely. The resistance conferring genes in many cases co-occur on plasmids with genes conferring tolerance to heavy metals. Hence, even in the absence of antibiotics in the environment, emergence of antibiotic resistant strains is possible in the presence of heavy metals (e.g., mercury) in the environment. Evidences of co-occurrence of antibiotic and metal tolerance in various natural isolates of bacteria have been highlighted.

Antibiotics are one of the most significant achievements of modern science. The rampancy of deadly infections (typhoid, pneumonia, tuberculosis) witnessed during the 19^th century disappeared to a large extent following discovery of a number of therapeutically useful antibiotics (e.g., penicillin, streptomycin, chloramphenicol, tetracycline) in the 20^th century. Unfortunately, these magic bullets failed to maintain their efficacy because of subsequent emergence of the pathogenic strains, which were immune to their inhibitory action. The problem of antibiotic resistance of bacteria has assumed a gruesome proportion at present. Some scientists find reasons to apprehend that we might be pushed back to a situation that we had to live with in the pre-antibiotic era.

Antibiotic resistance is caused by spontaneous mutation and it spreads to other bacteria by various mechanisms of gene transfer (e.g., conjugation, transduction and transformation). Mobile genetic elements also play a significant role in dissemination of resistance conferring genes (resistance genes). Paradoxically, it is the widespread and imprudent use of antibiotics that is promoting the emergence of antibiotic-resistant bacterial strains (Levy, 1992). A substantial portion of antibiotics used by humans or applied to animals and plants for prophylactic and therapeutic purpose is going to the soil. Antibiotic-resistant strains occur in a very small number in naturally occurring population of bacteria and they are kept suppressed by a huge number of antibiotic-susceptible bacteria. Antibiotics added to the soil annihilate these harmless organisms, thus favoring the selection of the resistant strains. The genes conferring resistance bear the potentialities of being transmitted to human and animal pathogens also by the mechanism of horizontal gene transfer. That is how antibiotic resistance spreads due to excessive use of antibiotics in farming and aquaculture. The presence of antibiotic resistance in aquatic isolates of bacteria is frequently reported. A couple of years ago resistance to a number of therapeutically useful antibiotics (amikacin, ampicillin, chloramphenicol, gentamicin, tetracycline and some others) was detected in some bacterial samples collected from the river Torsa which passes through China (Tibet), Bhutan and India before entering Bangladesh (Mukherjee and Chakraborty, 2006). Similar observations were reported involving some freshwater samples of Enterobacteriaceae, collected in Brazil (Lima-Bittencourt et al., 2007). Importance of in-depth studies on this problem hardly needs to be over-emphasized (Smith, 2008).

Biotechnology Journal, Antibiotic resistance, Metal tolerance, Plasmids, Gene transfer, Mercury, typhoid, pneumonia, tuberculosis, penicillin, streptomycin, chloramphenicol, tetracycline, pathogenic strains, spontaneous mutation, conjugation, transduction, Mobile genetic elements, harmless organisms, farming and aquaculture, aquatic isolates of bacteria, amikacin, ampicillin, chloramphenicol, gentamicin, tetracycline,Lima-Bittencourt