The effects of antibiotics on the growth of Escherichia coli have been found in several studies and some recent studies have also focused on the tolerance and reduced growth levels in bacteria to examine the molecular changes that allow such as change. E. Coli and anti-microbial agents: In a study by Dixon et al (2004), the antibacterial effects of microcystin, which is, a cyanotoxin produced by Microcystis aeruginosa have been discussed. In the presence of microcystin, the inhibitory values for a range of hydrophobic inhibitors were significantly reduced. Dixon and his colleagues studied the direct effects of appropriate concentrations of microcystin on the integrity of bacterial inner and outer membranes and found that the presence of microcystin affects the permeability levels of entero-bacterial outer membranes.
Tolerance to anti-microbial agents in seen in bacteria, which shows a slower growth rate or which no longer, multiplies. This is very common in the E. coli bacteria, which shows a decreased growth rate after being exposed to antibiotics. In clinical infections bacteria tend to multiply slowly and extended periods of anti-microbial chemotherapy are needed to eradicate these organisms and achieve complete cure. Hu and Coates (2005) used transposon mutagenesis to understand the molecular basis of antibiotic tolerance. The authors screened 5000 Escherichia coli mutants to see reductions of kanamycin tolerance in the late stationery phase and found that 4935 mutants were able to grow to the late stationery phase. The mutant KS639 was most sensitive to kanamycin. This variety of mutant showed an increased sensitivity to kanamycin and gentamicin, ciprofloxacin and rifampicin. From the data obtained it was seen that a mutant lacking intergenic regions showed reduced tolerance to kanamycin. The studies show that interegenic regions in the E coli may be responsible for anti-microbial agents.
In a study that tend to examine the effects of ciprofloxacin on E coli growth, Lueng et al studied the effects of the uptake and release of ciprofloxacin from a hydrophilic stent in an antibiotic solution and the effects of a ciprofloxacin loaded stent in inhibiting the growth of E.coli adherence were tested. The authors immersed segments of (hydrophilic stent) HS in 5 ml of ciprofloxacin solutions for 24 hours and ciprofloxacin remained in solution measure determined the uptake by the HS. CHS (ciprofloxacin-loaded stent) was placed in 5 ml of water for 24 hours and the released ciprofloxacin was measured. CHS was placed on culture plates with E coli and incubated and diameters of the inhibited zones were measured. CHS 0.5 cm in length was incubated in separate 5 ml E coli suspensions. This E coli was measured and compared with control HS. The results showed that zonal inhibition to growth of Escherichia coli was, proportional to the concentration of ciprofloxacin. Accordingly the authors concluded that ‘there was a free exchange (uptake and release) of ciprofloxacin along a concentration gradient between the antibiotic solution and HS. CHS reduced the number of adhered E coli, but the effect was short-livedâ€™.
Strains of E. coli and Resistance to Antibiotics:
Strains of Escherichia coli that are capable of contaminating raw milk can show heightened resistance to anti-microbial drugs. The susceptibility of E. coli that originates in milk and milk products, meat and several antibiotics such as cotrimoxazole, streptomycin, cephalothin, neomycin and chloramphenicol, erythromycin, ampicillin and amikacin. The minimal inhibition concentrations were detected using a standard micro-dilution method. Babak et al (2004) stated the necessity to identify bacterial strains that have acquired potentially transmissible resistance to anti-microbial drugs. The study by Babak and his colleagues differentiated two kinds of E coli strains, one that is susceptible to the adverse effects of antibiotics and another that is resistant to anti-microbial drugs.
There is a global expansion of bacterial resistance to anti-microbial agents such as methicillin and vancomycin with the Staphylococcus aureus showing increased resistance to methicillin and decreased sensitivity to vancomycin. The plague bacillus possesses a plasmid that is transferable to E. coli and has multiple antibiotic resistances. Vancomycin resistant enterococci are constantly transmitted to resistant organisms. These resistant strains have been effectively studied by McCormick (1998) to delineate the antimicrobial-resistant bacterial pathogens.
Escherichia coli was found in cattle faeces and novobiocin was used in the isolation method when samples of E coli were separated in different occasions. This study by Tutenel et al (2003) effectively links the isolation of E. coli O157 samples using the antibiotic novobiocin suggesting the adverse effects of antibiotics on bacterial growth or survival.
In a recent study by Chartone-Souza et al (2005), a tetracycline-platinum complex was synthesized which was found to be as effective as tetracycline itself in inhibiting bacterial growth of E coli and in this particular study two Escherichia coli sensitive bacterial strains. This tetracycline complex is six times more potent that tetracycline against E Coli HB101/pBR322, a bacterial strain that has developed a resistance to tetracycline. According to Chartone-Souza and others their study is extremely important given the fact that emergent resistance strains of E coli have made it difficult to treat bacterial infections with tetracycline.
From the studies discussed above, we see two distinct trends of the effects of antibiotics on the growth of E. coli. Antibiotics can develop increasingly resistant mutant strains of bacteria or can inhibit the growth of a particular strain. Whatever the results are, there have been numerous studies that have substantiated the fact that antibiotics have considerable adverse effects of the growth of E. coli and other bacterial strains.