Friday, September 18, 2009

Plasmid-mediated multiple antibiotic resistance of Escherichia coli in crude and treated wastewater used in agriculture.

Plasmid-mediated multiple antibiotic resistance of Escherichia coli in crude and treated wastewater used in agriculture.

S. Pignato, M. A. Coniglio, G. Faro, F. X. Weill and G. Giammanco
Journal of Water and Health Vol 07 No 2 pp 251–258

Antibiotic resistant bacteria strains are permanent threat to human populations. Genes encoding antibiotic resistance are commonly located on mobile genetic elements like bacterial plasmids. Horizontal gene transfer (HGT) occurs in the environmental condition. The main mechanism of HGT seems to be bacterial conjugation. This process requires direct contact between plasmid bearing, donor strain and plasmid free recipients. The frequency of conjugation depends on a number of different factors. One of them is the cell density. As we could imagine possibility of meeting of two bacterial cells living in 1ml of water is much lower than if there are millions of different cells occupying the same volume. Density of bacterial populations in environmental samples varies markedly depending on sampling sites. Usually is not very high ~106 cfu/ml. One of the places where bacterial population reaches high densities are wastewater treatment plants. So study of spread antibiotic resistance encoding bacterial plasmids in the waste water is very important.
In presented paper authors pointed out that the guidelines for cleaning water used for irrigation requires treatments to remove pathogens that can cause enteric infections for crop consumers, producers and handlers. According to the microbiological guidelines for safe use of wastewater in agriculture developed by the World Health Organization(WHO) less than 0.1 intestinal nematode eggs must be detected in 1 litre, while up to 1,000 faecal coliform bacteria per 100 ml can be tolerated for unrestricted irrigation. In the United States, much stricter wastewater quality standards for irrigation are recommended by the Environmental Protection Agency but, lacking federal standards for the quality of reclaimed water, individual states have developed guidelines mainly based on the daily monitoring of faecal coliform bacteria on a single, 100-ml sample, assuming a predictive relationship between indicator microorganisms and pathogen presence.
Although wastewater treatments proved to be effective in eliminating Salmonella spp. and in reaching WHO microbiological standards for safe use of wastewater in agriculture, they were ineffective in reducing significantly the frequency of plasmid-mediated multiple antibiotic resistance in surviving E. coli.
It was shown that 22.71%, 19.41%, 16.84% and 14.28% out of 273 isolates were resistant respectively to ampicillin, tetracycline, sulfamethoxazole, and streptomycin. Some other antibiotic resistant strains were detected at low frequency (trimetoprim – 9.15%; nalidixic acid – 8%; chloramphenicol – 5.12% and kanamycin – 2.93%). Also multiple antibiotic resistance was present in 24.17% of the isolates. Antibiotic resistance was detected to be transferred by conjugation from 54% resistant strains. Three different plasmids with the sizes of 125kb, 54kb and 60kb were isolated from those strains. Also some other mobile elements like class 1 integrons were detected in resistant strains.

Since multiple antibiotic-resistant bacteria carrying integrons and conjugative R plasmids can constitute a reservoir of antibiotic-resistance genes in wastewater reclaimed for irrigation, risks for public health should be considered. Bacterial strains carrying R plasmids and integrons could contaminate crops irrigated with reclaimed wastewater and transfer their resistances to the consumers’ intestinal bacteria. So we should remember to wash every vegetables and fruits before we will eat them…

Jarek Krol

1 comment:

Jessica Dark said...

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